Symposium Organizers
Christopher Richardson, University of Maryland
Jeffrey McCallum, University of Melbourne
Javad Shabani, The City College of New York
Clare Yu, University of California, Irvine
Symposium Support
Google Inc.
IBM Corp.
Microsoft
Quantum Science and Technology | IOP Publishing
EM08.01: Semiconductor Quantum Dot Qubit Materials
Session Chairs
Monday PM, November 27, 2017
Hynes, Level 1, Room 111
8:30 AM - EM08.01.01
Two-Dimensional Hole Gas in MBE Grown Si/Al/Si System
Aruna Ramanayaka 1 3 , Hyun Soo Kim 2 3 , Ke Tang 2 3 , Joseph Hagmann 3 , Curt Richter 3 , M.D. Stewart 3 , Joshua Pomeroy 3
1 , Joint Quantum Institute, Gaithersburg, Maryland, United States, 3 , National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 2 , University of Maryland, College Park, College Park, Maryland, United States
Show AbstractWe have grown and electrically characterized a two-dimensional (2D) hole gas formed using MBE grown Si/Al/Si system that is complementary to P doped 2D system in Si. Realization of 2D systems are important as precursors for other low dimensional devices such as nanowires, quantum dots, single atom devices, etc. Acceptor based qubits in Si have been proposed due to positive advantages over electron based qubits, such as large spin-orbit coupling, and absence of valley degeneracy. Additionally, Al-Si superlattices are also predicted to be superconducting, directly merging the benefits of both super and semiconducting quantum information processing devices. Therefore, realization of a 2D hole system in Si has the potential to become a promising material system in several areas of research. Although several attempts at creating a 2D hole system have been reported, these studies did not report electrical characterizations. Our first Hall measurements of Al delta doped Si devices at 4 K indicate that the buried Al in Si acts as acceptors, and the charge carrier concentration and the mobility is approximately 8x1013 cm-2 and 14 cm2/Vs respectively. Hall effect measurements in a tilted B-field support the conclusion that the dopants are in fact confined to a two-dimensional space. Approximately 20% of the dopants are activated based upon comparison of scanning tunneling microscopy and measured carrier concertation. STM images taken during the fabrication process will be presented along with the electrical results.
8:45 AM - EM08.01.02
Characterization of the Impact of Gate-Induced Strain on Silicon MOS Quantum Dot Performance
Ryan Stein 1 2 , Joshua Pomeroy 3 , Neil Zimmerman 3 , M.D. Stewart 3
1 , University of Maryland, College Park, Maryland, United States, 2 , Joint Quantum Institute, College Park, Maryland, United States, 3 , National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Show AbstractThe operation characteristics of quantum information devices built using gate defined quantum dots in silicon are extremely sensitive to disorder in the local environment of the quantum dot. In the Si MOS system, this disorder can be caused by several different sources including charge defects in the oxide, substrate impurities, and strain. The coefficient of thermal expansion (CTE) mismatch between typical MOS gate materials, such as aluminum, and the underlying silicon substrate is capable of inducing strain that modifies the local silicon conduction band. For quantum dot devices measured at cryogenic temperatures, this local modification of the conduction band is strong enough to lead to the formation of unintentional quantum dots and to affect the tunnel coupling between dots [1,2]. To realize the potential of quantum devices, gate-induced strain must be understood so as to be mitigated or exploited.
In this work, we investigate the role of gate-induced strain in quantum dot devices by comparing measurements of the 4-terminal I-V characteristics of tunnel barrier devices at 2K. The devices are fabricated on bulk silicon wafers with Al and poly-silicon gate electrodes separated by tunnel gap lengths ranging from 20-40nm and gate widths ranging from 50 to 500 nm. We will compare the measured transport characteristics between different devices with the goal of quantifying the level of strain and validating simulations.
[1] Thorbeck, T., Zimmerman, N.M., AIP Adv., 5, 087107, (2015).
[2] Park, J., et.al., APL Materials, 4, 066102, (2016).
9:00 AM - EM08.01.03
Metal Gate-Induced Crystallographic Strain and Tilts in GaAs Quantum Dot Devices
Anastasios Pateras 1 , Joonkyu Park 1 , Youngjun Ahn 1 , Jack Tilka 1 , Martin Holt 2 , Christian Reichl 3 , Juan Pablo Dehollain 4 , Uditendu Mukhopadhyay 4 , Lieven Vandersypen 4 , Paul Evans 1
1 Materials Science & Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States, 2 , Argonne National Laboratory, Lemont, Illinois, United States, 3 Solid State Physics Laboratory, ETH, Zurich Switzerland, 4 Kavli Institute of NanoScience, Delft University of Technology, Delft Netherlands
Show AbstractThe realization of single-electron quantum spin devices in epitaxially grown, lattice-matched GaAs/AlGaAs heterostructures involves the fabrication of metallic gates with dimensions of a few tens of nanometers on the surface of the heterostructure. Metal deposition induces interface stresses that elastically distort the layers underneath the gates and in the region where the quantum dot is electrostatically defined, potentially complicating the energy landscape of quantum electronic devices. The gate-induced distortions are expected to affect the energies and coherence of electronic states in the quantum dot by generating perturbation terms in the electronic Hamiltonian. The spatial distribution of stress imparted by the gates has been empirically estimated from magnetoresistance measurements in lateral surface superlattices, but can significantly differ in quantum dot heterostructures due to the geometry of the gate patterns, and because the residual stress in the metallic gates depends on several growth parameters. Synchrotron coherent x-ray nanobeam diffraction characterization provides the sensitivity and spatial resolution to determine the spatial distribution and magnitude of lattice distortions at the relevant scales. The small lattice mismatch between the AlGaAs and GaAs component layers, the presence of several thick layers with thicknesses larger than the x-ray extinction depth, and a 450 mm-thick GaAs substrate make the characterization of GaAs/AlGaAs heterostructures a challenging nanodiffraction problem. By considering dynamical x-ray scattering effects, we have reproduced the features observed in the experimental diffraction patterns, measured lattice strain on the order of 10-4 at the depth where a two-dimensional electron gas forms, and found tilts with a depth-averaged value up to 0.04° in the AlGaAs layers defining the two-dimensional electron gas. By fitting the experimentally measured tilts with an elasticity theory mechanical model we have found residual stresses of 50-100 MPa in the metallic gates. The characterization approach can be used to understand structural effects in a large class of quantum materials systems, bridging the gap between fabrication and characterization. The distortion due to gate electrodes needs to be considered in the design of quantum devices that require precisely controlled characteristics.
9:15 AM - EM08.01.04
Analysis of Advanced Ultra-Pure Ga for High Mobility in GaAs/AlGaAs 2DEGs
Kyungjean Min 1 , Geoffrey Gardner 1 , David Johnson 1 , Kevin Trumble 1
1 , Purdue University, West Lafayette, Indiana, United States
Show AbstractImpurities in the source Ga limit the electron mobility in GaAs/AlGaAs two-dimensional electron gas (2DEGs) grown by molecular beam epitaxy (MBE). A high mobility of 35 million cm2/Vs was recently observed when an 8N Ga (total nominal impurity concentration of ~10 ppb) source was used compared to 25 million cm2/Vs for a 7N Ga source. In both these cases, the Ga has been further purified by in-situ distillation within the MBE. This research focuses on further increasing the purity of Ga by zone refining and distillation to ultra-pure levels which are not commercially available, thus allowing new materials related to quantum computing to be realized. However, quantification of the specific impurities at such low levels is particularly challenging. Inductively Coupled Plasma Mass Spectrometry (ICP-MS) was employed to measure the concentration of impurities in the ultra-pure Ga. Elemental analysis of Ga before and after an MBE growth campaign in which in-situ distillation was performed will be presented. Discussed are the challenges in producing ultra-high purity Ga and elemental quantification by unique sample preparation methods using ICP-MS.
9:30 AM - *EM08.01.05
Prospects for GaAs Based Spin Qubits
Hendrik Bluhm 1 2
1 , RWTH Aachen University, Aachen Germany, 2 , Forschungszentrum Jülich GmbH, Jülich Germany
Show AbstractGaAs/AlGaAs heterostructures have played a pioneering role for semiconductor electron spin qubits, but have the drawback of unavoidable decoherence due to the interaction of the qubit electrons with nuclear spins of the host lattice. Si-based qubit implementations have by now demonstrated the possibility to completely avoid this problem by isotopic purification. On the other hand, the small effective mass, the high quality of heterostructures, and a single conduction band valley with a direct band gap still make GaAs-based approaches an interesting alternative. The dephasing mechanisms arising from the hyperfine interaction are now mostly well understood, and effective methods to reduce their effects have been developed. Building on these advancements, we have recently demonstrated fidelities for single qubit operations of about 99.5%, thus bringing a key figure of merit to the threshold required for quantum error correction. Simulations based on measured noise levels indicate that fidelities as high as 99.8% should be possible and imply good prospects for two-qubit gates of similar quality. While the procedures required to reach these values imply some overhead and the compatibility with advanced semiconductor technology will be an important criterion, these results show that the hyperfine interaction does not fundamentally preclude the use of GaAs-based spin qubits as a platform for quantum processors. A specific advantage is the possibility of optical coupling.
10:30 AM - EM08.01.06
The MOS Interface for Singlet-Triplet Quantum Bits
Malcolm Carroll 1 , Ryan Jock 1 , Martin Rudolph 1 , Noah Jacobson 1 , Patrick Harvey-Collard 1 2 , John Gamble 1 , Daniel Ward 1 , Stephen Carr 1 , Andrew Mounce 1 , Amir Shirkhorshidian 1 3 , Michael Lilly 1
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 , Universite de Sherbrooke, Sherbrooke, Quebec, Canada, 3 , The University of New Mexico, Albuquerque, New Mexico, United States
Show AbstractElectron spins in silicon metal-oxide-semiconductor (MOS) offer a potential path to implement quantum information science. Despite silicon’s opportunity to provide a near magnetic “vacuum”, fundamental doubts have been raised about the MOS interface (e.g., trap disorder and noise). In this talk, I will present characterization of the MOS interface using singlet-triplet (ST) quantum bits (qubit). The ST qubit is an excellent characterization platform. The two coupled spins can be tuned to directly probe most central properties including: valley splitting (VS), g-factor non-uniformity, charge and “magnetic” noise. Disorder is still present although an increasing number of instances from this lab and others show that design and tuning can lead regularly to single electron QDs. We will present highly tunable QDs that are fabricated without metals in the nanostructure region, minimizing potential complications of metal electrodes related to charge noise [1], stress [2], and modifications of the local magnetic field [3]. We find that the MOS interface in these QDs provides a tunable VS [4] with charge noise properties comparable to other semiconductor ST qubits [5]. We also examine the effect of introducing impurities (i.e., donors). These shallow states can be tuned in to resonance to form a useful qubit [6] or alternatively tuned out of resonance where they are undetected by other qubit’s performance. For magnetic noise, we find the effective Overhauser field variation consistent with the anticipated magnitudes from the 29Si concentration. We also observe a strong and tunable g-factor non-uniformity between QD locations. This particular effect is advantageously used to control a ST qubit and raises interesting future prospects.
Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.
[1] Zimmerman et al., Nanotechnology 25, 405201 (2014)
[2] Thorbeck et al., AIP Advances 5, 087107 (2015)
[3] Underwood, F52.8, APS March Meeting (2017)
[4] Gamble et al., APL 109, 253101 (2016)
[5] Rudolph et al, arXiv 1705.05887 (2016)
[6] Harvey-Collard et al., arXiv 1512.01606 (2015)
10:45 AM - EM08.01.07
Measurements of Valley Splitting in Novel Si/SiGe Heterostructures
Samuel Neyens 1 , Ryan Foote 1 , T. Knapp 1 , Brandur Thorgrimsson 1 , Thomas McJunkin 1 , Lieven Vandersypen 2 , Payam Amin 3 , Nicole Thomas 3 , James Clarke 3 , Donald Savage 1 , Max Lagally 1 , Mark Friesen 1 , Susan Coppersmith 1 , Mark Eriksson 1
1 , University of Wisconsin-Madison, Madison, Wisconsin, United States, 2 , Delft University of Technology, Delft Netherlands, 3 , Intel Corporation, Hillsboro, Oregon, United States
Show AbstractFor quantum dot qubits in Si/SiGe heterostructures, valley splitting is an important parameter for device performance. For single-spin and singlet-triplet qubits, for example, valley splitting should be large so that extra valley states do not present a leakage path from qubit states. For the quantum dot hybrid qubit, the valley splitting sets the qubit energy splitting itself, and so should be in a workable range (~40 μeV or ~10 GHz). Theoretical predictions for valley splitting in ideal Si/SiGe heterostructures have been on the order of 1 meV [1]; however, due to disorder, real Si/SiGe heterostructures often yield valley splittings on the order of tens of μeV or lower and which are also difficult to predict. We investigate a technique, based on a recent theoretical proposal [2], to enhance valley splitting in Si/SiGe heterostructures. We measure valley splitting in two novel heterostructures that are grown with an interfacial layer of Ge between the Si quantum well and the SiGe barrier, as well as in a control sample with no interfacial layer. For one of the experimental heterostructures, the CVD growth was interrupted prior to growth of the interfacial Ge layer, to improve its compositional abruptness. We measure the valley splitting using an activation energy technique: we measure the depth of the longitudinal resistance minima as a function of temperature in Hall bar devices at filling factors ν = 3 and ν = 5, in the first and second Landau level, respectively. At ν = 5, the control sample has the highest valley splitting, but at ν = 3, the sample with the growth-interrupted interfacial Ge layer has the highest valley splitting, ~50% higher than that of the control sample at a range of carrier densities. We interpret these results in the context of two types of disorder, both of which suppress valley splitting. First, the growth-interrupted Ge sample is likely to have larger interface roughness than the control sample, so in lower magnetic confinement regimes (ν = 5), valley splitting is more suppressed in the Ge sample. With higher magnetic confinement (ν = 3), this suppression is mitigated. Second, the growth-interrupted Ge sample has less alloy disorder at the upper quantum well interface than the control sample, due to its high compositional abruptness and the high purity of the interfacial Ge layer. In summary, we find evidence that an interfacial Ge layer between the Si well and the SiGe barrier can enhance the valley splitting at confinement scales of ~15 nm.
[1] T. B. Boykin, G. Klimeck, M. A. Eriksson, M. Friesen, S. N. Coppersmith, P. von Allmen, F. Oyafuso, & S. Lee. Phys. Rev. B 70, 165325 (2004).
[2] L. Zhang, J.-W. Luo, A. Saraiva, B. Koiller, & A. Zunger. Nature Comm. 4, 2396 (2013).
