Symposium F.NM05—Emerging Materials for Quantum Information Technologies

Electron-phonon interaction plays an important role in the electronic properties of topological materials by affecting the dispersion and lifetime of phonon and electronic states. For example, Kohn anomaly describes the phonon softening phenomenon when the dielectric screening from conducting electrons in a metal is disrupted. Another evidence of strong electron-phonon interaction is Fano resonance, the interference between discrete and continuous states, featured by an asymmetric Breit-Wigner-Fano lineshape. In this work, we present the anisotropic Kohn anomaly and Fano resonance in a type-II Weyl semimetal candidate LaAlSi by polarized Raman spectroscopy. The B₁ phonon mode in our Raman spectra shows excitation-wavelength-dependent phonon-softening and lineshape asymmetry. In type-I Weyl semimetal TaP, abnormal symmetry breaking of another B₁ mode in polarized Raman spectroscopy has been observed, attributed to the quantum interference of degenerate carrier pockets with different symmetries. The experimental observations combined with density-functional theory (DFT) calculations provide valuable insights into microscopy scattering pathways and transport properties of the type-II Weyl semimetal candidate LaAlSi and type-I Weyl semimetal TaP.

Owing to the rapid development of quantum information technology including quantum communication, quantum sensor, and quantum computer, realization of quantum light source, an essential element for the photonic quantum information applications, have been required. There have been numerous efforts to obtain single photon sources as a quantum light source by using various solid-state material platforms including semiconductor quantum dots (QDs), defect-based color centers in diamond and SiC, QDs in two-dimensional material of WS₂ and WSe₂, and etc. III-V semiconductor QDs have been widely considered as a promising platform for the single photon source due to their various advantages of high generation efficiency of single photon, wavelength tunability, and favourable integration with various semiconductor-based photonic circuits. Single photon emission in 1.5 μm telecom-wavelength range was quite important because of compatible with quantum communication and photonic integrated circuits of silicon photonics. However, single photon emission in telecom-wavelength range was still challenge. InAs QDs are normally grown by Stranski–Krastanov growth mode with molecular-beam epitaxy or metal organic chemical vapor deposition (MOCVD). Because the diameter and height of the self-assemble InAs QDs are determined by strain originated from lattice mismatch with host materials, it was difficult to obtain telecom-wavelength single photon emission from InAs/GaAs or InAs/InP QDs. In this study, we obtained 1.5 μm telecom-wavelength single photon emission from InP-based InAs QDs by using strain-controlled layer of InAlGaAs. The single photon source was fabricated with circular bragg grating structure to obtain high collection efficiency of single photon emission. Moreover, we proposed integrated single photon devices to realize integrated quantum light source with photonic integrated circuits.
For the strain control during the growth of InAs QDs, tensile-strained InAlGaAs layers as a host material were grown on the InP substrate. The InAs QDs were grown by Stranski–Krastanov growth mode in MOCVD. lattice-mismatched InAlGaAs on InP showed 0.74 % tensile strain with InP, which was characterized by x-ray diffraction measurement. We monitored the structural properties of InAs QDs by atomic force microscopy and transmission electron microscopy with changing the strain-controlled layers of InAlGaAs, InGaAs, and InP and with changing the thickness of strain-controlled layers. To optimize the size and density of the QD, InAs samples were grown with various growth condition including the temperature, growth rate, and the III/V ratio. We found that the size of InAs QDs were strongly dependent on the density and As flow was critical for emission efficiency of QD and wetting layer. Through systematic study, we could obtain low enough density of InAs QD from 10⁷ /cm² to 10⁹ /cm² for fabrication of single photon emitter. We carried out the micro-photoluminescence measurement at 3.4 K for optical analysis of QDs. The single QD emission peak was exhibited from 1.5 to 1.6 μm wavelength. The second-order correlation function was measured for single photon analysis through Hanbury-Brown Twiss experiment. We designed circular bragg grating structures with InAs/InAlGaAs QD for collection efficiency and Purcell enhancement of cavity. Optical simulation of the QD in cavity was performed by finite-difference time domain simulation. The collection efficiency of the circular bragg grating structure was more than 80 % to objective lens of NA 0.7. We also designed the integrated single photon source with Si photonic circuit. InAlGaAs waveguide was designed as the nano tapered waveguide and SiO₂ interlayer was adopted to efficiently couple the single photon emission with Si waveguide. The QD-based telecom-wavelength single photon source of high collection efficiency and integrated single photon source could provide a solution for on-chip quantum photonic devices.

Atom qubits in silicon have demonstrated extremely long electron and nuclear spin coherence times [1,2] making silicon a promising semiconductor material for spin-based quantum information. The two-level spin state of single electrons bound to shallow phosphorus donors in silicon provide well defined, reproducible qubits [3] with low gate densities. An important challenge in these systems is the realisation of an architecture, where we can position donors within a crystalline environment with <20nm separation, individually address each donor, manipulate the electron spins using ESR techniques, read-out their spin states and perform both single and two qubit gates.

We have pioneered a unique strategy to build a scalable quantum computer in silicon atom by atom. We use scanning tunneling microscope lithography and ultra-high vacuum crystal growth to precisely position individual P donors in Si [4] aligned with nanoscale precision to local control gates [5] necessary to initialize, manipulate, and read-out the spin states [6-8].

During this talk I will focus on demonstrating fast, high fidelity single-shot spin read-out [9], ESR control of precisely-positioned P donors in Si [10] and our recent results of demonstrating a fast (~ns) two-qubit SWAP gate [11,12]. We have also published a 3D architecture for implementation of error corrrection using the surface code [13]. With important advances in control at the atomic-scale, I will attempt to highlight the benefits of single atom qubits in silicon.

References
[1] K. Saeedi et al., Science 342, 130 (2013).
[2] J. T. Muhonen et al., Nature Nanotechnology 9, 986 (2014).
[3] B.E. Kane, Nature 393, 133 (1998).
[4] M. Fuechsle et al., Nature Nanotechnology 7, 242 (2012).
[5] B. Weber et al., Science 335, 6064 (2012).
[6] H. Buch et al., Nature Communications 4, 2017 (2013).
[7] B. Weber et al., Nature Nanotechnology 9, 430 (2014).
[8] T. F. Watson et al., Science Advances 3, e1602811 (2017).
[9] D. Keith et al., Physical Review X 9, 041003 (2019)
[10] S. Hile et al., Science Advances 4, eaaq1459 (2018).
[11] M.A. Broome et al., Nature Communications 9, 980 (2018).
[12] S. Gorman, Y. He et al., Nature 571, 371 (2019).
[13] C. Hill et al., Science Advances 1, e1500707 (2015).

Diborane (B₂H₆) is a promising precursor molecule for atomic precision p-type doping of silicon that has recently been experimentally demonstrated on the Si (100) surface [Škeren, et al., 2019]. We use first principles density functional theory (DFT) calculations to determine the reaction pathway of diborane dissociating into incorporated boron at the silicon surface. We show that diborane must overcome an energy barrier to even adsorb onto the bare silicon surface, explaining the low sticking coefficient seen in previous studies. Heating can be used to increase the initial adsorption rate. From there, the diborane has a roughly half chance of splitting into two BH₃ particles or merely losing hydrogen to form a boron dimer such as B₂H₄. Dimers of boron are likely electrically inactive, thus making it crucial that the diborane split for final incorporation of an electrically active acceptor. The dissociation process proceeds with high energy barriers, necessitating the use of high temperatures for incorporation. Using the barriers calculated from DFT, we parameterize a Kinetic Monte Carlo model, predicting incorporation statistics of diborane as a function of initial depassivation geometry, dose pressure, and anneal temperature. Finally, we present a theoretical comparison of diborane to other potential acceptor precursor molecules.

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-NA-0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

Scanning tunneling microscopy and hydrogen depassivation lithography (HDL) have enabled tip-defined placement of dopant atoms in Si for Atomic Precision Advanced Manufacturing (APAM) [1-2] of conventional and quantum electronic devices. Although it affords greater placement accuracy than ion implantation, APAM has historically required high-temperature processing incompatible with microelectronics fabrication workflows. Recent process developments have reduced the thermal budget for APAM sample prep, the highest temperature step of the process, to temperatures comparable to activation for ion implantation. Reduced temperature prep opens the door to integrating APAM near the end of front-end-of-line (FEOL) fabrication thereby integrating atomic-scale dopant placement with conventional microelectronic processes. Further reducing the thermal budget for key parts of the APAM process flow – sample preparation, hydrogen termination, dopant incorporation, and silicon capping – may open the door to adding dopants during back-end-of-line (BEOL) fabrication for the first time.

Here, we discuss techniques for FEOL and BEOL APAM integration of donor and acceptor dopants in Silicon. We demonstrate room-temperature Si(100) hydrogen passivation without compromising HDL and reduced-temperature Si encapsulation without significantly compromising electron transport. In an effort to reduce surface preparation temperatures and achieve BEOL CMOS compatibility, we explore both wet chemical and ion beam processes for low-temperature Si(100) 2×1 preparation and discuss the limitations of each. Thermal budget considerations explored here are one of the key challenges which must be addressed to enable APAM integration. We conclude that hydrogen passivation, HDL, dopant introduction, and Si encapsulation can be made compatible with BEOL integration. Future challenges remain for sample cleaning, but preliminary results are encouraging and suggest, with continued development, the possibility of APAM BEOL CMOS integration.

This work was supported by the Laboratory Directed Research and Development Program at Sandia National Laboratories, and was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. DOE, Office of Basic Energy Sciences user facility. Sandia National Laboratories is 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 under contract DE-NA-0003525. The views expressed do not necessary represent the views of the U.S. DOE or the United States Government.

[1] D.R. Ward et al., Appl. Phys. Lett. 111, 193101 (2017)
[2] D.R. Ward et al., EDFA Magazine 22, 1 (2020)

Recently, new devices for future classical and quantum computation technologies have been demonstrated using scanning tunneling microscopy (STM) and hydrogen depassivation lithography (HDL) to position phosphorous or arsenic atoms as donors in silicon with atomic-scale precision.[1,2] Although precursors such as phosphine (PH₃) and arsine (AsH₃) serve satisfactorily as gaseous sources for donors, the potential counterpart acceptor precursors such as diborane (B₂H₆) and alane (aluminum hydride, AlH₃) as sources have proven to be more challenging when combined with the aforesaid atomic-scale STM-HDL technology. In particular, alanes are energetically stable only at temperatures T ≤ 120 K. On the other hand, recent DFT calculations [3] show a stable route exists for AlH₃ dissociation and Al incorporation (following selective surface hydrogen desorption by STM). Moreover, the DFT results show the overall process of AlH₃ surface interactions on Si parallels PH₃; hence, atomically precise acceptor doping should be possible using alanes. With these constraints in mind, we have developed a new type of low-temperature alane precursor source that provides p-type dopants that are compatible with atomic-scale fabrication techniques. Specifically, infrared data from fast Fourier transform spectroscopy, examining alane formation and specie type, will be presented as a function of temperature. In addition, electronic transport characteristics from van der Pauw and Hall effect measurements as a function of temperature are planned for presentation.