11:00 AM - EM08.01.08
Isotopically Engineered Si-28/SiGe Heterostructures for Quantum Computing
Satoru Miyamoto 1 , Yusuke Hoshi 2 3 , Noritaka Usami 2 , Kohei Itoh 1
1 School of Fundamental Science and Technology and Spintronics Research Center, Keio University, Yokohama Japan, 2 Graduate School of Engineering, Nagoya University, Nagoya Japan, 3 Institute of Industrial Science, University of Tokyo, Tokyo Japan
Show AbstractIsotope engineering in semiconductor matrixes such as silicon and diamond challenges a material limit imposed at the atomic level for quantum computing [1]. In particular, isotope purification of silicon by zero-nuclear-spin 28Si prolongs coherence of spin qubits because 29Si nuclear spins constituting natural Si act as a dominant source of magnetic noise behind qubit operation. Indeed, isotopically enriched 28Si crystals have been proven to achieve coherence times exceeding seconds for a spin ensemble of phosphorus-bound electrons [2]. More recently, based on strong compatibility with state-of-the-art Si nano-devices, an emerging benchmark for quantum computer was build with single phosphorus qubits in a 28Si epitaxial layer [3]. Extraordinary coherence of single electron-spin qubit was also ensured for 28Si quantum dots (QDs) thanks to the absence of host nuclear spins [4]. Both types of spin qubits placed near the surface could suffer temporal fluctuation from a variety of charge traps at a gate-oxide interface. Meanwhile, Si/SiGe quantum-well (QW) structures allow investigation of the Si-based QDs with keeping a large separation from the charge traps [5]. As the coherence time is considered to be dominated by the 29Si nuclear spins [6], isotope enrichment of Si-QW layers is vigorously pushed forward to provide a buried-type nuclear-spin-free system. In this instance, since the roughness at Si/SiGe hetero-interface causes unnecessary QDs, the stacked structures require to be tailored with reduced surface and interface roughness.
Here the Si-28/SiGe QW structures are epitaxially grown onto chemical-mechanically polished SiGe virtual substrates by employing isotopically purified 28SiH4 gas-source, and the resulting structures are closely investigated from the aspect of material science. Atomic force microscope analyses on the grown surface represent a strain-induced cross-hatch pattern with surface roughness lowered at the level of ~1 nm. The Si-QW interface is found to compose an atomic-level fluctuation for the structures grown at controlled temperatures. Furthermore, a combined study with x-ray diffraction and Raman scattering spectroscopy reveals that the 28Si-QW layer is coherently strained with experiencing a fully relaxed SiGe buffer layer. The coherence time of a single-electron spin for a QD formed in the Si-28 layer is extended significantly as will be discussed in the conference.
The work has been supported by the KAKENHI (S) No. 26220602, JSPS Core-to-Core Program, and Spintronics Research Network of Japan.
[1] K. M. Itoh and H. Watanabe, MRS Communications 4, 143 (2014).
[2] A. M. Tyryshkin et al., Nat. Mater. 11, 143 (2012).
[3] J. T. Muhonen et al., Nat. Nanotechnol. 9, 986 (2014).
[4] M. Veldhorst et al., Nat. Nanotechnol. 9, 981 (2014).
[5] E. Kawakami et al., Nat. Nanotechnol. 9, 666 (2014); K. Takeda et al., Sci. Adv. 2, e1600694 (2016).
[6] K. Eng et al., Sci. Adv. 1, e1500214 (2015); J. Yoneda et al., (submitted).
11:15 AM - EM08.01.09
Magnetotransport Measurements of MOS Material for Quantum Dots
Stephen Carr 1 , Daniel Ward 1 , John Gamble 1 , Tammy Pluym 1 , Albert Grine 1 , John Anderson 1 , Michael Lilly 1 , Malcolm Carroll 1
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractSilicon has been and continues to be the dominant host material for classical information processing. Over the past decade, considerable effort has been invested in investigating silicon as a host material for quantum information processing [1,2]. Remarkable advances have been achieved in the coherent quantum control of individual donors [1,2], quantum dots [1,2], and hybrid donor-quantum dot devices [3,4] using silicon-based material systems. Experimental characterization of silicon-based material systems directed toward devices for classical information processing is well established, whereas materials characterization for devices directed toward quantum information processing is an active area of research [5,6,7].
Here we present cryogenic magnetotransport measurements of metal-oxide-silicon (MOS) starting material used for the fabrication of MOS quantum dot devices. The devices used for the MOS materials characterization are enhancement-mode metal-oxide-semiconductor-field-effect-transistors (MOSFETs) patterned as Hall bar structures. We extract parameters such as threshold voltage, peak mobility, critical density, and interface roughness from measurements of more than 30 devices fabricated using a process with as many as 20 variable fabrication parameters. We identify trends in the measured parameters related to the fabrication parameters, including several conditions that produce peak mobilities greater than 20,000 (cm2/Vs) and critical percolation [8] densities as low as 1.0 x 1011 (cm-2), measured at a temperature of 4 (K). In addition, some parameters such as the interface roughness are correlated to valley splitting [6] measurements in quantum dots fabricated from the same starting material [3,4].
[1] F.A. Zwanenburg, et al., Rev. Mod. Phys. 85, 961 (2013).
[2] J.J.L. Morton et al., Nature 479, 345 (2011).
[3] M. Rudolph, et al., International Electron Device Meetings, Dec. 2016, San Francisco. e-print arXiv:1705.05887v1.
[4] P. Harvey-Collard, et al., e-print arXiv:1512.01606v2.
[5] J.S. Kim, et al., Appl. Phys. Lett. 110, 123505 (2017).
[6] J.K. Gamble, et al., Appl. Phys. Lett. 109, 253101 (2016).
[7] X. Mi, et al., Phys. Rev. B 92, 035304 (2015).
[8] L. A. Tracy, et al., Phys. Rev. B 79, 235307 (2009).
Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.
EM08.02: Donor Qubit Materials
Session Chairs
Monday PM, November 27, 2017
Hynes, Level 1, Room 111
1:30 PM - *EM08.02.01
Atomic-Precision Fabrication and Metrology for Epitaxial Si Quantum Devices
Justin Koepke 1 , David Scrymgeour 1 , Peter Schultz 1 , Andrew Baczewski 1 , Richard Muller 1 , Ezra Bussmann 1 2 , Shashank Misra 1 , Malcolm Carroll 1
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 Center for Integrated Nanotechnologies, Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractSi electronics research is focusing more and more on devices wherein quantum mechanics governs function and a capacity to fabricate and characterize material layer-by-layer or atom-by-atom is relevant to understand structure-function relationships. For example, in Si quantum computing evidence indicates that atomic-scale disorder interacts with subtle quantum degeneracies impacting coherence and gating of electron spin qubits [1-4]. This talk is about our work using layer-by-layer and atom-by-atom scanning tunneling microscopy (STM) and molecular beam epitaxy (MBE) to advance materials engineering for Si quantum devices. First, atomic-precision fabrication via STM can be used to address atomic-scale disorder issues in Si:P donor-based qubits [2,3]. We report methods to fabricate single donors with the STM at nearly 50% yield, opening the door to systematic studies of their environment. Second, in the strained-Si/SiGe quantum dot platform, conduction band valley degeneracy enhanced by heterointerface roughness is a decoherence mechanism [1,4]. We are using the STM and MBE to revisit Si/SiGe thin-film growth issues governing interface roughness from a layer-by-layer viewpoint to understand how to make smoother interfaces. STM provides atomic resolution views of the growth surface (interface) where elastic effects force nanoscale roughness and varying atomic step densities commensurate with the topography.
Acknowledgement
This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office
of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia
National Laboratories is a multi-mission laboratory managed and operated by National Technology and
Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for
the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA-0003525.
References
[1] F. A. Zwanenburg et al., Rev. Mod. Phys. 85, 961 (2013)
[2] J. Salfi et al. Nature Materials 13, 605 (2014)
[3] J.K. Gamble et al. Phys. Rev. B 91, 235318 (2015)
[4] M. Borselli et al. Appl. Phys. Lett. 98, 123118 (2011)
2:00 PM - EM08.02.02
Understanding Phosphorus Donor Insertion on Si(001) from PH3 Dissociation in Dimer Windows Formed by Hydrogen Lithography
Peter Schultz 1 , Andrew Baczewski 1 , Richard Muller 1 , Justin Koepke 1 , Ezra Bussmann 1 , Robert Simsonson 1
1 , Sandia National Labs, Albuquerque, New Mexico, United States
Show AbstractSuccess of donor-based qubits in silicon depends upon achieving atomic scale control of donor placement in Si. Hydrogen lithography with scanning tunneling microscopy (STM) is one auspicious path to obtain precise control of donor placement, with PH3 adsorption and dissociation limited to precisely located H-depassivated dimer windows on the Si(001) surface prepared with STM. Optimizing the yield efficiency of this approach is crucial to a successful process, which requires a deep understanding of the fundamental chemical processes, energetics and kinetics, of PH3 dissociation at the surfaces that result in insertion of a P donor into the silicon. Using density functional theory (DFT) calculations to develop a reaction network that captures the essential mechanisms for P insertion, informing kinetic Monte Carlo (kMC) simulations of the insertion chemistry, we are able to deduce the fundamental processes that govern P insertion into dimer windows and describe the insertion statistics, and the effective yield, observed in STM with surprising accuracy, e.g. a ~2:1 ratio of single vs. zero P incorporation in a three-dimer window. The multiscale approach, using DFT to identify and quantify the chemical processes and kMC to evolve the chemistry to predict the consequences in macroscopic simulations of insertion yield, provides greater confidence in the understanding of the P donor placement and yields insights needed to improve the incorporation process. — Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525.
2:15 PM - EM08.02.03
Atomic Manipulation of Si Atoms via Electron Beams—Observations, Mechanism and Control
Ondrej Dyck 1 , Eva Zarkadoula 1 , Panchapakesan Ganesh 1 , Miguel Fuentes-Cabrera 1 , Andrew Lupini 1 , Bethany Hudack 1 , Artem Maksov 2 , Sergei Kalinin 1 , Stephen Jesse 1
1 , Oak Ridge National Lab, Oak Ridge, Tennessee, United States, 2 , University of Tennessee, Knoxville, Knoxville, Tennessee, United States
Show AbstractDevelopment and broad implementation of solid state quantum computing devices necessitates the capability to assemble matter atom by atom in 3D, to enable multiple qubit structures, and controlled defect centers with defined optical, optoelectronic, or spintronic functionalities. To date, the approaches for fabrication of these were based on a combination of scanning tunneling microscopy atomic manipulation with advanced surface science techniques. Here, we demonstrate that the sub-atomically focused beam of a scanning transmission electron microscope can be employed to manipulate atoms in 3D. In a prototypical Si matrix, we demonstrate that the beam can induce crystallization, amorphization, and dopant atom motion. Combined with the custom-developed beam manipulation and feedback system, it allows for the guidance of atomic configurations and creation of custom structures defined by local phase variations. The mechanisms involved in atomic manipulation are explored using a combination of molecular dynamics and two-temperature formalism. We demonstrate that this approach can be extended for manipulation of individual Si atoms on reactive and non-reactive 2D supports. Further perspectives of e-beams as a new paradigm for fabrication of atomic-scale qubit devices will be discussed.
This research was conducted at and partially supported SVK the Center for Nanophase Materials Sciences, which is a US DOE Office of Science User Facility. Research for OD, SJ was sponsored by Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy. A.M. acknowledges fellowship support from the UT/ORNL Bredesen Center for Interdisciplinary Research and Graduate Education.
2:30 PM - *EM08.02.04
Quantum Tunneling Microscopy of an Atomic Scale Device in Silicon
Benoit Voisin 1 , Joseph Salfi 1 , J. Bocquel 1 , M. Usman 2 , A. Tankasala 3 , L. Hutin 4 , M. Vinet 4 , C. Leroyer 4 , Brett Johnson 2 , Jeffrey McCallum 2 , Rajib Rahman 3 , Michelle Simmons 1 , Lloyd Hollenberg 2 , S. Rogge 1
1 , CQC2T- UNSW, Kensington, New South Wales, Australia, 2 , CQC2T - Uni Melbourne, Melbourne, Victoria, Australia, 3 , Purdue University, Purdue, Indiana, United States, 4 , CEA-LETI, Minatec Campus, Grenoble France
Show AbstractSilicon is posed to take a major role in the rise of quantum technologies, with the ability to isolate and control electrons in nanoscale devices [1]. This opens the way to spin-based quantum computing schemes and the exploration of many-body physics using quantum simulators [2]. For donor-based approaches offering some of the longest coherence times in the solid-state, the small spatial extent of the localized wavefunctions makes interactions and device behavior unusually dependent on donor locations in the lattice. While STM imaging [3] can provide absolute dopant locations [4], correlating dopant positions with device behaviour has not been possible yet to date.
Here we present a novel path to experimentally access and manipulate interacting donor wavefunctions in a functioning device, using scanning tunneling microscopy (STM). We have designed an atomically precise device using a hybrid of STM lithography [5] performed at low temperature and top-down implantation. The STM dot wavefunction is probed in real space using the same STM in the Coulomb blockade regime where the chemical potential and occupation number can be independently tuned. Moreover the STM tip used as a moveable electrode offers a unique way of tuning tunnel rates over a wide range, desirable for instance in readout schemes.
A large degree of control of donor interactions is essential in silicon, where also valley degrees of freedom play a key role. Besides direct exchange interactions, hybrid systems made from donors and quantum dots have been proposed, notably to mediate exchange over relatively large distances [6]. STM allows mapping with atomic resolution the exchange interaction between a donor and a quantum dot, revealing no lattice aperiodic oscillations. This evidences a valley filtering effect with the quantum dot containing only 2 out of the 6 valleys existing for the donor, result supported by tight-binding/full-configuration interaction calculations. The effect of strain on valleys was also investigated: STM images of donors implanted in 0.7% strained silicon show a dominant contribution of the z-valleys. These results highlight how tuning valleys can assist to achieve uniform couplings in silicon.
Combining atomic precision device fabrication with spatially resolved spectroscopy will be applicable to a broad range of quantum systems, from qubit coupling to many-body physics of spin arrays.
[1] F. Zwanenburg et al., RMP 115 165301 (2015)
[2] J. Salfi et al., Nature Comms. 7, 11342 (2016)
[3] J. Salfi et al., Nature Materials 13, 605-610 (2014)
[4] M. Usman et al., Nature Nano. 11, 763-768 (2016)
[5] M. Fuechsle et al., Nature Nano. 7, 242-246 (2012)
[6] V. Srinivasa et al., PRL 114 226803 (2015)
Symposium Organizers
Christopher Richardson, University of Maryland
Jeffrey McCallum, University of Melbourne
Javad Shabani, The City College of New York
Clare Yu, University of California, Irvine
Symposium Support
Google Inc.