REFERENCES
1. Schofield, S. R.; Curson, N. J.; Simmons, M. Y.; Rueß, F. J.; Hallam, T.; Oberbeck, L.; Clark, R. G. Atomically Precise Placement of Single Dopants in Si. Phys. Rev. Lett. 91, 136104 (2003).
2. Stock, Taylor J. Z.; Warschkow, Oliver; Constantinou, Procopios C.; Li, Juerong; Fearn, Sarah; Crane, Eleanor; Hofmann, Emily V. S.; Kölker, Alexander; McKenzie, David R.; Schofield, Steven R.; Curson, Neil J., Atomic-Scale Patterning of Arsenic in Silicon by Scanning Tunneling Microscopy, ACS Nano, 14(3), 3316-3327 (2020).
3. Smith, Richard; Bowler, David, R., Reaction paths of alane dissociation on the Si(001) surface, Journal of Physics: Condensed Matter 30(10), 105002 (2018).

Efforts to grow Si (100) with a supersaturated Al delta layer in pursuit of a superconducting phase are hampered by an ambiguity in the surface Al atom density as measured by STM (scanning tunneling microscopy). An apparent consensus for decades has been that Al tends to form surface dimers, which can appear as single sphere in positive bias, in 2 x 2 structure at low temperature and saturates at 0.5 monolayer (3.4 x 10¹⁴ cm^-2) before Al cluster formation (monolayer is in reference to Si(100) site density). However, our recent SIMS (secondary ion mass spectroscopy) analysis shows that the 2D number density of Al on Si (100) is actually 0.25 monolayer (1.7 x 10¹⁴ cm^-2), half of the prior STM density, indicating the possibility of another Al phase on Si (100) surface. In addition, mesa etched hall bar devices fabricated find p-type carriers with consistent densities ≈ 1.5 x 10¹⁴ cm^-2 and mobility of ≈ 20 cm²/V s. Developing a complete phase diagram of the surface doping will enable optimal Al dosing above the expected superconducting critical density of 4 at. % with a minimum number of mixed Al clusters in the doped lattice.

The fabrication of n-type dopant-based devices such as the ‘single atom transistor’ [1] and 2D Quantum Metamaterials[2] uses STM lithography to create patterns of reactive Si dangling bonds with a background of passive H-terminated Si. Phosphine (PH3) is used as a precursor molecule for the P dopant atoms adsorbing extremely selectively onto the bare Si atoms which make up the device pattern. For p-type dopants, the Group III equivalents of phosphine, the alanes, are thought to react on the surface in a similar way [3], but are not stable at room temperature. Diborane has a very low sticking coefficient onto bare Si, forcing the use of very large doses to achieve a good density of B dopants[4]. Here we are exploring alkyl dopant sources, including Trimethyl Al, Triethyl Al and Tri-isobutyl Al, which are widely used in III-V CVD reactors. Recently, we have focused on Triethyl Al, as it has a much higher sticking coefficient than Tri-isobutyl Al and is preferred over Trimethyl Al for lower carbon contamination of the grown films. On Si, ethylene is known to desorb from the clean Si surface at about 400°C, which promises a lower carbon incorporation rate.
We have conducted adsorption and selectivity experiments with Triethyl Al on bare and H-terminated Si(001) surfaces. At room temperature, we find that the sticking coefficient of the molecule is low even on the bare Si(001) surface. There is no apparent adsorption on the H-terminated surface. However, we find that patterns drawn on the H-terminated surface after a TEAl dose will slowly fill with adsorbates over time, indicating the presence on the surface of mobile molecules, which are weakly bound and invisible to the STM. By performing the adsorption at elevated temperatures, we were able to remove these mobile molecules, and at the same time, the reaction of the bare surface to these molecules was greatly increased. After adsorption, the ethyl groups were driven off the surface, and the Al atoms incorporated into the surface, using a high-temperature anneal to around 400°C. This recipe was used to create delta layers of Al, which were then buried using the same process as used to bury P delta layers [1]. SIMS data of these delta layers show good confinement of Al to the delta layer. We continue to refine the adsorption and annealing process to increase the peak concentration of the Al in the delta layer, minimise segregation into the overgrowth, as well as to further reduce the amount of carbon trapped at the delta layer.
Considering the bigger picture, if a suitable process can be found for Triethyl Al, this process is likely to be compatible with other standard CVD dopant precursors, such as TriEthyl Ga, and similar precursors for other elements, such as Zn or Er, which would greatly broaden the applicability of the atomically-precise dopant placement process.

1: M. Fuechsle, J. A. Miwa, S. Mahapatra, H. Ryu, S. Lee, O. Warschkow, L. C. L. Hollenberg, G. Klimeck, and M. Y. Simmons Nat Nano 7 242-246 (2012)
2: https://www.zyvexlabs.com/2d-workshop/workshop-overview/
3: R. Smith and D.R. Bowler, J. Phys. Condens. Matter 30, 105002 (2018).
4: T. Skeren, S. Koster, B. Douhard, C. Fleischmann and A. Fuhrer, Arxiv: 1912.06188 (2019)

In recent years it has become possible to construct near-atomically perfect phosphorus-based structures, 2D materials and quantum devices, embedded in silicon using STM bottom-up fabrication. If this technology can be developed to a sufficient degree of accuracy and reproducibility, it will have very clear applications for quantum computing and quantum information architectures in addition to realizing the promise of scaling electronics to the atomic limit. In the current implementation of the bottom-up fabrication technique, the STM is used to generate regions of clean silicon by selective removal of H atoms from a hydrogen terminated Si (100) surface. This is followed by introduction of phosphine (PH3) gas which chemically bonds only to depassivated sites. Subsequent thermal processing allows for reactions to take place in which P atoms substitutionally replace Si surface atoms so that spacings between electronic components (defined by P) are controlled to within a few lattice sites. Individual components can be scaled down to the point that they are composed of few P atom clusters. However, to date, deterministic control over the number of P atoms incorporated at a site has not been achieved in a reliable manner. Results from previous studies have found that for targeted single-atom sites, there is approximately a 20% chance of failure to incorporate any atoms (i.e. the site is left empty).