IBM Corp.
Microsoft
Quantum Science and Technology | IOP Publishing
EM08.03: Molecular Qubits and Wide Bandgap Materials
Session Chairs
Tuesday AM, November 28, 2017
Hynes, Level 1, Room 111
8:00 AM - EM08.03.01
Efficient Extraction of Sub-Poissonian Light from an NV Center in a Diamond Parabolic Reflector
Noel Wan 1 , Donggyu Kim 1 , Sara Mouradian 1 , Benjamin Lienhard 1 , Michael Walsh 1 , Brendan Shields 2 , Tim Schroder 1 3 , Dirk Englund 1
1 , MIT, Cambridge, Massachusetts, United States, 2 , University of Basel, Basel Switzerland, 3 , Niels Bohr Institute, University of Copenhagen, Copenhagen Denmark
Show AbstractWe introduce and experimentally demonstrate a parabolic reflector coupled to a single nitrogen-vacancy center in diamond. We describe a novel fabrication process that enables the creation of this monolithic all-diamond structure on the surface of bulk diamond. Using this device, we achieve fluorescence detection rates as high as 4.6 x 10^6 counts per second from a single NV center. This highly efficient optical interface outcouples up to 48% of the emitted photons, permitting the observation of sub-Poissonian light. We discuss further device improvements and prospects for integration with the wide variety of emitters in diamond and in other materials.
8:15 AM - *EM08.03.02
Spin Control and Dynamics in Engineered Molecular Systems
Dane McCamey 1
1 Department of Physics, University of New South Wales, Sydney, New South Wales, Australia
Show AbstractSinglet exciton fission is a process in which an optically prepared singlet state splits into two triplet excitons with (anti-)correlated spins. If harnessed efficiently, this process can be exploited to enhance the photocurrent of solar cells, raising the limiting power conversion efficiency from 33.7% to 45.9% under 1 Sun [1]m, which has motivated interest in this field. Fission is usually studied via optical spectroscopic techniques, such as pump-probe transient absorption, which drive electronic transitions and give details about the rate and yield of fission. However experimental insight into the nature of the triplet-pair state generated upon fission is lacking, as such approaches do not have the resolution required to identify the small spin couplings which influence these processes.
Transient electron paramagnetic resonance (EPR) spectroscopy involves continually measuring an EPR signal, usually following optical excitation of a spin system with transient properties. This is particularly useful in characterising singlet fission systems, as it allows us to drive transitions of the different spin species which contribute to the fission process. The spectral resolution this provides allows us to unambiguously determine the nature and spin dynamics of the triplet-pair states on the fission reaction coordinate.
By applying this approach to a range of novel molecules comprising pentacene dimers with engineered coupling, we show that singlet fission proceeds via a strongly coupled Quintet (S=2) state, before dissociating to two correlated but uncoupled Triplet (S=1) states [3]. The assignment of spin states is confirmed by the relative ratios of the coherent nutation frequency of the resolved spectral features.
Alongside the applications in solar energy generation, singlet fission provides access to an engineerable molecular platform in which to study and exploit fundamental physics of correlated spin systems. In this vein we will discuss coherence and spin lifetimes of the generated triplet states. Finally, we will discuss approaches to obtaining control of the coupling between the correlated spins which are prepared via fission. We will show that modification of the coupling between dimers has a strong impact on the fission process, but that fission proceeds in dimers with both homoconjugated and non-conjugated bridges [4]. We will also describe approaches to optical dynamical control of the coupling between the weakly-coupled pairs of triplets.
[1] Tayebjee, M.J.Y., et al., J. Phys. Chem. Lett. 3, 2749-2754 (2012)
[2] Sanders, S.N., et al., J. Am. Chem. Soc. 137, 8965-8972 (2015)
[3] Tayebjee, M. J. T. et al, Nature Physics 13, 182 (2017)
[4] Kumarasamy, E. et al. submitted for publication (2017)
8:45 AM - EM08.03.03
Wide-Bandgap AlN Photonics Platform for Visible to UV Spectrum Quantum Information Processing
Tsung-Ju Lu 1 , Hyeongrak Choi 1 , Michael Fanto 2 , Jeffrey Steidle 2 , Paul Thomas 2 , Sara Mouradian 1 , Wei Kong 1 , Jeehwan Kim 1 , Stefan Preble 2 , Dirk Englund 1
1 , MIT, Cambridge, Massachusetts, United States, 2 , Rochester Institute of Technology, Rochester, New York, United States
Show AbstractThere have been many advances in photonic integration of single photon sources and quantum memories. However, the photonic integrated circuits (PICs) in previous demonstrations all have some type of limitation that prevents them from being used as an universal photonics platform for quantum information processing and quantum computation. Perhaps the biggest constraint is a small transparency window in the material that prevents ultraviolet (UV) or visible (VIS) wavelength operations, missing out on potential applications with color centers like nitrogen vacancy centers (NVs) in diamond or trapped ions. Another common issue is unwanted background fluorescence in the material, which degrades the purity of the single photon generation.
Aluminum nitride (AlN) has a wide-bandgap of 6.015 eV, offering a broad transparent window down to the ultraviolet regime (< 400 nm) without suffering from absorption loss. In addition, AlN’s electro-optic and piezoelectric properties enable on-chip optical modulation, which is necessary for fully functional active integrated quantum devices. Furthermore, its high thermal conductivity and small thermo-optic coefficient benefit cryogenic operations, which is necessary for manipulation and control of color centers and trapped ions. AlN also has low background fluorescence, making it ideal for single photon quantum applications.
Here, we report the development of a wide-bandgap PIC platform based on commercially available c-axis oriented nearly crystalline AlN grown on top of a sapphire substrate. Background fluorescence measurement are presented and compared with conventional VIS photonics platform based on silicon nitride. X-ray diffraction shows low defect densities compared to previously used AlN grown on top of SiO2, which can potentially give a lower propagation loss. To demonstrate the viability of this platform for quantum photonics applications, we also show various integrated optic devices using AlN-on-sapphire.
We report ring resonators with quality factor (Q) of ~140,000 at 637 nm wavelength and Q on the order of 104 at 369 nm wavelength, which corresponds to low loss of ~6 dB/cm in VIS and record low loss in UV. We also show on-chip 50/50 split beamsplitters using directional couplers, as well as broadband grating couplers for coupling light off-chip to APDs for single photon detection. For integration of NVs in diamond nanowires with the AlN-on-sapphire photonics platform, we also demonstrate a distributed Bragg reflector with adiabatic tapering that has experimentally measured > 15 dB attenuation at 532 nm wavelength to be used for green excitation pump filtering.
The AlN-on-sapphire photonics platform opens up new possibilities in integrated quantum optics with trapped ions or atom-like color centers in solids, as well as classical applications including UV-Raman spectroscopy.
9:00 AM - *EM08.03.04
Dopant-Vacancy Centers Spin in Diamond—Ab Initio Theory
Adam Gali 1 2 , Gergo Thiering 1
1 , Hungarian Academy of Sciences, Budapest Hungary, 2 Department of Atomic Physics, Budapest University of Technology and Economics, Budapest Hungary
Show AbstractThe electronic structure of dopant-vacancy defects in diamond will be discussed by means of group theory and ab initio simulations. Particularly, the role of electron-phonon interaction in the effective spin-orbit coupling will be explained for nitrogen-vacancy (NV) center (negatively charged NV defect), and its consequences on the optical electron spin polarization of the center. New results on emerging new complexes, silicon-vacancy (SiV) and germanium-vacancy (GeV) will be also presented. We discuss the spin-orbit splitting of the negatively charged SiV and GeV center, and show the complex physics of the electron-phonon wave functions of the neutral SiV and GeV defects. Particularly, their spin and optical properties will be discussed in detail.
EM08.04/EM06.07: Joint Session: NV Ensembles and Architectures
Session Chairs
Mutsuko Hatano
Shashank Misra
Tuesday PM, November 28, 2017
Hynes, Level 3, Room 300
10:00 AM - *EM08.04.01/EM06.07.01
Creating Quantum Materials with Spins in Semiconductors
Brian Zhou 1 , David Awschalom 1
1 , University of Chicago, Chicago, Illinois, United States
Show AbstractThere is a growing interest in exploiting the quantum properties of electronic and nuclear spins for the manipulation and storage of information in the solid state. Although conventional electronics avoid disorder, recent efforts embrace materials with incorporated defects whose special electronic and nuclear spin states allow the processing of information in a fundamentally different manner because of their explicitly quantum nature [1]. These defects possess desirable qualities – their spin states can be controlled at and above room temperature, they can reside in a material host amenable to microfabrication, and they can have an optical interface near the telecom bands. Here we focus on recent developments that exploit precise quantum control techniques to explore coherent spin dynamics and interactions. In particular, we manipulate and measure the geometric (Berry) phase of a single spin in diamond using all-optical control techniques [2], and investigate the robustness of this control pathway to noise as well as its viability for implementations of photonic networks of quantum states. Separately, we find that defect-based electronic states in silicon carbide can be isolated and read with high fidelity at the single spin level with long spin coherence times [3], can achieve near-unity nuclear polarization [4] and be robustly entangled at room temperature [5]. Finally, we identify and characterize a new class of optically controllable defect spin based on chromium impurities in both silicon carbide and gallium nitride [6].
[1] D.D. Awschalom, L.C. Bassett, A.S. Dzurak, E.L. Hu and J.R. Petta, Science 339, 1174 (2013).
[2] B. B. Zhou et al., Nature Phys. 13, 330 (2017); B. B. Zhou et al., arXiv:1705.00654.
[3] D. J. Christle et al., Phys. Rev. X7, 021046 (2017).
[4] A. L. Falk, P. V. Klimov, et al., Physical Review Letters 114, 247603 (2015).
[5] P. V. Klimov, A. L. Falk, D. J. Christle, V. V. Dobrovitski, and D. D. Awschalom, Science Advances 1, e1501015 (2015).
[6] W. F. Koehl et al., Editors Suggestion, Phys. Rev. B 95, 035207 (2017).
10:30 AM - *EM08.04.02/EM06.07.02
Coherent Optical Control of Silicon-Vacancy Center Spins in Diamond
Christoph Becher 1
1 , Saarland University, Saarbruecken Germany
Show AbstractColor centers in diamond, i.e. atomic-scale, optically active defects in the diamond lattice, have received large recent attention as versatile tools for solid-state-based quantum technologies ranging from quantum information processing to quantum-enhanced sensing and metrology. They provide individually addressable spins with very long coherence times, narrow optical spectra and bright single-photon emission. However, identifying a spin impurity which combines all of these favorable properties still remains a challenge.
In this context, the negatively charged SiV center in diamond features an advantageous electronic structure and superior spectral properties [1]: At liquid helium temperatures, the SiV exhibits a narrow zero phonon line (ZPL) with a four-line fine structure and lifetime-limited linewidths on the order of 120MHz [2]. Furthermore, due to its small Huang-Rhys factor, up to 80% of the fluorescence is emitted via the ZPL. Moreover, the SiV offers an optically accessible Λ-type level structure with a large orbital level splitting and previous studies determined the ground state coherence time of the center to be on the order of 40ns [3].
The specific level structure of the SiV and the spin-dependent fluorescence enable all-optical coherent control of its internal states; the large ground state splitting furthermore allows for using broadband, ultrafast laser pulses for coherent manipulation. We here report on all-optical coherent control of the orbital degree of freedom based on Raman transitions [4]. These experiments can be extended to the spin degree of freedom for SiV centers in strong magnetic fields [5].
A limitation of current experiments is the short ground state coherence time which is due to phonon-induced transitions between the orbital states. In order to extend the coherence time we follow two routes: 1. We report on coherent control experiments at very low temperatures (mK) where phonons are frozen out. Despite a large increase in spin relaxation time (T1) we find that the spin coherence time (T2*) is still short which is attributed to coupling to a local spin bath. 2. We investigate phonon engineering as a tool to reduce the phonon density of states (PDOS) at the frequency of the ground state splitting (50GHz). In particular, we find that diamond nanowire structures with diameter < 200nm exhibit phonon confinement effects, leading to a reduced PDOS at 50GHz. At the same time the nanowire structures provide high photon collection efficiencies enabling efficient spin-photon interfacing. The combination of low-temperature operation and efficient optical interfacing are promising techniques for scaling up the coupling of SiVs in a quantum network.
References
[1] C. Hepp et al., Phys. Rev. Lett. 112, 036405 (2014).
[2] L. J. Rogers et al., Nat. Commun. 5, 4739 (2014).
[3] B. Pingault et al., Phys. Rev. Lett. 113, 263601 (2014).
[4] J. N. Becker et al., Nat. Commun. 7, 13512 (2016).
[5] B. Pingault et al., Nat. Commun. 8, 15579 (2017).
11:00 AM - EM08.04.03/EM06.07.03
GaN Nanowires as Electrically Active Waveguides for Nitrogen Vacancy Center Read-Out and Charge State Control
Martin Hetzl 1 , Jakob Wierzbowski 1 , Theresa Hoffmann 1 , Verena Zuerbig 2 , Christoph Nebel 2 , Martin Stutzmann 1
1 Walter Schottky Institute, TU Munich, Garching Germany, 2 , Fraunhofer Institute for Applied Solid State Physics IAF, Freiburg Germany
Show AbstractNitrogen vacancy centers (NVs) in diamond are promising candidates for quantum computing applications. However, due to charge state instabilities of surface-near NVs, their optical and spin coherence times are fluctuating. In addition, the large refractive index of diamond is not beneficial for the optical read-out of NVs.
We demonstrate the deterministic epitaxial growth of GaN nanowires (NWs) on diamond (111) surfaces in an n/p-heterodiode structure acting as efficient nano-waveguide for optical read-out and, at the same time, as electrical nano-contacts to control the charge state of single or ensembles of NVs close to the diamond surface. The NWs are fabricated via plasma-assisted molecular beam epitaxy in the selective area growth mode, which allows the implementation of NW arrays with predefined NW diameters, lengths and periods. By employing finite difference time domain (FDTD) simulations, we optimize these parameters to efficiently guide the excitation laser light into the diamond substrate and extract the NV photoluminescence through the GaN nanowires, respectively. This alllows an enhancement of the PL signal of the NVs by over one order of magnitude at room temperature and also at 10 K.
In order to prevent blinking of the NVs during laser excitation, the charge state of surface-near NVs is stabilized and adjusted by applying a voltage to the n-GaN NW/p-diamond nano-diodes.
11:15 AM - *EM08.04.04/EM06.07.04
Photophysics of Electronic Transitions on NV Centre in Diamond—Towards Scalable Quantum Chip Architecture
Milos Nesladek 1 , Michal Gulka 1 2 , Emilie Bourgeois 1
1 , imec, Division IMOMEC and Hasselt University, Diepenbeek Belgium, 2 Biomedical Engineering, Czech Technical University, Kladno Czechia
Show AbstractScalable principles for quantum state readout are one of the key- open questions in quantum technology. Building on the recent results of photoelectric detection of magnetic resonances (PDMR) [1,2] we review the prospects of PDMR technique for solid-state qubit devices in diamond. One of the advantages of PDMR over optically detected magnetic resonances (ODMR) are high detection rates ~ 5 x 109 s-1, significantly exceeding standard ODMR. Consequently, PDMR might enable single shot readout of individual NV centres and provide a fast data acquisition essential for quantum sensing as well as quantum computation devices. To achieve this goal the photoelectric gain associated with 2-photon ionization scheme is to be optimised. In this work we discuss photophysics of the transitions on NV centre and address several quantum readout scenarios for obtaining highest signal/noise ratio. We discuss the trapping and recombination kinetics and emphasize the role of point defects in the diamond lattice on the influence of the magnetic resonance contrast. In specific configuration the normally observed negative PDMR contrast can be reverted to positive. We demonstrate pulsed PDMR measurements, compatible with coherent spin manipulation realized on quantum chips and discuss prospects of the chip design.
[1] E. Bourgeois et al Nat. Comm. 6, 8577 (2015).
[2] M. Gulka et al, Phys. Rev. Applied 7, 044032, (2017)
11:45 AM - EM08.04.05/EM06.07.05
Realization of Nano-Tesla Sensitivity in Wide Field Dynamical Decoupling by Delta-Doped NV Centers
Kosuke Mizuno 1 , Hitoshi Ishiwata 1 , Makoto Nakajima 1 , Takayuki Iwasaki 1 , Mutsuko Hatano 1
1 , Tokyo Institute of Technology, Meguro Japan
Show AbstractNanoscale magnetic resonance imaging (MRI) could be realized using nitrogen-vacancy (NV) center in diamond with optical spatial resolution and high magnetic sensitivity. This technique could be used to explorer cell membrane structures that were impossible to investigate using conventional MRI technique due to its limited spatial resolution. Capability of nanoscale MRI could be further extended by using our perfectly aligned high density delta-doped NV centers [1] for improved signal-to-noise ratio and high contrast that could be obtained uniformly over wide area. We constructed a wide-field magnetometer with 20 micrometer square observation area by initializing NV centers through epi-illumination and detecting the fluorescence by charge coupled device (CCD) camera. In this study, we propose realization of nano-tesla sensitivity in wide field by using an image intensified CCD (ICCD) camera and a high power laser excitation.
Inhomogeneity is inevitable with the wide field technique, which arise from spatial distribution of laser power, microwave intensity, density and coherence property of NV center. Inhomogeneity leads to reduced operation fidelity and change in measured values. We investigated the effect of laser and microwave intensity on XY8 dynamical decoupling measurement using delta-doped NV centers. Normalizing the fluorescence intensity eliminates the effect of laser power inhomogeneity. Conversely, microwave power determines operation fidelity directly and requires more careful designing.
Secondly, previous study of wide-field NV center technique [2] is using CCD camera with a lower signal-to-noise ratio and a lower shutter speed than photo diodes. It requires many repetitive measurements during single exposure. CCD camera operate with readout time of a few milliseconds which is approximately 10 times longer than the interaction time of NV center limiting its sensitivity in wide field imaging. In contrast, ICCD camera with nanosecond time resolution allows higher signal-to-noise ratio by extracting data within first 300 ns of fluorescence and reduce back of laser excitation compared to CCD camera. Combining 5 W high power laser and delta-doped CVD diamond [1], numerical estimate of the sensitivity reaches 80 nT/Hz1/2. Since fluctuating magnetic field from the nuclear spins 9 nm above surface is approximately 350 nT-rms [3]. Numerical estimate provides possibility of nuclear spin detection by using wide-field technique. Nanoscale MRI technique shows possibility to investigate cell membrane structures and eventually clarify interaction between proteins at cell membrane.
Acknowledgements:
This work was supported by CREST, JST and JSPS KAKENHI Grant (No. JP17H01262)
[1] Ishiwata et al., arXiv:1704.03642 (2017).
[2] DeVience et al., Nat. Nanotechnol. 10, 129 (2015).
[3] Pham et al., Phys. Rev. B 93, 45425 (2016).
EM08.05: Single Photon Emitters and Photon Manipulation
Session Chairs
Tuesday PM, November 28, 2017
Hynes, Level 1, Room 111
1:30 PM - EM08.05.01
Optical Absorption and Emission Mechanisms of Single Defects in Hexagonal Boron Nitride
Nicholas Jungwirth 1 , Gregory Fuchs 1
1 , Cornell University, Ithaca, New York, United States
Show AbstractWide bandgap semiconductors host point defects, or color centers, that can feature optical and spin properties that are useful for applications in quantum optics, precision sensing, and quantum information technology. Some color centers, such as the nitrogen vacancy (NV) center in diamond, are bright enough to be investigated in the single defect limit using single-molecule microscopy techniques. While diamond is the most celebrated host material, the last several years have witnessed the discovery of defect-based single photon sources in SiC, ZnO, GaN, WSe2, WS2, and hexagonal boron nitride (h-BN). The latter three materials exist as two-dimensional monolayers and layered solids, thus offering the possibility of integrating single-photon sources with van der Waals heterostructure devices for tuning and other control. Defect emission in h-BN can be ultrabright, have a narrow linewidth, be tuned, and remain photostable up to 800 K. In this work we investigate the polarization selection rules of sharp zero-phonon lines (ZPLs) from isolated defects in h-BN and compare our findings with the predictions of a Huang-Rhys model involving two electronic states. Our survey, which spans the spectral range ~550-740 nm, reveals that, in disagreement with a two-level model, the absorption and emission dipoles are often misaligned. We relate the dipole misalignment angle (Δθ) of a ZPL to its energy shift from the excitation energy (ΔE) and find that when ΔE corresponds to an allowed h-BN phonon frequency Δθ≈0° and that 0°≤Δθ≤90° when ΔE exceeds the maximum allowed h-BN phonon frequency. Consequently, a two-level Huang-Rhys model succeeds at describing excitations mediated by the creation of one optical phonon but fails at describing excitations that require the creation of multiple phonons. We propose that direct excitations requiring the creation of multiple phonons are inefficient due to the low Huang-Rhys factors in h-BN and that these ZPLs are instead excited indirectly via an intermediate electronic state. This hypothesis is corroborated by polarization measurements of an individual ZPL excited with two distinct wavelengths that indicate a single ZPL may be excited by multiple mechanisms. These findings provide new insight on the nature of the optical cycle of novel defect-based single photon sources in h-BN.
1:45 PM - EM08.05.02
Tailoring Color Center Positioning and Emission through CVD-Grown Self-Aligned SiC:O Nanowire Photonic Crystal Structures
Natasha Tabassum 1 , Vasileios Nikas 1 , Brian Ford 1 , Edward Crawford 2 , Stefania Castelleto 3 , Spyros Gallis 1
1 Colleges of Nanoscale Science and Engineering, State University of New York Polytechnic Institute, Albany, New York, United States, 2 , GLOBALFOUNDRIES Corp, East Fishkill, New York, United States, 3 School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, Victoria, Australia
Show AbstractSilicon-based nanosystems with high integration functionality and scalability, such as silicon carbide (SiC) nanowire (NW) arrays, and the deterministic positioning of color centers into these nanosystems are fundamental building blocks towards the implementation of devices in the emerging field of quantum technologies. Herein, we report on an innovative CVD-synthesis route for realizing SiC NW arrays doped with oxygen and erbium (Er3+) or chromium (Cr3+). A key advantage of the CVD-synthesis process is that the geometry and the periodicity of the NWs can be tailored during the fabrication process. The arrays of the 20 nm-thick SiC:O NWs are grown in a self-aligned manner at predetermined positions, thus, enabling the fabrication and engineering of photonic crystal (PC) structures. The ultrathin NW PC structure not only facilitates the on-demand placement of color centers (e.g. Er, Cr) but is pivotal in tailoring the emission properties of these centers. Finite-difference time-domain (FDTD) calculations revealed that the extraction efficiency could be substantially enhanced in such ultrathin NW PC architectures. For example, it was found that the extraction efficiency for the 1538-nm emission of Er3+ ions, which is the telecommunication wavelength used in optical fibers, is 34% in NW PC structures; an order of magnitude higher than the reference with ~3.3%. Through a combinational and systematic photoluminescence (PL), time-resolved PL (TRPL) and power-dependence PL (PDPL) spectroscopy, defects and PC geometry effects on color center emission yield were studied. To this end, the Er3+ PL intensity from the NW PC architecture was found to be enhanced by approximately 60 times compared to a representative reference sample without a PC structure. Furthermore, the Er3+ emission was found to be modulated linearly with the pitch of the NW array. Characterization of emission of single color centers integrated into these NW PC structures was also carried out using continuous-wave and pulsed excitation single photon confocal microscopy. Pertinent single photon emission and optical detectable magnetic resonance results will be presented.
2:00 PM - EM08.05.03
Tailoring the Band-Gap of Single-Photon Emitters with Static Strain
Andrea Giunto 1 , Luca Francaviglia 1 , Wonjong Kim 1 , Gözde Tutuncuoglu 1 , Martin Friedl 1 , Anna Fontcuberta i Morral 1
1 LMSC, EPFL, Lausanne, VD, Switzerland
Show AbstractThe possibility of using photons as qubits for quantum computers is only one of the reasons that attracts attention towards single-photon emitters. Quantum dots (QD) can show high-quality single-photon emitting properties, and, when embedded in nanowires (NWs), the latter’s light-guiding properties can be exploited for an optimal coupling of the emitted light.
In this work, we investigate GaAs-AlGaAs core-shell NWs embedding Al-poor AlGaAs QDs, which generate due to Al oscillations in the NW shell [1,2]. The purpose of the project is to shift the emission energy of the single-photon-emitting QDs for conducting experiments of storage of light in Rb vapor [3]. In addition, Raman spectroscopy is employed to gain a deeper understanding of the origins of the shift.
The emission energy can be tailored by applying a mechanical strain to the NW [4], or, more efficiently, through the deposition of a dielectric envelope capping the NWs [5,6]. In our case, QDs show as-grown emission energies ranging from 1.78, to 1.87 eV. Setting the target to the absorption energy for Rb vapor of 1.59 eV, a red-shift in the QD emission energy is achieved through the application of tensile strain generated by a SiO2 capping.
Increasing thicknesses of SiO2 are deposited via Plasma-Enhanced Chemical Vapor Deposition (PECVD) over the as-grown NWs. A clear trend is observed with photoluminescence (PL) measurements, with growing red-shifts up to 130 meV, and a saturation level reached for 350-nm-thick capping. These observations are made for two different systems. First, a red-shift is directly observed in NWs arranged in regular arrays, whose emission spectra are recorded before and after SiO2 deposition from the same NWs (the disposition in arrays allows to recognize the NWs even at the optical microscope). Secondly, as-grown NW forests are inspected, and their emission is studied statistically. The above-mentioned trend is clearly visible, despite the variety of dimensions and growth directions in the forest, and the consequent different thicknesses of the deposited dielectric. The fact that the red-shift trend can be observed statistically even on such a variety of QDs under different strain conditions proves the success of the method.
Finally, the origins of strain are elucidated by performing Raman spectroscopy. Understanding the sources of strain allows the employment of different coatings able to provide even larger emission shifts.
1. Heiss et al., Nature Materials 12, no. 5 (2013)
2. Mancini et al., Applied Physics Letters 105, no. 24 (2014)
3. Phillips et al., Physical Review Letters 86, no. 5 (2001)
4. Montinaro et al., Nano Letters 14, no. 8 (2014)
5. Bouwes Bavinck et al., Nano Letters 12, no. 12 (2012)
6. Stepanov et al., Nano Letters 16, no. 5 (2016)
2:15 PM - EM08.05.04
Deterministic Creation and Electrical Driving of Quantum Emitters in Atomically Thin Semiconductors
Dhiren Kara 1 , Carmen Palacios-Berraquero 1 , Alejandro Montblanch 1 , Matteo Barbone 1 , Pawel Latawiec 3 , Marko Loncar 3 , Andrea Ferrari 2 , Mete Atature 1
1 , Cavendish Laboratory, University of Cambridge, Cambridge United Kingdom, 3 , John A. Paulson School of Engineering and Applied Science, Harvard University, Cambridge, Massachusetts, United States, 2 , Cambridge Graphene Centre, University of Cambridge, Cambridge United Kingdom
Show AbstractQuantum emitters (QEs) have been recently observed in tungsten diselenide (WSe2), a member of the 2-dimensional transition metal dichalcogenides (2D-TMDs). This is particularly exciting since the 2D nature of TMDs makes them ideal for interfacing with photonic structures and building optoelectronic devices in the form of van der Waals heterostructures. TMDs also offer dangling-bond free surfaces, removing the issue of charge noise and traps faced by close-to-surface QEs in bulk crystals. We have created QE arrays by transferring the host TMD onto a SiO2 substrate patterned with nanopillars. This could be essential for scalable technology and sheds light on their as yet unknown origin. Further we have constructed a heterostructure consisting of TMD, graphene and hexagonal boron nitride to implement a 2D light emitting diode device design. With this, we have demonstrated electrically-driven single photon emission in both WSe2 and WS2 [2]. This demonstrates that quantum emission is robust in a structure, ubiquitous to the 2D-TMD family and available at different emission wavelengths across the visible spectrum. I will finally discuss our current efforts to charge a QE with an electron (or hole), with the aim of providing a long-lived spin state for use as a qubit.