In this talk, we detail our progress towards achieving true single atom per site control. We write patterns intended for single P atom incorporation using feedback controlled lithography (FCL) so that adsorption sites are atomically identical. In a subset of patterned sites, we investigate STM-based manipulation of the deposited phosphine species. These results are combined with a density functional theory analysis in which precise adsorption configurations and reaction products can be identified via STM image simulation to gain insights into the chemistry of phosphine adsorption at FCL-tailored sites. We find that for perfectly patterned structures, sub-nanometer precision in placement of individual P atoms is achieved and that there is evidence of the presence of the P atoms after the thermal processing step in which P is incorporated. While technically demanding, these results suggest a path to fabricate devices with absolute certainty in the number of P atoms comprising key components, enabling precision control over their electronic structure and, therefore, quantum properties.

Donor-based quantum materials in silicon are a promising candidate for scalable solid-state quantum computing and analog quantum simulation. Central to the scalability and performance of this quantum materials system is the precision control and physical understanding of materials properties at the atomic scale. Recent advances in atomically precise fabrication in silicon using scanning tunneling microscopy (STM) have demonstrated success in atomic-scale control of tunneling and atom-by-atom construction of single and few-donor quantum devices, enabling donor-based quantum meta-materials and high-fidelity two-qubit gates. In this presentation, we demonstrate the construction of a 3×3 artificial square lattice using precision patterned few-donor quantum dots. We probe the resonant quantum transport and Hubbard properties of the donor-array towards the possibilities of using the donor-based artificial lattice as a Hubbard quantum simulator.

The Hubbard model has been one of the primary models for capturing the essential physics and materials properties in condensed matter and emerging quantum systems. Due to its natural long-range Coulomb interactions, accessible low-temperatures, and atomic-scale nature, an artificial lattice of donor clusters made of one or multiple donor atoms in silicon is advantageous in simulating Hubbard many-body problems, especially in strong interaction regimes. In this presentation, we focus on the atomic-scale fabrication and low-temperature transport characterization of an STM-patterned 3×3 array of few-donor quantum dots. By embedding the dot-array system in the electrostatic environment of the real transport device and solving the extended Hubbard model, we analyze the Coulomb blockade and quantum transport features through the donor array. We estimate the microscopic structure and tunnel coupling of individual quantum dots at each artificial lattice site by measuring single-electron tunneling spectroscopy through single and few-donor quantum dots with atomically defined tunnel gaps. Combining experimental and simulated results, we evaluate the potential impact of disorder and atomic-scale imperfection in the donor array on single-electron transport and the possibility of using quantum transport as a sensitive probe for understanding Hubbard physics in donor-based quantum materials systems.

The technology required to fabricate atomic-scale quantum devices in silicon has advanced significantly over the past decade to produce a variety of dopant-based devices, including qubits, atomic wires, tunnel junctions, and single electron transistors. Nearly all such devices made to date have been phosphorous-based and achieved through the careful control of the reaction PH₃ with the silicon surface. While the PH₃/Si system has been well studied, little progress has been made towards the realization of analogous acceptor-based devices. With the promise of hole-based qubits with extremely long coherence times and strong coupling to electric fields, along with the potential to achieve superconductivity in Si, there are tremendous opportunities to advance Si-based quantum computing upon the identification of an ideal gaseous precursor. Here, we present results on the use of BCl₃ and AlCl₃ as dopant precursors for the formation of acceptor-doped, delta layers in silicon. Surface reactions were studied with scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), and density functional theory (DFT) calculations to elucidate the reaction pathways for B and Al dopants leading towards dopant incorporation into the Si surface. SIMS measurements confirm B and Al concentrations in excess of 10²⁰ cm^-3. These results provide a major step in the further development of atomic-scale quantum devices in Si. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

Silicon carbide (SiC) is a technologically mature semiconductor material that is host to a variety of different solid-state spin defects. These spin qubits not only boast long coherences but also a natural spin-photon interface that can mediate entanglement at a distance. However, as for any solid-state quantum technology, issues of charge noise and magnetic fluctuations are prevalent. In this talk, we present recent advances in minimizing and controlling these sources of noise.

We first discuss our ability to mitigate charge noise in commercial SiC electronic devices [1]. Surprisingly, the integration of single spins into these devices results in THz-scale tunability of a single divacancy spin’s optical interface while simultaneously eliminating spectral diffusion and blinking. This results in an electrically-tunable single photon source that can be tuned over 40,000 linewidths, constituting a near-ideal spin-photon interface at wafers scales near the telecommunications band.

We then describe the control and optimization of the nuclear spin environment for these spin qubits [2]. In general, nuclear spins are both a cause of decoherence and a valuable resource for electron spins in the solid-state. While isotopic purification removes noise, it also reduces the availability of nuclear spin quantum memories that are vital to many quantum technologies. Guided by ab-initio theory, we strike this balance and find an optimal isotopic concentration that maximizes the number of available quantum memories. Experimentally, we demonstrate extended coherences and the first entanglement and control of single nuclear spins in silicon carbide through isotopic engineering.

The elimination of electrical noise combined with finely tuning the nuclear environment results in a material platform for quantum technology that combines a stable spin-photon interface with an electron spin that can leverage entanglement with local nuclear spin qubit clusters. We show that this electron spin state has long coherence times (T₂>2.3 ms, T₂*>370 μs), state-of-the-art control fidelities (>99.984%), and displays readout and initialization contrast exceeding 99%, providing an exciting pathway toward scalable quantum information technologies in the solid-state.

This work was done in collaboration with Mykyta Onizhuk, He Ma, Gary Wolfowicz, Peter Mintun, Alex Crook, Hiroshi Abe, Jawad Ul Hassan, Nguyen Son, Takeshi Ohshima and Giulia Galli.