[1] C. Palacios-Berraquero, D. M. Kara, A. R.-P. Montblanch et. al., Large-scale quantum-emitter arrays in atomically thin semiconductors. Nat. Commun. 8, 15093, doi:10.1038/ncomms15093 (2017)
[2] C. Palacios-Berraquero, M. Barbone, D. M. Kara et. al., Atomically thin quantum light-emitting diodes. Nat. Commun. 7, 12978, doi: 10.1038/ncomms12978 (2016)
3:00 PM - EM08.05.05
Strong Light-Matter Interaction in an Ultrasmall Mode Volume Photonic Crystal Cavity—Towards Room-Temperature Indistinguishable Single Photon Sources
Hyeongrak Choi 1 , Mikkel Heuck 2 , Dirk Englund 1
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Photonics Engineering, Technical University of Denmark, Lyngby Denmark
Show AbstractPhotonic crystal nanocavity design with engineered electromagnetic boundary conditions can reach very small mode volume, greatly enhancing light-matter interaction. For example, we recently predicted that a silicon-air 1D photon crystal cavity can reach an optical mode volume of approaching 7*10^-5 of a cubic optical wavelength. Embedding highly nonlinear materials, such as PTS-polydiacetylene, into this cavity may enable Kerr nonlinearities at the levels of few photons (H. Choi et al, PRL 118, 223605 (2017)). Here, we present our recent progress on a diamond cavity design that promises a highly efficient source of nearly indistinguishable single-photon, even at room temperature.
3:15 PM - *EM08.05.06
The Electronic and Vibronic Properties of Single Photon Emitters in Hexagonal Boron Nitride
Gregory Fuchs 1
1 , Cornell University, Ithaca, New York, United States
Show AbstractIsolated point defects in wide bandgap materials can serve as sources of single photons for applications in quantum optics, precision sensing, and quantum information processing. Along with the discovery of new material hosts of “quantum defects”, there is a corresponding opportunity to uncover new functionality if we can understand defect states and properties. The recent discovery of bright, photo-stable, and spectrally-narrow single photon emission from defects in hexagonal boron nitride (h-BN) offers a new toolbox for quantum optics because of the possibilities for easily integrating single-photon sources into van der Waals heterostructures for electrical gating, and into pre-patterned optical structures for optical enhancement. I will present our investigations of h-BN defects with the goal of understanding their fundamental electronic and vibronic properties. We first perform a survey of optically isolated h-BN defects and we observe zero-phonon line (ZPL) energies in a range that exceeds 500 meV. Next we study the temperature dependence of the ZPL spectral width and amplitude. We find that the results are consistent with a lattice vibration model that considers piezoelectric coupling with phonons in the defects two-dimensional h-BN sheet. We also examine the mechanisms of optical absorption and emission within the Huang-Rhys model. We find that if the energy difference ΔE between the exciting laser energy and the ZPL energy is less than 200 meV, the largest phonon energy in h-BN, then the polarization axes for absorption and emission are nearly always aligned as expected. However, if ΔE is greater than 200 meV, the polarization axes for absorption and emission can be misaligned by any angle between 0 and 90°. This observation reveals the presence of two optical absorption mechanisms for h-BN defects – either through direct, phonon-mediated optical absorption between or through indirect optical absorption via an intermediate electronic state. These results provide new insights into the electronic and vibronic processes within the optical cycle of single photon emitters hosted in a 2-dimensional insulator.
3:45 PM - EM08.05.07
Optical Manipulation and the Temporal Evolution of a Spin-Valley Qubit in Single-Layer Transition Metal Dichalcogenides
Parijat Sengupta 1 , Junxia Shi 2
1 , Boston University, Boston, Massachusetts, United States, 2 Electrical and Computer Engineering, University of Illinois, Chicago, Illinois, United States
Show AbstractQuantum information processing (QIP) includes in its ambit the fields of quantum communication, metrology, and cryptography. The foundational element of these fields is the quantum bit (qubit) that has been designed in multiple flavours, primarily with the use of single photons and linear-optical elements, and implemented in the formulation of QIP protocols [1]. The methods of optical QIP when paired with solid-state systems via the interaction of photons with atoms permit possibilities harnessing the quantum degrees-of-freedom (DOFs) usually defined by the electron's charge and spin. In addition to these DOFs, materials with valley structures in momentum space allow an additional index. The valley-dependent index as a DOF primarily manifests in the two-dimensional (2D) honeycomb lattices with a broken spatial inversion symmetry. For instance, the single-layer 2D transition metal dichalcogenides (TMDCs) MX2 (M = Mo, W; X = S, Se) with such a DOF are characterized by two degenerate valley extrema, K and K’, that are connected by time-reversal-symmetry (TRS) [2].
Apart from valley-polarized currents, the edges, K and K’, exhibit a finite out-of-plane Berry curvature and carry an orbital magnetization enabling an exclusive absorption of left- or right-circularly polarized light. Additionally, a large spin-orbit coupling (soc) that preserves the out-of-plane spin splits the valley states into a Kramer’s doublet with the spin-up (down) state at K (K’) valley separated from their opposite spin counterparts by the soc energy. This valley-specific delineation leads to spin-valley locking where two DOFs (spin and valley) are coupled. The interaction of light and the ensemble of electrons with such a coupled-DOF serve to setup a qubit whose optical manipulation and temporal evolution on the Bloch sphere (BS) is the basis of suggested QIP operations. We assign the ground (valence) and excited (conduction) energy levels the binary states of 0 and 1.
Working within the Floquet framework [3] that treats light as a periodic perturbation, we establish the time-evolution operator (TOE) of the two-state system as a function of the Rabi frequency. The TOE-governed qubit rotation on the BS is dynamically-controllable by the degree of elliptical-polarization of the incident light and the time for which the pulse is applied. The extent of superposition of the two states, vital to QIP, is accomplished by a modulation of the detuning frequency. As a useful aside, we propose an arrangement to tune the valley Hall current (normally zero by TRS [4]) by setting up a Linblad equation of carrier excitation from the valleys [5].
[1] P. Kok and W. Lovett, Intr. to Opt. Quant. Info. Processing, Cambr. U. Press, 2010
[2] J. Schaibley, H. Yu, et al., Nature Rev. Materials 1, 16055, 2014
[3] M. Claasen, C. Jia, B. Moritz, et al Nature 7, 13074, 2016
[4] M. Yamamoto, Y. Shimazaki, et al., J. Phys. Soc. Jpn. 84, 121006, 2015
[5] P. Sengupta, Y. Tan, and J. Shi, arXiv: 1703.06593, 2017
EM08.06: Poster Session: Emerging Qubit Materials
Session Chairs
Wednesday AM, November 29, 2017
Hynes, Level 1, Hall B
8:00 PM - EM08.06.01
Ambipolar Transport in Mn2CoAl Spin Gapless Semiconductors by Ionic Liquid Gating
Kenji Ueda 1 , S. Hirose 1 , M. Mori 1 , H. Asano 1
1 , Nagoya University, Nagoya Japan
Show AbstractSpin gapless semiconductors (SGS) are magnetic semiconductors with zero gap at the Fermi level (EF) in one spin channel and the usual energy gap in the other spin channel. Due to their special band characters, the carriers are fully spin-polarized and have very high mobility, and they are promising for fabricating novel spintronic devices. Highly spin-polarized electrons or holes can be produced by tuning the EF of SGSs because they are expected to have ambipolar characteristics. Theoretical calculations have predicted many compounds to be SGS, however no material has yet been established as an SGS. The inverse Heusler compound Mn2CoAl (MCA) is one of the most promising candidates for an SGS because SGS-like transport properties (linear MR, high mobility, etc.) have already been reported for bulk MCA. We believe the SGS-like transport properties do not constitute strong enough evidence for an SGS; rather, the main indicators of an SGS, namely ambipolar transport and full spin-polarization, must be sought after. In this study, we have tried observing the ambipolar transport properties of MCA by using the ionic liquid gating technique for the first time [1].
MCA films were fabricated on MgAl2O4 (MAO) substrates at 400°C by ion beam assisted sputtering. Hall bar patterns using thin MCA films (~15 nm) were prepared, and the channel areas and side-gate electrodes were covered by small droplets of an ionic liquid (DEME-TFSI) to form liquid-gated electronic double-layer transistors (EDLT). Transport properties such as longitudinal (ρxx) and Hall resistivity (ρxy) were measured by using the same Hall bar patterns.
MCA films were epitaxially grown on MAO (MCA (001)[110] // MAO (001)[100]). They showed magnetic hysteresis with saturation magnetization (Ms) of ~295 emu/cc, which is comparable to the Ms of bulk MCA. The hole concentration and mobility of the MCA films was ~5×1020 cm-3 and ~10 cm2/Vs at 4 K, respectively, which were comparable to the reported values for MCA films.
For the EDLT, zero-field drain-source resistance (RDS) exhibited a sharp maximum at a gate voltage (VG) of ~0.6 V, and the drain-source current changed its polarity from positive to negative (ambipolar characters) at VG= ~0.6 V. A similarly sharp maximum and current polarity change have often been observed for field-effect-induced ambipolar transport in graphene, which is a typical gapless semiconductor. The mobility showed a maximum value of 45 cm2/Vs at VG = ~0.6 V, which is one order of magnitude higher than the minimum values, while the carrier concentration showed a minimum with carrier polarity change from p- to n-type, which also corroborate ambipolar transport in MCA. The observed ambipolar characteristics are among the most significant features of SGS and strongly support the gapless features of MCA. From these results, we can conclude MCA is a gapless semiconductor with finite spin-polarization.
Ref. [1] K. Ueda et al., Appl. Phys. Lett. 110 (2017) 202405.
8:00 PM - EM08.06.02
Flying Qubit Investigations for Semiconductor Heterostructure Qubit Implementations
Kan Xie 1 , Gaurab Panda 1 , Haozhi Dong 1 , Virginia Ayres 1 , Harry Shaw 3 , Deborah Preston 2 , Manohar Deshpande 4
1 , Michigan State University, East Lansing, Michigan, United States, 3 Goddard Space Flight Center, NASA, Greenbelt, Maryland, United States, 2 Advanced Special Studies, University of Maryland, College Park, Maryland, United States, 4 Goddard Space Flight Center, NASA, Greenbelt, Maryland, United States
Show AbstractLocal environments that can improve quantum state lifetime and coherence times could provide increased flexibility for quantum information device implementations. A “flying qubit” approach based on Surface Acoustic Wave (SAW) interactions has been theoretically [1] and experimentally [2] investigated for spin-entangled electrons in semiconductor 2D-1D heterostructures, and for 0D quantum dots, with the conclusion that it is feasible. In recent research, analysis of SAW wave longitudinal component effects on 1D current densities in semiconductor 2D-1D heterostructure devices indicated that shifts in quantum channel accessibility could lead to device optimization strategies [3]. In the present research, analytical models for transverse and longitudinal SAW wave interactions with 2D-1D semiconductor heterostructure transport channels are combined with realistic COMSOL simulations of wave properties and scattering to examine the effects of materials bandgap, channel width and quantum layer stack location on transport and coherence times.
[1] CHW Barnes, JM Shilton, AM Robinson. Quantum computation using electrons trapped by surface acoustic waves. Physical Review B 62 (2000) 8410-8419.
[2] RPG McNeil, M Kataoka, CJB Ford, CHW Barnes, D Anderson, GAC. Jones, I Farrer, DA Ritchie. On-demand single-electron transfer between distant quantum dots. Nature 477 (2010) 439-442.
[3] H Dong, G Panda, K Xie, VM Ayres, HC Shaw, D Preston, MD Deshpande. Flying Qubit Investigations For Heterostructure-based Qubit Implementations. Proceedings of the 17th International Conference on Nanotechnology, Pittsburgh, PA, USA, July 25-28, 2017, in press.
8:00 PM - EM08.06.03
Spin Filter Semiconductors for Advanced Quantum Spintronic Devices
Gregory Stephen 1 , Michelle Jamer 2 , Ioana-Gianina Buda 1 , Christopher Lane 1 , Lucas Prescott 1 , Stanislaw Kaprzyk 1 3 , Bernardo Barbiellini 1 , Arun Bansil 1 , Laura Lewis 1 , Don Heiman 1
1 , Northeastern University, Boston, Massachusetts, United States, 2 , National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 3 , AGH University of Science and Technology, Krakow Poland
Show AbstractNext-generation quantum devices will rely on precise control of carrier spins. Spin Filter Materials (SFMs) provide promising advantages for spin manipulation. SFMs are semiconductors having a different bandgap for each spin, whereby the spin degeneracy of the band structure is split by a magnetic exchange energy. When a SFM is used as a tunnel barrier in a spin filter device it effectively blocks one spin direction, thus producing a highly spin-polarized output. The SFM Heusler compound CrVTiAl has two important advantages for quantum spin devices: (1) when used as a spin-dependent tunneling barrier, only nonmagnetic metallic contacts are required; and (2) it is a compensated ferrimagnet with near-zero magnetic moment, thus not producing detrimental fringing fields at nearby devices. The magnetic moment of CrVTiAl is exceptionally small (10-3 μB/f.u. at μ0H = 1 T), temperature independent and linear in field. [2] In addition, CrVTiAl is predicted to have a high Curie temperature (TC > 1000 K), [1] unlike the SFM material EuS which has TC = 16 K. Ab initio calculations show that the spin-dependent bandgap can be preserved even with moderate disorder. Thin films of CrVTiAl have been synthesized on a variety of substrates by magnetron sputtering and have been post-processed with annealing. The temperature-dependent resistivity, ρ(T), shows characteristic small-bandgap semiconducting behavior. Modeling ρ(T) yields a thermal activation energy of ΔE = 0.4 eV, similar to the predicted majority bandgap. Magnetotransport measurements also show a large change in the mobility and an order of magnitude decrease in the carrier concentration above room temperature. The existence of an energy gap in CrVTiAl in accordance with the DFT predictions is an important step towards the realization of a room temperature spin filter device. Work supported by NSF-ECCS-1402738.
[1] I. Galanakis, K. Ozdogan, and E. Sasioglu, J. Phys. Condens. Matter 26, 86003 (2014).
[2] G.M. Stephen, I. McDonald, B. Lejeune, L.H. Lewis, and D. Heiman, App. Phys. Lett. 109, 2320 (2016).
8:00 PM - EM08.06.04
Size-Dependent Magnetic Domain Structure in MnAs Nanoclucters Selectively Grown on Si (111) Substrates Covered with Different Dielectric Mask Designs
Ryoma Horiguchi 1 , Masaya Iida 1 , Kohei Morita 1 , Shinjiro Hara 1
1 , Research Center for Integrated Quantum Electronics, Hokkaido University, Sapporo Japan
Show AbstractNuclear spins in semiconductors are promising for quantum bits since the quantum mechanical superposition states can be maintained for a relatively long time. Polarized electron spin injection from magnetic materials into semiconductors enables us to control nuclear spin polarization by hyperfine interactions. For realizing high injection efficiency of electron spins in future quantum information devices based on Si technologies, therefore, high-quality heterojunction between semiconductors and magnetic materials is required. We have fabricated ferromagnetic MnAs nanoclusters (NCs) with atomically-abrupt heterointerfaces after AlGaAs buffer layer growth selectively on Si (111) wafers, which have SiO2 or SiON mask layers with two different patterns with periodical circular mask openings. Patterning types of the mask layers were designed as follows: (I) SiO2 or SiON films outside 100 x 100 μm2 square regions with the openings were removed and (II) SiO2 or SiON films were removed only for the openings. MnAs/AlGaAs NCs were grown by selective-area metal-organic vapor phase epitaxy at 800 oC only on Si surface in the openings. MnAs was grown for 10 min for the patterning type (I) and 1 min for (II).