[1] C. P. Anderson*, A. Bourassa*, K. C. Miao, G. Wolfowicz, P. J. Mintun, A. L. Crook, H. Abe, J. U. Hassan, N. T. Son, T. Oshima, D. D. Awschalom, “Electrical and optical control of single spins integrated in scalable semiconductor devices,” Science 336, 6470, 1225-1230 (2019)

[2] A. Bourassa*, C. P. Anderson*, K. C. Miao, M. Onizhuk, H. Ma, A. Crook, H. Abe, J. Ul-Hassan, T. Ohshima, N. T. Son, G. Galli, D. D. Awschalom, “Entanglement and control of single quantum memories in isotopically engineered silicon carbide,” arXiv 2005.07602 (2020)

The nitrogen vacancy (NV) center in diamond exhibits spin-dependent fluorescence and long spin coherence times under ambient conditions, enabling applications in quantum information processing and sensing. NV centers near the surface can have strong interactions with external materials and spins, enabling new forms of nanoscale spectroscopy. However, NV spin coherence degrades within 100 nanometers of the surface, suggesting that diamond surfaces are plagued with ubiquitous defects. Prior work on characterizing near-surface noise has primarily relied on using NV centers themselves as probes; while this has the advantage of exquisite sensitivity, it provides only indirect information about the origin of the noise. I will describe our recent efforts to use X-ray and photoelectron spectroscopies, diffraction techniques, and morphology characterization to understand sources of noise at the diamond surface. By correlating this spectroscopic data with single spin measurements, we have been able to devise new surface processing and termination techniques to stabilize shallow NV centers within 5 nm of the surface with coherence times exceeding 100 ms [1]. Specifically, we are able to demonstrate reversible and reproducible control over the top layer of atoms. These highly coherent, shallow NV centers will provide a platform for sensing and imaging down to the scale of single atoms.

In fact, many platforms for quantum technologies are limited by noise and loss arising from uncontrolled defects at surfaces and interfaces, including superconducting qubits, trapped ions, and semiconductor quantum dots. Our approach for correlating surface spectroscopy techniques with single qubit measurements to realize directed improvements is generally applicable to many systems, and I will describe our recent efforts to tackle noise and microwave losses in superconducting qubits. Building large, useful quantum systems based on transmon qubits will require significant improvements in qubit relaxation and coherence times, which are orders of magnitude shorter than limits imposed by bulk properties of the constituent materials. This indicates that relaxation likely originates from uncontrolled surfaces, interfaces, and contaminants. Previous efforts to improve qubit lifetimes have focused primarily on designs that minimize contributions from surfaces. However, significant improvements in the lifetime of two-dimensional transmon qubits have remained elusive for several years. We have fabricated two-dimensional transmon qubits that have both lifetimes and coherence times with dynamical decoupling exceeding 0.3 milliseconds by replacing niobium with tantalum in the device [2]. We have observed increased lifetimes for many devices, indicating that these material improvements are robust, paving the way for higher gate fidelities in multi-qubit processors.

References
1. S. Sangtawesin, B. L. Dwyer, S. Srinivasan, J. J. Allred, L. V. H. Rodgers, K. De Greve, A. Stacey, N. Dontschuk, K. M. O'Donnell, D. Hu, D. A. Evans, C. Jaye, D. A. Fischer, M. L. Markham, D. J. Twitchen, H. Park, M. D. Lukin, N. P. de Leon, "Origins of diamond surface noise probed by correlating single spin measurements with surface spectroscopy," Physical Review X 9, 031052 (2019).
2. A. P. M. Place, L. V. H. Rodgers, P. Mundada, B. M. Smitham, M. Fitzpatrick, Z. Leng, A. Premkumar, J. Bryon, S. Sussman, G. Cheng, T. Madhavan, H. K. Babla, B. Jaeck, A. Gyenis, N. Yao, R. J. Cava, N. P. de Leon, A. A. Houck, "New material platform for superconducting transmon qubits with coherence times exceeding 0.3 milliseconds," arXiv:2003.00024.