To estimate the size of MnAs NCs, the acceleration voltage, Vacc, dependence of backscattered electron images, whose contrasts depend on solid composition of materials, was observed by scanning electron microscopy. At Vacc = 0.5 kV, dark contrasts due to MnAs layers and bright contrasts due to AlGaAs layers were observed in the regions where MnAs/AlGaAs NCs were formed. At Vacc = 2.0 kV, only the bright contrasts were observed in the NC regions in (I). The penetration length of electrons at Vacc = 2.0 kV was calculated to be 44 nm for MnAs. Therefore, the thickness of MnAs layer was estimated to be approximately 40 nm. We also observed different contrasts brighter than contrasts due to Si substrates near the NCs in the cases of (I) with SiO2 and SiON and (II) with SiO2. Judging from the contrasts observed above and energy dispersive X-ray spectroscopy in our previous study, we concluded that MnSi alloys were formed near the Si substrate surface owing to unintentional Mn diffusion into the substrate during the NC growth. In (II) with SiON, the contrasts due to MnSi have never been observed. Next, we characterized magnetic domains in MnAs NCs by magnetic force microscopy at room temperature under zero-field condition. Before applying external magnetic fields, B, single and multiple magnetic domains were observed in most of the MnAs NCs with an area of 4 x 104 nm2 or less and 6 x 104 nm2 or more, respectively, in (II) with SiO2. In the case of (I) with SiON, in particular, after applying B of 1500 Gauss, the change from multiple to single magnetic domains was observed in relatively large NCs. In summary, the results imply that the patterning type (II) with SiON is promising for tuning magnetic domains in the NCs with no contamination by MnSi in Si substrates.
Symposium Organizers
Christopher Richardson, University of Maryland
Jeffrey McCallum, University of Melbourne
Javad Shabani, The City College of New York
Clare Yu, University of California, Irvine
Symposium Support
Google Inc.
IBM Corp.
Microsoft
Quantum Science and Technology | IOP Publishing
EM08.07: Superconducting Qubit Materials I
Session Chairs
Wednesday AM, November 29, 2017
Hynes, Level 1, Room 111
9:00 AM - EM08.07.01
Improving Initial AlOx Stability to Improve Qubit Performance by Reducing TLSs
Yanxue Hong 1 2 , Z. Barcikowski 3 2 , Aruna Ramanayaka 2 4 , Ryan Stein 3 2 , Roy Murray 2 4 , M.D. Stewart 2 , Neil Zimmerman 2 , Joshua Pomeroy 2
1 Electrical and Computer Engineering, University of Maryland, College Park, Maryland, United States, 2 , National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 3 Materials Science and Engineering, University of Maryland, College Park, Maryland, United States, 4 , Joint Quantum Institution, College Park, Maryland, United States
Show AbstractAluminum oxide (AlOx) is a critical material for quantum information (QI), often employed as the gate insulator in Si-based QI devices and almost ubiquitously as a tunnel barrier for superconducting QI architectures. However, AlOx is frequently reported to have a broad distribution of electrically active defects, including two-level systems (TLSs)[1]. TLSs, believed to arise from the initial nonequilibrium structure of AlOx[2], have been shown to be the dominant decoherence source in superconducting qubits[3]. By fabricating AlOx tunnel barriers with initial oxidation profile closer to equilibrium, and thus reducing TLSs, we have demonstrated that relaxation-driven resistance drift is substantially mitigated by confining AlOx tunnel barrier with Co electrodes having a smaller free energy of oxidation in large area tunnel junctions (≈3000 μm2). Co/ AlOx /Co (confined) tunnel junction resistance increased by (32 ± 6) % over 5400 hours, while Co/Al/ AlOx /Co (unconfined) tunnel junction resistance increased by (85 ± 23) % over 5200 hours. Relatively higher and more uniform potential barrier heights, extracted from WKB (Wentzel-Kramers-Brillouin) transport modelling, suggests the as-fabricated tunnel barrier is closer to an equilibrium state and thus less prone to relaxation-related aging. To further evaluate the electrical stability, single-electron transistors (SETs) are fabricated with the plasma oxidized Co/ AlOx /Co (confined) tunnel junctions. A more sensitive measure of the material instability can be made with the SETs. Metal-based SETs made with thermal AlOx suffer from long-term charge offset drift, which is revealed to originate from a large number of interacting TLSs located in the AlOx[4, 5]. Here we are developing SETs by double-angle deposition as a path toward low-capacitance and small-area Co/ AlOx /Co tunnel junctions. Better charge offset stability is expected to be measured on these devices than on typical thermally oxidized devices with unconfined oxygen.
1. Oh, S., et al., Low-leakage superconducting tunnel junctions with a single-crystal Al2O3 barrier. Superconductor Science and Technology, 2005. 18(10): p. 1396.
2. Simmonds, R.W., et al., Decoherence in Josephson phase qubits from junction resonators. Physical Review Letters, 2004. 93(7): p. 077003.
3. Martinis, J.M., et al., Decoherence in Josephson qubits from dielectric loss. Physical review letters, 2005. 95(21): p. 210503.
4. Stewart, M.D. and N.M. Zimmerman, Stability of Single Electron Devices: Charge Offset Drift. Applied Sciences, 2016. 6(7): p. 187.
5. Zimmerman, N.M., et al., Why the long-term charge offset drift in Si single-electron tunneling transistors is much smaller (better) than in metal-based ones: Two-level fluctuator stability. Journal of Applied Physics, 2008. 104(3): p. 033710.
9:15 AM - *EM08.07.02
Reducing 1/f Noise in Superconducting Resonators by Surface Spin Desorption
Lara Faoro 1
1 , French National Center for Scientific Research, Paris France
Show AbstractReducing noise and decoherence in solid state quantum devices will enable enhanced performance of a wide range of sensors and circuits, however, such efforts have been largely inhibited by the lack of knowledge about the origin of this noise and decoherence. Here we correlate measurements of frequency noise and loss in superconducting resonators made from NbN on Al2O3 with ultrasensitive in-situ electron spin resonance (ESR) measurements on the same devices[1]. We find that after removing a large fraction of surface spins the magnitude of the frequency noise is reduced by an equivalent amount (~10 times). Our data is in excellent agreement with a model for strongly interacting two-level systems[2,3], allowing us to attribute the origin of the frequency noise, arising from electric dipolar couplings, to ESR-active slow two-level fluctuators on the surface of our device. The chemical fingerprint of the ESR spectrum together with noise and loss data enables a whole new route towards identification and elimination of sources of noise in solid state quantum circuits, and here we show that surface spins directly affect the performance of high-Q superconducting resonators.
[1] S.E. de Graaf et al., Phys. Rev. Lett. 118, 057703 (2017).
[2] L. Faoro et al., Phys. Rev. B 91, 014201 (2015).
[3] J. Burnett et al., Nature Communications 5, 4119 (2014).
9:45 AM - EM08.07.03
Ab Initio Investigation of Interacting Surface Spin Dynamics as a Flux Noise Origin in Superconducting Qubits
Keith Ray 1 , Nicholas Materise 1 , Jonathan DuBois 1 , Vincenzo Lordi 1
1 , Lawrence Livermore National Laboratory, Livermore, California, United States
Show AbstractThe microscopic origins of magnetic flux noise observed in superconducting qubits have not been fully characterized. Among other surface adsorbates, defects, and impurities, paramagnetic O2 has been identified experimentally as a likely flux noise source [Phys. Rev. Applied 6, 041001 (2016)]. Furthermore, computational studies [PRL 112, 017001 (2014)] of magnetic spins induced by molecules adsorbed on bare Al-terminated Al2O3 demonstrated the possibility of nearly degenerate adsorbate magnetic states. We present a density functional theory investigation of magnetic noise associated with Al2O3 surfaces likely to be encountered in experiment. Motivated by noise models involving spin clusters on the surface, we calculate the exchange interaction between paramagnetic O2, as well as the magnetic state energy splitting and anisotropy, on Al-terminated Al2O3. We use the calculated quantities to parametrize Monte Carlo models that characterize the magnetic phases of the system as a function of applied field, temperature, and O2 coverage, while taking into account the disorder of absorbed O2 molecules that define a spin lattice. Simulations using the Landau–Lifshitz–Gilbert equation allow us to investigate the spin dynamics, which we use to parametrize a finite-element model of a macroscopic flux qubit device.
Prepared by LLNL under Contract DE-AC52-07NA27344.
10:30 AM - *EM08.07.04
Quantum Engineering of Superconducting Qubits
William Oliver 1
1 Lincoln Laboratory and Department of Physics, Massachusetts Institute of Technology, Boston, Massachusetts, United States
Show AbstractWe revisit the materials, fabrication, and design of the superconducting flux qubit. By adding a high-Q capacitor, we dramatically improve its reproducibility, anharmonicity, and coherence, achieving T1 = 55 ms and T2 = 90 ms [1]. We identify quasiparticles as a leading cause of temporal variability in T1. We introduce and demonstrate a stochastic control technique that effectively pumps away these quasiparticles and thereby stabilizes and improves T1 [2]. We discuss the 3D integration of this qubit into architectures of interest for quantum computing applications [3].
For more information:
[1] F. Yan et al., Nature Communications 7, 12964 (2016)
[2] S. Gustavsson et al., Science 354, 1573 (2016)
[3] D. Rosenberg et al., arXiv:1706.04116 (2017)
11:00 AM - EM08.07.05
Self Aligned Superconducting Coplanar Waveguides with Ground Plane Crossovers for Quantum Information Readouts, Bus Resonators and Kinetic Inductance Traveling Wave Amplifiers
David Pappas 1 , Mustafa Bal 1 , Xian Wu 1
1 , National Institute of Standards and Technology, Boulder, Colorado, United States
Show AbstractSuperconducting transmission lines and resonators based on coplanar waveguides (CPWs) have become important tools in superconducting quantum circuits. To improve yield, tune the impedance, and suppress unwanted slotline modes it is desirable to implement an architecture with narrow (sub-micron) gaps that intrinsically are immune to shorts and with ground-plane crossovers above the center line. We demonstrate a simple, self-aligned method to fabricate stitched-ground planes with very narrow gaps and center lines, down to 100 nm in a CPW geometry using optical lithography in NbTiN, TiN, and NbN based circuits. These devices are useful over a wide range of impedances due to the high capacitance per unit length. Measurements of resulting transmission lines and resonators will be presented
11:15 AM - EM08.07.06
Air Bridge Crossovers for Strongly Coupled Superconducting Qubits
Jonilyn Yoder 1 , David Kim 1 , Peter Baldo 1 , Rabindra Das 1 , Alexandra Day 1 , George Fitch 1 , Eric Holihan 1 , David Hover 1 2 , Justin Mallek 1 , Alexander Melville 1 , Danna Rosenberg 1 , Gabriel Samach 1 , Steven Weber 1 , Donna Yost 1 , William Oliver 1 3
1 , MIT Lincoln Laboratory, Lexington, Massachusetts, United States, 2 , California Institute of Technology, Los Angeles, California, United States, 3 Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractSuperconducting qubits are lithographically-defined electronic circuits containing Josephson tunnel junctions that behave as “artificial atoms” when cooled to milliKelvin temperatures. The superconducting qubit coherence time – which is a key metric to characterize their quantum mechanical performance – has improved significantly in the last 15 years. These improvements have been driven by advances in the materials, fabrication, and design of superconducting qubits.
Here I will describe our work to fabricate strongly coupled high-coherence superconducting qubits. I will discuss a process for fabricating superconducting air bridge crossovers, a feature that improves qubit performance by (1) bridging gaps in the ground plane to reduce spurious electromagnetic modes and crosstalk and (2) increasing coupling strength between connected qubits by enabling large mutual inductances. The air-bridge fabrication process is fully compatible with the qubit fabrication process, and it includes mixed patterning using both photolithography and electron beam lithography. The process has led to working superconducting qubit circuits with superior performance. In addition, we are working towards further scaling of our superconducting circuits using 3D integration with indium bump bonding between chips.
This research was funded in part by the Office of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA) and by the Assistant Secretary of Defense for Research & Engineering under Air Force Contract No. FA8721-05-C-0002. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of ODNI, IARPA, or the US Government.
11:30 AM - EM08.07.07
Decontamination and Passivation of Superconducting Al Resonators with Supercritical NF3
Chris Barrett 1 , Rodrigo Guerrero 1 , Bruce Arey 1 , Shutthanandan Vaithiyalingam 1 , Marvin Warner 1 , Ashish Alexander 2 , Chris Weddle 2 , Christopher Richardson 2
1 , Pacific Northwest National Laboratory, Richland, Washington, United States, 2 , Laboratory for Physical Sciences, College Park, Maryland, United States
Show AbstractThe inherent surface contamination encountered with microfabrication and packaging of superconducting devices designed for quantum computing continues to limit qubit coherence times.1 Here, a method and system of abating participation of surface contamination on coplanar waveguide resonators using supercritical NF3 is introduced. Application of NF3 in its supercritical form, as opposed to a plasma, is explored as a low temperature, less-invasive means of device processing, mitigating any further artifact formation and extending greater levels of control over the chemical treatment.2 Previous efforts using NMP based solvents carried by supercritical CO2 have been shown to reduce the spread of measured internal quality factor (Qi) in aluminum resonators, with Qi greater than one million at the single photon level.3 Supercritical treatments were also found to be compatible with the cleaning of fully packaged devices. However, subsequent chemical analysis of processed devices has indicated that carbon is a persistent surface adsorbate that would appear to contribute to losses. With this in mind, the oxidizing behavior of supercritical NF3 is being explored to both scavenge residual carbonaceous species, while also fluorinating the underlying native oxide surfaces. Using observations from x-ray photoelectron spectroscopy and helium ion microscopy, ongoing efforts to evaluate and exploit the cleaning potential of NF3, and subsequent passivation of aluminum surfaces will be discussed at length.
(1) C. J. K. Richardson et al., 2016 Supercond. Sci. Technol. 29 064003
(2) S. J. Molloy et al., 1998 US Patent 5,849,639
(3) C. A. Barrett et al., 2017 Bulletin of the American Physical Society
11:45 AM - EM08.07.08
Characterizing Loss in Superconduct Bump Bonds at DC and Microwave Frequencies
Brooks Foxen 1 , Josh Mutus 2 , Brian Burkett 2 , Yu Chen 2 , Evan Jeffrey 2 , Erik Lucero 2 , Anthony Megrant 2 , John Martinis 2 1
1 , University of California, Santa Barbara, Santa Barbara, California, United States, 2 , Google Quantum Hardware, Santa Barbara, California, United States
Show AbstractAs superconducting qubit circuits grow in complexity, device fabrication must extend beyond planar geometries to route control signals. In the absence of low loss integrated dielectrics, flip-chip bump bonds provide a path forward. We report on our development of superconducting indium bumps with critical currents and yields sufficient to support large qubit systems. We report upper bound resistance measurements below Tc, critical current distributions both within a single device and across devices, as well as microwave loss using coplanar waveguide resonators. Additionally we report on our high-throughput cryogenic DC measurement system.
EM08.08: Superconducting Qubit Materials II
Session Chairs
Wednesday PM, November 29, 2017
Hynes, Level 1, Room 111
1:30 PM - EM08.08.01
Impact of Trenching on Superconducting TiN Coplanar Waveguide Resonators
Alexander Melville 1 , Wayne Woods 1 , Greg Calusine 1 , David Kim 1 , Xhovalin Miloshi 1 , Arjan Sevi 1 2 , Jonilyn Yoder 1 , William Oliver 1 2 3
1 , MIT Lincoln Laboratory, Lexington, Massachusetts, United States, 2 Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractDissipation from two-level systems (TLS) at metal-substrate, metal-air, and substrate-air interfaces reduces the quality factor of superconducting resonators probed at the single-photon limit. Trenching into the substrate reshapes the electric field distribution within the resonator and, thereby, the participation of the various interfaces. Targeting geometries that lower the participation of lossy regions serves to improve the resonator quality factor.
In this work, we fabricated titanium nitride (TiN) coplanar waveguide resonators with varying geometries. In particular, we focus on the impact of trenching by maintaining a nominally identical fabrication process for all samples, varying only the etch time to control the trench depth. We measured the internal quality factor of these resonators in a dilution refrigerator as a function of circulating field strength, from the single-photon limit (low-power) to >10,000 photons (high-power). In parallel, we performed electromagnetic simulations to determine the relative electric-field participation for the various interfaces for the measured resonator geometries. By combining the simulation and measurement results, we developed a model for the losses at the various interfaces that can predict resonator quality factors in the single-photon limit for geometries beyond our current data set. In addition, we will compare the effect of trenching for aluminum and TiN resonators.
This material is based upon work supported by the Department of Defense under Air Force Contract No. FA8721-05-C-0002 and/or FA8702-15-D-0001. Any opinions, findings, conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the Department of Defense.
1:45 PM - EM08.08.02
Growth and Physical Properties of High Quality, Epitaxial Titanium Nitride Prepared by Controllably-Unbalanced Reactive Magnetron Epitaxy
Amber Reed 1 , Hadley Smith 1 2 , Rachel Adams 1 2 , Shin Mou 1 , Lawrence Grazulis 1 3 , Michael McConney 1 , Kurt Eyink 1 , Ashish Alexander 4 , Chris Weddle 4 , Christopher Richardson 4 , Augustine Urbas 1
1 Materials and Manufacturing Directorate, Air Force Research Lab, WPAFB, Ohio, United States, 2 , University of Dayton, Dayton, Ohio, United States, 3 , University of Dayton Research Institute, Dayton, Ohio, United States, 4 , University of Maryland, College Park, Maryland, United States
Show AbstractIn this work, we demonstrate the synthesis of epitaxial titanium nitride (TiN) on (001)-oriented sapphire (Al2O3) substrates using controllably unbalanced reactive magnetron sputtering. Atomic force microscopy reveal remarkably smooth surface morphologies, root mean square roughnesses < 210 pm, and step flow growth. Microstructural characterization was performed using high-resolution X-ray diffraction, reciprocal lattice mapping, and cross sectional transmission electron microscopy and indicate TiN grows (111)-oriented on Al2O3(0001) single crystal substrates. Temperature-dependent Hall measurements show that TiN layers exhibit a room temperature resistivity of < 14 μΩ cm-1 and carrier concentrations of ~ 1022 cm-2. The measured critical temperature, TC ~ 5.5 K, for our TiN, is comparable to values of up to 6 K, reported by Sprengler, Kaiser, Christensen and Muller-Vogt for single TiN prepared using chemical vapor deposition1. We investigate the role of TiN microstructure and stoichiometry, which are determined strongly by deposition conditions (e.g. substrate temperature and nitrogen gas fraction) on determining TiN temperature-dependent electrical properties. Finally, we discuss the implications our results have on TiN as a superconductor for quantum information devices.
1 W. Spengler, R. Kaiser, A. N. Christensen, and G. Muller-Vogt, "Raman scattering, superconductivity and phonon density of states of stoichiometric and nonstoichiometric TiN. [Comparison with TiC]," Physical Review B (Solid State) 17 (3), 1095-1101 (1978).
2:00 PM - *EM08.08.03
Towards Reliable, Extensible Superconducting Qubit Circuits
Sami Rosenblatt 1 , Jared Hertzberg 1 , Markus Brink 1 , Vivekananda Adiga 1 , Jerry Chow 1
1 , IBM T.J. Watson Research Center, Yorktown Heights, New York, United States
Show AbstractSuperconducting quantum processors with 5 to 16 qubits have recently been fabricated and deployed to the cloud on the IBM Q Experience [1,2]. While developing this hardware, new challenges in operating multiqubit chips have been uncovered resulting from classical and quantum crosstalk. In order to achieve extensibility, there is a need for reducing process variability and leveraging alternative types of qubits. Process yield windows can be improved, for example, by reducing sources of systematic spread in Josephson junction fabrication, such as lithography, tunnel resistance, and qubit design. Room-temperature testing of thousands of junctions is routinely utilized in screening hardware for baseline control, further improving chip yield. Target qubit frequency spreads of 1% or lower can then be used to sort qubits by frequency in an extensible manner. Short of realizing this goal, hybrid approaches utilizing different qubit types, such as Capacitively Shunted Flux Qubits (CSFQs) and Transmons, offer a path where extensibility can be achieved while maintaining high gate fidelities.
Work performed in collaboration with D. Pappas, R. McDermott, B. Plourde, A. Houck, and the IBM Q Experience Team, and supported in part by IARPA under contract No. (W911NF-16-1-0114-FE).
[1] https://www.research.ibm.com/ibm-q/
[2] https://github.com/IBM/qiskit-qx-info/tree/master/backends/ibmqx3
3:30 PM - EM08.08
Quantum Materials Panel Discussion
Show Abstract
Symposium Organizers
Christopher Richardson, University of Maryland
Jeffrey McCallum, University of Melbourne
Javad Shabani, The City College of New York
Clare Yu, University of California, Irvine
Symposium Support
Google Inc.
IBM Corp.
Microsoft
Quantum Science and Technology | IOP Publishing
EM08.09: Qubit Noise from Materials and Interfaces
Session Chairs
Thursday AM, November 30, 2017
Hynes, Level 1, Room 111
8:45 AM - *EM08.09.01
Probing Noisy Surfaces with Trapped Ions
Jeremy Sage 1 , Colin Bruzewicz 1 , Amy Greene 1 2 , William Loh 1 , Robert McConnell 1 , Jonathan Sedlacek 1 , Jules Stuart 1 2 , John Chiaverini 1
1 Quantum Information and Integrated Nanosystems Group, MIT Lincoln Laboratory, Lexington, Massachusetts, United States, 2 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractTrapped ions are one of the leading modalities for large-scale quantum information processing. Their internal electronic states allow for long quantum coherence times and their external quantum states of motion can be utilized for high-fidelity control of ion-ion interactions. In many systems of trapped ions, the fidelity of this control is currently limited by decoherence of the ions’ external states due to electric-field noise coming from nearby material surfaces. The physical origin of this noise is not well-understood; however, an effort to gain insight into the noise source is crucial to its mitigation. In this talk, I will discuss experiments done to systematically probe the noise with trapped ions. These experiments are enabled by utilizing the ions’ exquisite sensitivity to electric fields to our advantage.
9:15 AM - EM08.09.02
Ab Initio Study of Electric Field Noise Heating in Ion Traps Caused by Electrode Surface Adsorbates
Brenda Rubenstein 2 , Keith Ray 1 , Wenze Gu 2 , Vincenzo Lordi 1
2 , Brown University, Providence, Rhode Island, United States, 1 , Lawrence Livermore National Laboratory, Livermore, California, United States
Show AbstractTrapped ions, due to their relative isolation, allow unique access to the quantum world. Although they are promising candidates for the fundamental building blocks of a quantum computer, their function in this role depends on coherence times long enough to implement error correcting codes. A significant contribution to decoherence is motional heating of the trapped ion from its vibrational ground state, caused by electric field noise. We present simulations characterizing the electric field noise generated by a variety of trap electrode surface adsorbates. Using density functional theory, we accurately calculate the chemically specific adsorbate-electrode interaction potential and induced dipole, for both out-of-plane and in-plane modes, which determines the noise spectrum. We then utilize classical simulations to determine the effect of adsorbate diffusion. The temperature, frequency, and coverage dependence of the electric field noise is discussed and compared with experiment.
Prepared by LLNL under Contract DE-AC52-07NA27344.
9:30 AM - *EM08.09.03
Shielding Spin Qubits in Semiconductors from Electric Noise
Guido Burkard 1
1 , Department of Physics, University of Konstanz, Germany, Konstanz Germany
Show AbstractWith the use of nuclear-spin-free materials such as silicon, germanium, and carbon, spin-based quantum bits (qubits) now rank among the most coherent systems for quantum information processing. The frontier has therefore shifted to the ubiquitous electric noise which is limiting the coherence of spin qubits. Electrically controlled multi-electron qubits (e.g. the resonant exchange qubit) possess a sizeable electric dipole and are especially affected by electric noise. By investigating the energy landscape of the resonant exchange qubit and other three-electron qubits, we have demonstrated the existence of special operating points (sweet spots) in the control parameter space where the qubit is protected against electrical noise. This opens new possibilities to exploit the benefits of electric control in hybrid quantum systems consisting of semiconductor spins embedded into an electromagnetic cavity. Our theoretial analysis shows that the resonant-exchange qubit can be efficiently coupled to a superconducting microwave resonator. In this context, we also describe the dispersive microwave detection of materials properties such as the valley splitting in silicon quantum dots, and the prospect of using cavity QED to couple remote spin qubits in an extended quantum register. Similarly, we discuss how the coupling of the electron and nuclear spin of defect centers in wide-gap semiconductors to an optical cavity lends itself to the implementation of two-qubit quantum gates for defect spin qubits.
EM08.10: Semiconductor-Superconductor and Topological Qubit Materials I
Session Chairs
Thursday PM, November 30, 2017
Hynes, Level 1, Room 111
10:30 AM - *EM08.10.01
Coherent Superconductor-Semiconductor Quantum Circuits
Karl Petersson 1 2
1 , Microsoft, Copenhagen Denmark, 2 Niels Bohr Institute, University of Copenhagen, Copenhagen Denmark
Show AbstractThe recent development of semiconductors with epitaxial superconducting Al contacts offers new approaches to realizing coherent superconducting quantum devices. In particular, we have demonstrated superconducting transmon qubits with Josephson junctions based on hybrid superconductor-semiconductor nanowire materials. These gate tunable transmons (gatemons) have the potential advantage that they can be readily controlled through local electrostatic gating of the junction element. I will discuss progress in understanding and optimizing gatemon qubits, as well as approaches to scaling the materials. I will also discuss how these hybrid materials might be used to realize novel qubits that are intrinsically protected against sources of decoherence.
11:00 AM - EM08.10.02
InAsSb Nanowires with Epitaxial Aluminum as a Platform for Topological Quantum Computation
Mihir Pendharkar 1 , Sasa Gazibegovic 2 , Jay Logan 1 , Roy Op het Veld 2 , Hao Zhang 3 , Marcel Verheijen 2 , Diana Car 2 , Elham Fadaly 2 , Di Xu 3 , Michiel de Moor 3 , Nick van Loo 3 , Daniel Pennachio 1 , Joon Sue Lee 1 , Borzoyeh Shojaei 1 , Leo Kouwenhoven 3 , Erik Bakkers 2 , Chris Palmstrom 1
1 , University of California, Santa Barbara, Santa Barbara, California, United States, 2 , Technische Universiteit Eindhoven, Eindhoven Netherlands, 3 , Delft University of Technology, Delft Netherlands
Show AbstractIII-V semiconductors with strong spin orbit coupling have gained significant interest in the field of topological quantum computation, with the prediction and subsequent realization of Majorana fermions in InSb nanowires. As compared to InAs and InSb, certain compositions of InAs(x)Sb(1-x) have been shown to exhibit a further reduction in band gap and electron effective mass along with an enhancement in spin orbit interaction, making it a favorable material system for quantum information processing.
This work focusses on Molecular Beam Epitaxy growth, device fabrication and low temperature electrical characterization of InAsSb nanowires with in-situ evaporated Aluminum islands. The superconductor-semiconductor interface has been studied with Scanning Transmission Electron Microscopy and composition determined with Energy Dispersive X-Ray Spectroscopy. These nanowires demonstrate gate controlled depletion and induced superconductivity in InAsSb at low temperatures, paving the way for realization of novel topological phenomenon.
The authors would like to acknowledge the support of Microsoft Research Station Q.
11:15 AM - *EM08.10.03
A Hybrid Superconducting Nanowire Laser
Maja Cassidy 1 , Willemijn Uilhoorn 2 , James Kroll 2 , Damaz de Jong 2 , David van Woekom 2 , Jesper Nygard 3 , Peter Krogstrup 3 , Leo Kouwenhoven 2
1 , The University of Sydney, Sydney, New South Wales, Australia, 2 , Delft University of Technology, Delft Netherlands, 3 , University of Copenhagen, Copenhagen Denmark
Show AbstractWe demonstrate lasing from a nanowire Josephson junction strongly coupled to a superconducting cavity. The emission frequency f is set by the frequency of the fundamental mode of the cavity, while the gain medium consists of a Josephson junction formed in a proximitized InAs/Al nanowire (P. Krogstrup et al, Nature Materials 14, 400–406 (2015)). A dc voltage bias Vb applied across the Josephson junction results in photon emission to the cavity when the energy of the emitted photon equals a multiple of the cavity frequency (nhf = 2eVb) via the ac Josephson effect. Increased emission intensity and laser coherence is observed up to the n = 4 emission peak, while emission at higher frequencies is suppressed due to the excitation of subgap states in the nanowire. Real time analysis of the emission statistics shows an emission coherence exceeding 300 us. Our hybrid nanowire laser displays improved coherence compared to recently demonstrated SIS junction lasers (M.C. Cassidy et al, Science355, 939-942 (2017)), and allows for modulating the emission with an electrostatic gate.