The central challenge in building quantum computers and long-range quantum networks is distributing entanglement across multiple individually-controllable quantum memories. While proof-of-concept experiments are possible with today’s optical components, scaling these systems to tens, hundreds, or thousands of individually controllable quantum memories requires very-large-scale integration of electronic and photonic circuits.
Diamond color centers (CC) have emerged as leading solid state “artificial atom” qubits, having recently surpassed the entanglement-to-decoherence threshold for deterministic delivery of quantum entanglement[1] as well as the first demonstration of “useful” quantum-memory-enhanced communication beating any direct-transmission optical channel[2]. A central challenge now lies in the development of quantum information processing architectures to improve entanglement rates, reduce decoherence, and to scale systems for applications ranging from quantum repeaters to general-purpose quantum computers.
To answer this challenge, we are developing photonic integrated circuits (PICs) with integrated quantum memories based on diamond CCs. In particular, this talk discusses our recent demonstration of a PIC with 128 waveguide-integrated silicon vacancy (SiV) and germanium vacancy (GeV) color centers[3].
Secondly, this talk will review recent advances on spin and optical properties of recently discovered tin-vacancy (SnV) CCs which have significantly larger ground state orbital splitting than the lighter Group IV-vacancy complexes. We observe that even at modest temperatures near ~ 4K, the SnV possesses the essential attributes for a spin-photon interface: optical transitions approaching the lifetime limit and a long-lived electronic spin state[4].
The talk will finish with an outlook on resource-performance analyses for large-scale quantum repeaters [3,5,6], modular quantum computers[7,8], and coherent interfaces to phonons and microwave photons[9].
[1] P. C. Humphreys, N. Kalb, J. P. J. Morits, R. N. Schouten, R. F. L. Vermeulen, D. J. Twitchen, M. Markham, and R. Hanson, Nature 558, 268 (2018).
[2] M. K. Bhaskar, R. Riedinger, B. Machielse, D. S. Levonian, C. T. Nguyen, E. N. Knall, H. Park, D. Englund, M. Lončar, D. D. Sukachev, and M. D. Lukin, arXiv [quant-Ph] (2019).
[3] N. H. Wan, T.-J. Lu, K. C. Chen, M. P. Walsh, M. E. Trusheim, L. De Santis, E. A. Bersin, I. B. Harris, S. L. Mouradian, I. R. Christen, E. S. Bielejec, and D. Englund, arXiv [quant-Ph] (2019); to appear in Nature (2020)
[4] M. E. Trusheim, B. Pingault, N. H. Wan, M. Gündoğan, L. De Santis, R. Debroux, D. Gangloff, C. Purser, K. C. Chen, M. Walsh, J. J. Rose, J. N. Becker, B. Lienhard, E. Bersin, I. Paradeisanos, G. Wang, D. Lyzwa, A. R.-P. Montblanch, G. Malladi, H. Bakhru, A. C. Ferrari, I. A. Walmsley, M. Atatüre, and D. Englund, Phys. Rev. Lett. 124, 023602 (2020).
[5] Y. Lee, E. Bersin, A. Dahlberg, S. Wehner, and D. Englund, arXiv [quant-Ph] (2020).
[6] K. C. Chen, E. Bersin, and D. Englund, arXiv [quant-Ph] (2020).
[7] H. Choi, D. Zhu, Y. Yoon, and D. Englund, Phys. Rev. Lett. 122, 183602 (2019).
[8] H. Choi, M. Pant, S. Guha, and D. Englund, Npj Quantum Information 5, 104 (2019).
[9] T. Neuman, M. Eichenfield, M. Trusheim, L. Hackett, P. Narang, and D. Englund, arXiv [quant-Ph] (2020).

The optical and electronic properties of defects in solid-state materials can be tuned with external fields, as well as with sculpted electromagnetic environments, such as cavities and waveguides. Here, we add to this toolbox by computationally demonstrating that defects, often described as “artificial atoms,” become “artificial molecules” when placed within a few unit cells of each other. Specifically in monolayer hexagonal boron nitride, we observe dissociation curves and hybridization of defect orbitals into bonding and antibonding orbitals, as well as changes in frequency and intensity of dipole-allowed transitions. We calculate the energetics of cis and trans analogs of paired out-of-plane defects and find that paired in-plane defects interact more strongly than out-of-plane ones. We envision leveraging this chemical degree of freedom to more precisely engineer defects as quantum memories and quantum emitters for quantum information applications.

Single photon emitters (SPEs) in solids have emerged as promising candidates for various applications such as quantum technology, nanophotonics and biological imaging. Silicon-vacancy color centers in diamond exhibit high-brightness, stable photo-stability and room-temperature quantum emission, but the precise localization, controllable formation, and direct observation of SiV centers within nanometer scale still remain elusive. Here, we use grow ultrasmall high-quality nanodiamonds using plasma-enhanced vapor deposition (PECVD) from molecular diamondoids as seedings, and incorporated SiV centers during growth. We directly observe SiV emission from 20 nm nanodiamond particles using a combination of cathodoluminescence (CL) and cryogenic transmission electron diffraction (cryo-TEM). Our results provide the controlled growth and direct observation of stable SiV emission in nanometer-scale CVD grown nanodiamonds. These findings are important for fundamental understanding of color center formation mechanism and stability in nanoscale, and are promising for the realization of precise localization of color centers for various quantum applications.

Defects in diamond are leading solid-state candidates for a range of quantum technologies. Recent research has focused on identifying new color centers with properties that reach beyond the limitations of the well-known nitrogen-vacancy (NV^-) center. Group IV-vacancy centers have been a primary focus due to their symmetry-protected optical transitions and long-lived spin degree of freedom. A detailed understanding of the electronic structure of these emitters is paramount to their use in spin-photon systems. Here we detail the ground- and excited-state properties of the neutral group IV color centers using first principles methods. In particular, we capture the product Jahn-Teller (pJT) effect [1] resulting from the simultaneous Jahn-Teller interactions in two defect orbitals. The significant pJT effects found suggest the excited state is a coupled electron-phonon (vibronic) system, which can have important implications on electronic observables. In particular, we present a nonperturbative approach to capturing the effective spin-orbit splitting [2], which we find to be significantly quenched as a result of this pJT interaction. The lowest optically-active ³E_u states are weakly spin-split into ms-resolved states. Our spin-resolved level structure predictions will be useful in understanding the finer details of the SiV⁰ as well as provide additional fingerprints for experimental identification of the other group IV neutral emitters.

[1] Harris, I., Ciccarino, C.J., Flick, J., Englund, D.R., and Narang, P., arXiv:1907.12548 (2019).
[2] Ciccarino, C.J., Flick, J., Harris, I., Trusheim, M.E., Englund, D.R. and Narang, P, arXiv:2001.07743 (2020).

Hexagonal boron nitride has been found to host single-photon emitters across a range of energies. Among the observed emitters is a group in the visible spectrum, known as the 2 eV emitters, that are notoriously heterogeneous. Extensive experimental efforts have uncovered a wealth of interesting properties; most notably, the 2 eV emitters are highly desirable for quantum information applications, having minimal coupling to phonons and spin-dependent transitions. Despite these efforts, the microscopic origin of the emitters is the subject of intense debate. We propose that boron dangling bonds are the source of the observed emission. We apply first-principles calculations, based on density functional theory with a hybrid functional, to characterize the properties of these dangling bonds and demonstrate that they provide a consistent understanding of the experimental observations. In the negative charge state, the boron dangling bond is occupied with two electrons. Excitation of one electron into a localized p_z state results in optical emission with a zero-phonon line of 2.06 eV. The transition is linearly polarized, and the coupling to phonons is characterized by a Huang-Rhys factor of 2.3. Proximity of the states to the conduction band allows for an indirect excitation mechanism for sufficiently large energies. This unique feature explains the misalignment of the absorptive and emissive dipole observed in experiment. While the ground state is a singlet state, we predict the existence of a metastable triplet state, which allows for spin-dependent transitions. All of these features are in line with experimental observations, indicating that boron dangling bonds are the likely origin of the observed emission.

Quantum computing has made tremendous progress in recent years, including both experimental advances and hardware developments. Quantum processors have scaled significantly in size, as measured by the number of quantum bits (qubits) connected on a chip, with devices incorporating 10's of qubits available today. An example of a publicly available quantum system is the IBM Q Experience (www.research.ibm.com/ibm-q). While the goal of fault-tolerant universal quantum computing is still some time away, early applications and demonstrations can be implemented on smaller scale near-term quantum systems. Among the different hardware implementations, Josephson-junction-based superconducting quantum circuits are a promising technology to scale quantum processors. Together with increasing the number of qubits, other metrics need to evolve and improve as well, such as qubit quality, hardware integration, and system level performance. In this talk, I will review the current status and discuss some of the remaining challenges.

Two level system (TLS) loss in dielectric materials and interfaces remains at the forefront of materials research in quantum information science. The identification of low loss fabrication techniques, materials, and thin film dielectrics is critical to achieving scalable architectures for superconducting quantum computing. Superconducting microwave resonators provide a convenient qubit proxy for assessing loss performance and studying loss mechanisms such as TLS loss, non-equilibrium quasiparticles, and magnetic flux vortices. In this talk, an overview of design considerations for accurate resonator loss measurements will be given, summarizing techniques that have been evolving for over two decades, and will conclude with recommendations for future measurements in this field.

In this talk, we present the demonstration of a superconducting transmon qubit realized using a graphene-based weak-link junction. The graphene is encapsulated by hexagonal Boron Nitride (hBN), forming a van der Waals heterostructure. Applying a voltage to a backgate in proximity to the weak-link junction enables voltage tunability of the qubit frequency. We present the coherent control of this qubit, and discuss the promise and the challenges if building superconducting qubits with such van der Waals heterostructures.

Several niobium titanium nitride ternary superconducting alloys were grown to assess the superconducting properties of an alloy which can be tuned to grow pseudomorphically with an insulating layer. The goal is to engineer a superconductor that may decrease charge noise and has improved critical current across the interface of a Josephson junction. This alloy can be paired with wide-bandgap nitrides to create an oxygen-free heterostructure. Nb_xTi_1-xN thin films were grown using plasma assisted molecular beam epitaxy on C-plane sapphire substrates, across the compositional range 0≤x≤1. Sample topography is verified using reflection high energy electron diffraction and ex-situ atomic force microscopy. The surface exhibits a river-rock-type topography across the compositional range with RMS roughness values under one nanometer. The structure is analyzed using x-ray diffraction and Rutherford backscattering spectroscopy, which shows the change in composition causes a corresponding change in lattice constants. All films are completely relaxed, and consistent with an abrupt metamorphic growth mode. Variations in the alloy phase and corresponding superconducting properties will be discussed.

The non-linear building blocks of superconducting qubits are Josephson Junctions (JJs). The current state-of-the-art is qubits fabricated with JJs that are made from Al/AlOx/Al using the double-angle evaporation and oxidation method. The need to fabricate higher quality, lower loss, and more uniform superconducting devices motivates this work to focus on making epitaxial Josephson junction heterostructures using transition metal nitride superconductors and a wide-bandgap nitride dielectric on a low-loss substrate.

Aluminum nitride and Titanium nitride are two foundation materials in the construction of an all nitride JJ. We use plasma assisted molecular beam epitaxy (PAMBE) with varying metal-nitrogen growth flux ratios from nitrogen rich to metal rich growth conditions. The optimization of both epitaxial materials is guided by characterization of the bulk by x-ray diffraction (XRD), chemical profile using energy dispersive x-ray spectroscopy (EDX), ‘near’ surface structure using electron backscatter diffraction (EBSD) and surface topography via atomic force microscopy (AFM).

Superconducting TiN epitaxial growth parameters and their correlation to structure are reported along with temperature dependent resistivity and critical temperature. Superconducting microwave loss is a key parameter for quantum information applications, and will be explored for all films and correlated to structure using coplanar waveguide resonators.

Due to the significantly longer superconducting coherence length along the a-axis of YBa₂Cu₃O_7−δ (YBCO) than along the c-axis, the growth of a-axis YBCO films is important for Josephson junctions, provided they can be made sufficiently smooth.[1] The necessity for smoothness is so Josephson junctions, regardless of their position on the wafer, have the same thickness of the tunnel barrier.[2,3] With the goal of making smoother a-axis YBCO films, we utilize ozone-assisted molecular beam epitaxy (MBE) to grow a-axis YBCO films and perform comprehensive structural, morphological, and electrical characterization. The a-axis films are grown on (100) LaAlO₃ substrates. The structural quality and a-axis content of the samples is examined using four-circle x-ray diffraction (XRD). The morphology of the film surfaces is assessed by atomic force microscopy (AFM). Leveraging a new temperature-ramping model, we achieve high structural quality and improved surface roughness of a-axis YBCO films. In-situ reflection high-energy electron diffraction (RHEED) substantiates the gradually improved crystallinity during the temperature ramping. Ex-situ XRD measurements prove the films to have >99% a-axis content, while AFM measurements reveal improved surface quality with a root-mean-square (rms) roughness of < 2 nm. Resistivity vs. temperature measurements exhibit the onset of the superconducting transition at T_c ~ 87 K.