11:45 AM - EM08.10.04
Defect-Free Bi1-xSbx Nanowires on Si by MBE
Daya Dhungana 1 , Pier-Franscesco Fazzini 2 , Filadelfo Cristiano 1 , Sebastian Plissard 1
1 , LAAS-CNRS, Toulouse France, 2 , LPCNO-INSA, Toulouse France
Show Abstract1D nanostructures of Bismuth Antimonide (Bi1-xSbx) alloys, especially nanowires, are promising for quantum computing, thermoelectrics and spintronics.1 Varying Sb composition (x) , Bi1-xSbx is suggested to behave as: a semi-metal (x<0.07), an indirect bandgap semiconductor (0.070.22).1 Thus, a precise control of this parameter hold promises for future quantum devices. For instance, if x=0.03, 3D Dirac cones should be observed in the structure and could be used to host Majorana zero mode when coupled with a superconducting contact.2 If 0.080.23, the high electron mobility and strong spin-orbit interactions make it an interesting candidate for spintronics.1,3
Hence excellent quality of 1D nanostructure, with control over Sb content, is necessary in order to understand and engineer the material. This study is the initial step on addressing these existing hurdles and explore opportunities. Epitaxial Bi1-xSbx nanowires with controlled Sb concentrations are integrated on Si(001) and Si(111) substrates for the first time.
The process starts with the removal of native oxide from unpatterned Si(001) and Si(111) wafers with the help of hydrofluoric acid (HF 5%). The degassing of substrates at 200 °C follows next. Then self-catalyzed growth of Bi1-xSbx nanowires (varying x) occurs on these substrates in a solid source molecular beam epitaxy (MBE) system. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) characterizations are carried out for morphology, composition and crystallographic studies. Electrical characterizations will be performed to extract intrinsic properties (electron mobilities, resistivity etc.).
SEM characterizations confirm high density of Bi1-xSbx nanowires with diameters around 25 nm and lengths up to 15 μm. As expected, the composition has a significant impact on these nanostructures including density, morphology, and crystallography. Finally, the electrical characterizations will be presented.
(1) Tang, S.; Dresselhaus, M. S. J. Mater. Chem. C 2014, 2 (24), 4710.
(2) Hasan. Nat. Phys. 2011, 7 (1), 8–10.
(3) Zahid Hasan, M.; Xu, S. Y.; Neupane, M. In Topological Insulators: Fundamentals and Perspectives; 2015; pp 55–100.
EM08.11: Semiconductor-Superconductor and Topological Qubit Materials II
Session Chairs
Thursday PM, November 30, 2017
Hynes, Level 1, Room 111
1:30 PM - *EM08.11.01
Semiconductor-Superconductor Hybrid Structures for Topological Quantum Computing
Mike Manfra 1 3 4 , Tiantian Wang 1 2 , Anthony Hatke 1 2 , Nisede Thomas 1 2 , Sergei Gronin 1 2 , Geoffrey Gardner 1 2 3
1 Station Q Purdue and Department of Physics and Astronomy, Purdue University, West Lafayette, Indiana, United States, 3 School of Materials Engineering, Purdue University, West Lafayette, Indiana, United States, 4 School of Electrical and Computer Engineering , Purdue University, West Lafayette, Indiana, United States, 2 Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana, United States
Show AbstractQuantum computation requires initialization, manipulation, and readout of information that is stored in quantum-mechanical two-level systems – the qubits. At present many possible physical systems are explored as potential platforms for quantum computation, include superconducting qubits, trapped ions, and various semiconductor spin systems. A principal challenge to all quantum technologies is loss of information due to decoherence through uncontrolled interactions with the environment. It is theoretically conjectured that decoherence may be suppressed by storing information in non-local topological degrees of freedom that do not couple to local Hamiltonians. Possible physical realizations include even-denominator fractional quantum Hall states in a two-dimensional electron gas and topological superconductors. Creating optimized platforms for topological quantum computation presents interesting challenges for material science. In this talk I will describe our efforts using molecular beam epitaxy to build semiconductor and semiconductor-superconductor hybrid systems that may form the basis for future quantum technologies that are robust against decoherence. We focus on hybrid high-spin-orbit-coupling (SOC) semiconductor-superconductor interfaces (e.g. Al/InAs and Al/InSb) that are believed to support Majorana zero modes that may be topologically protected against decoherence. The interplay of material properties, and device operation will be discussed.
2:00 PM - EM08.11.02
Epitaxial Growth of Thin Films of the 3D Dirac Semimetal Cd3As2
Manik Goyal 1 , Timo Schumann 1 , David Kealhofer 1 , Luca Galletti 1 , Susanne Stemmer 1
1 , University of California, Santa Barbara, Santa Barbara, California, United States
Show AbstractTopological semimetals are a newly discovered class of materials, which have gathered tremendous attention to their unique transport properties and promise for future applications in quantum information technologies, for instance as platforms for topological superconductivity. Cadmium arsenide (Cd3As2), which was predicted to be a three-dimensional topological Dirac semimetal theoretically [1] and later confirmed experimentally [2], exhibits large electron mobility, ultrahigh magnetoresistance and other unusual transport characteristics [3]. While most experiments so far have been carried out on bulk crystals, thin films are needed for new devices and for engineering of novel topological phases.
This presentation focuses on the synthesis of high-quality epitaxial layers of Cd3As2 on (111) oriented III-V substrates by molecular beam epitaxy (MBE). MBE is an established technique to grow high quality, defect free thin layer with controlled strain. We demonstrate the feasibility of MBE to grow single phase Cd3As2, epitaxially aligned to the substrate and featuring smooth, and atomically stepped surfaces. Via scanning transmission electron microscopy, we directly image the atomic structure and confirm that the “vacancy ordering” that is characteristic of the I4acd Dirac semi metal phase. We show record-high room temperature carrier mobility of 20,000 cm2/Vs in Cd3As2 on GaSb [4,5]. The carrier mobility is reduced in films with lower thickness. This indicates that the mobility is limited by interface defects arising due to lattice mismatch between substrate and film. To overcome this problem, we developed the growth of lattice matched InxGa1-xSb buffer layers. We achieved an improvement in surface morphology and carrier mobility for thinner films. Additionally, we were able to stabilize the growth at higher substrate temperatures for thinner film with improved thickness control in comparison to growth of lattice-mismatched substrates. This increased temperature improves film quality, and hence electron mobility, due to increased adatom mobility during growth. To investigate the ability to impose controlled epitaxial strains into the Cd3As2 films, we grow films on InxGa1-xSb buffer layers with varying In/Ga ratio. In this talk, growth conditions, structural and transport properties of epitaxial unstrained and strained Cd3As2 films will be discussed.
Reference:
[1] Wang, Z. et al., Phys. Rev. B 88,125427 (2013).
[2] Sergey, B. et al., Phys. Rev. Lett. 113, 027603 (2014).
[3] Tian, L. Nature Materials 14 280–284(2015)
[4] Schumann, T., Goyal, M. et al. APL Materials 4, 126110 (2016).
[5] Schumann, T., Goyal, M. et al., arXiv:1706.03172
2:15 PM - EM08.11.03
Epitaxial Superconductor-Semiconductor Interfaces as a 2D Platform for Qubits
Joseph Yuan 1 2 , Kaushini Wickramasinghe 1 , William Mayer 1 , Aaron Somoroff 1 , Klea Dhimitri 3 , Tri Nguyen 1 , Javad Shabani 1 2
1 City College of New York, CUNY, New York City, New York, United States, 2 The Graduate Center, CUNY, New York, New York, United States, 3 Hunter College, CUNY, New York, New York, United States
Show AbstractHybrid superconductor-semiconductor materials offer a flexible platform for a number of quantum computation schemes [1]. We focus on material growth of a two dimensional electron gas (2DEG) confined to InAs surface quantum well. The proximity of the 2DEG to surface allows coupling it to epitaxial thin film of Al superconductor. We report on these epitaxial interfaces by analyzing the InAs, Al and combined transport properties. Using AFM imaging we benchmark quality of the interface and identify Al growth window as a function of substrate temperature and flux rates. Josephson junctions fabricated on these structures exhibit supercurrent with high interface (contact) transparency [2].
[1] M. Cheng, et al., Phys. Rev. B 85, 165124 (2012)
[2] J. Shabani, et al., Phys. Rev. B 93, 155402 (2016)
2:30 PM - EM08.11.04
Electron Microscopy Studies for Compositional and Structural Analysis of Potentially Topological Barium Niobium Sulfide Material Class
Cigdem Ozsoy Keskinbora 1 , Aravind Devarakonda 2 , Austin Akey 1 , Stephan Kraemer 1 , Joseph Checkelsky 2 , David Bell 1
1 , Harvard University, Cambridge, Massachusetts, United States, 2 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show Abstract2005 was an important milestone as a consequence the realization of the existence of a metallically conductive surface state in an insulator material1. Within a couples of years, the experimental evidences of the surface state followed the theory studies. A high spin orbit coupling creates edge states where quantum spin Hall Effect can exist in the absence of an external magnetic field. The discovery of the new phenomenon opened up an intensive discussions in condensed matter, and even very well-known conventional material systems such as Bi2(Te,Se)3, BiSb alloys etc., became “exotic” and highly investigated materials again.
The later stages brought up the next questions such as, can the topologically protected surface state exist in insulators, metals, and superconductors? Today, not only already known materials system are an investigation area for topological materials, but also new materials are being developed where the topological surface can be tuned, modified or controlled.
The exploration and synthesis of constitute only one aspect of the challenges in the development of new topological materials, another challenge is their characterization. Since the phenomena appears at very restricted and dedicated conditions, the characterization method must have very high sensitivity, resolution, localization and precision. Transmission electron microscopy is a powerful technique to investigate structural, compositional or electromagnetic properties of materials. Especially, recent implementation of aberration correction2 in the transmission electron microscopy made chemical and structural characterization with very high spatial resolution and sensitivity possible3. This in turn allows with spatial resolution in the range of picometers for characterization of topological materials, where small compositional variations have large effects on the material properties. The crystal structure and composition determine the electromagnetic properties of the materials. To be able to understand, analyze and control these properties, both crystal structure and composition need to be known precisely.
Here, in this study we summarize detailed electron microscopy studies on single crystal barium niobium sulfides that were synthesized by vapor transport and flux methods towards the realization of a new class of naturally heterostructured low-dimensional topological materials4. The findings of STEM, atomic resolution (S)TEM, EELS, EDX and inline electron holograph experiments for compositional analysis and determination of crystal structure will be reported.
References
1) Kane, C. L., and E. J. Mele, 2005, Phys. Rev. Lett. 95, 226801.
2) Rose, H., J. Electron Microsc. 2009, 58, 77–85
3) Krivanek, O. L. et al., Nature 2010, 464, 571–574
4) Kim, Y. et al., Phys. Rev. Lett. 115, 086802 (2015)
2:45 PM - EM08.11.05
Variations in Electric Switching and Transverse Resistance of GeTe/ Sb2Te3 Superlattices at Elevated Temperature Studied by Conductive Scanning Probe Microscopy
Leonid Bolotov 1 2 , Yuta Saito 1 2 , Tetsuya Tada 1 2 , Junji Tominaga 1 2
1 , National Institute of Advanced Industrial Science and Technology, Tsukuba Japan, 2 , Japan Science and Technology Agency, Kawaguchi, Saitama, Japan
Show AbstractPhase change superlattices (PCSL) composed of ultra-thin layers of ferroelectric GeTe and topological insulator Sb2Te3 are of practical interest as a component of novel quantum information and energy-efficient memory devices owing to its voltage-pulse-controlled electric resistance and unique magneto-optical properties. In particular, superlattices of GeTe(0.7nm)/Sb2Te3 (1nm) have showed a reversible electric switching between high (HRS) and low (LRS) resistance states at room temperature, caused by a structural phase transition involving atom displacement. [1-3] Here, we have studied temperature variations in electric switching and transverse resistance of superlattice grains using conductive scanning probe microscopy (SPM) to shed light on the underlying charge transport mechanism.
Superlattice samples were grown by a sputtering method [4] on Si and sapphire substrates, which includes a growth of layers of GeTe(0.7nm) and Sb2Te3 (1nm) with different number of periods at 230oC following deposition of a 50-nm-thick tungsten film (a backside electrode); Ge2Sb2Te5 alloy samples were prepared at RT for comparison. Point current-voltage (I-V) hysteresis and the Kelvin potential were measured with tantalum or platinum-coated SPM cantilevers (a top electrode) in a temperature range of 25 – 200oC in argon gas, thus, eliminating chemical degradation of the films at high temperature.
Fabricated PCSL films showed 2 kinds of grains having different Kelvin potential voltage. Both grain types showed a decrease of the Kelvin potential voltage by ~150mV from 25 to 150oC, reflecting significant change of the Fermi energy. Noticeable, at 25-80oC, I-V curves were non-linear with large current hysteresis (ON/OFF ratio ~100) and bipolar switching around +1.6 V and -1.4 V. At 130-200oC, I-V curves became linear with no hysteresis. The current hysteresis re-appeared after cooling. In contrast to the GeSbTe alloy, the PCSL showed large change in the thermal coefficient of resistance (TRC) above 80oC, the evidence of different structural phases of the PCSL film in different temperature regions. The observed variations in the Fermi energy and TRC will be discussed in term of a ferroelectric-like phase transition and an overlap of the PCSL electronic bands near the Fermi energy, which impact on transverse carrier transport in the PCSL film.
The work was supported by Japan Science and Technology Agency (JST/CREST, the grant number JPMJCR14F1).
[1] J. Tominaga et al., Adv. Mater. Interfaces 1 (2014) 1300027.
[2] J. Tominaga et al., Appl. Phys. Lett. 99 (2011) 152105.
[3] L. Bolotov et al., Scientific Reports 6 (2016) 33223.
[4] Y. Saito et al., AIP Adv. 6 (2016) 045220.