References
[1] J. B. Barner, C. T. Rogers, A. Inam, R. Ramesh, S. Bersey, Appl. Phys. Lett. 1991, 59, 742.
[2] Z. Trajanovic, I. Takeuchi, P. A. Warburton, C. J. Lobb, T. Venkatesan, Appl. Phys. Lett. 1995, 66, 1536.
[3] I. Takeuchi, P. A. Warburton, Z. Trajanovic, C. J. Lobb, Z. W. Dong, T. Venkatesan, M. A. Bari, W. E. Booij, E. J. Tarte, M. G. Blamire, Appl. Phys. Lett. 1996, 69, 112.

A metallic bilayer system can behave like two materials when thick, but in the thin film limit may exhibit properties that are a hybrid of the individual layers or dominated by one. As an example, the Nb/Al bilayer, in conjunction with AlOx, is a widely utilized material stack for quantum computing, where in the thin film limit (<100 nm) the Al superconducting properties are strongly perturbed by the proximity of the Nb. Each metal individually has liabilities: when used separately in tunnel junction devices, Al’s lower Tc requires lower operation temperatures but Nb oxides introduce a range of problematic features including magnetic scattering and dissipation. By choosing an appropriate Al thin film thickness on Nb, it may be possible to mitigate both to get 1) the higher Nb Tc and 2) better AlOx tunnel barrier. We have fabricated a series of Nb/Al/AlOx/normal metal tunnel junctions while varying the Al thin film thickness in order to investigate the tunnel junction properties with changing Al thickness. The normalized sub-gap conductance in tunneling spectra is found to strongly vary as a function of Al thickness, with sub-gap conductance exceeding normal state conductance in some cases, which is typically attributed to Andreev reflection. Tunnel junction spectra are well modeled within the Blonder-Tinkham-Klapwijk (BTK) theory and we discuss trends between BTK barrier strength Z and the apparent superconducting gap as the Al thin film thickness varies. Additionally, we will present temperature dependent tunneling spectra measured between 4 K and , and discuss the analysis of the data within the BTK theory.

Nanowires are filamentary crystals with a tailored diameter ranging from few to ~100 nm. The reduced dimensions and longitudinal morophology of these nanowires results in interesting optical and electrical properties and provides a great potential for many applications, including sensing, quantum computing and all those that involve photonics [1].
In all these potential applications, the deterministic positioning of nanowires and associated branches on a substrate is key.
In this talk we will present our strategy to obtain III-V nanowire networks in an organized fashion on a device substrate. We will consider different shapes of the nanostructure connections as well as strate-gies to increase the carrier mobility in views of ballistic 1D transport. We will address the growth and the optical and electronic properties as well as perspectives for quantum technology applications [2-4].



References:

[1] L. Güniat, P. Caroff, A. Fontcuberta i Morral, Chem. Revs. 119, 8958 (2019)
[2] G. Tütüncüoglu et al, Nanoscale 7, 19453 (2015)
[3] M. Friedl et al Nano Lett. 18, 2666 (2018)
[4] M. Fried. et al Nano Lett. 20, 3577 (2020)

In my talk, I will discuss low-dimensional condensed matter systems, in which topological properties could be engineered per demand. Majorana fermions can emerge in hybrid systems with proximity-induced superconducting pairing. I will present our numerical and analytical studies of such geometries with proximity effects [1-3]. In the second part of the talk, I will discuss an analytical model of a Rashba nanowire that is partially covered by and coupled to a thin superconducting layer, where the uncovered region of the nanowire forms a quantum dot [4,5]. Even if there is no topological superconducting phase possible, there is a trivial Andreev bound state that becomes pinned exponentially close to zero energy as a function of magnetic field strength when the length of the quantum dot is tuned with respect to its spin-orbit length such that a resonance condition of Fabry-Perot type is satisfied. In this case, the Andreev bound state remains pinned near zero energy for Zeeman energies that exceed the characteristic spacing between Andreev bound state levels but that are smaller than the spin-orbit energy of the quantum dot. Importantly, as the pinning of the Andreev bound state depends only on properties of the quantum dot, this behavior is unrelated to topological superconductivity.

References
[1] C. Reeg, D. Loss, and J. Klinovaja, Phys. Rev. B 97, 165425 (2018).
[2] C. Reeg, D. Loss, and J. Klinovaja, Phys. Rev. B 96, 125426 (2017).
[3] C. Reeg, J. Klinovaja, and D. Loss, Phys. Rev. B 96, 081301 (2017).
[4] C. Reeg, O. Dmytruk, D. Chevallier, D. Loss, and J. Klinovaja, Phys. Rev. B 98, 245407 (2018).
[5] O. Dmytruk, D. Chevallier, D. Loss, and J. Klinovaja, Phys. Rev. B 98, 165403 (2018).

Majorana modes are one of the most promising candidates for the creation of topological qubits in the long term. While evidence for Majorana modes has been obtained for Majorana modes in several systems such as semiconductor nanowires, semiconductor 2DEGs, iron-based superconductors, topological insulators as well as iron atom chains to name a few, evidence for a qubit degree of freedom is elusive. I will begin by reviewing what it means experimentally to observe a qubit. Following this we will discuss the central device challenge in this regard i.e. quasiparticle poisoning. I will then describe potential material dependence of this phenomena as well as how quasiparticle poisoning can manifest in various experiments related to topological superconductivity. Specifically, we will discuss how the observation of quantized conductance into a zero-bias conductance peak associated with a Majorana over a range of tunneling provides some bounds on quasiparticle poisoning times. The measurement of Coulomb blockade transport, which has been observed only for a limited number of superconductors can provide more precise information on such bounds.

With Quantum Supremacy upon us, and with major tech companies investing in quantum computing, one may wonder how much room is left there for academic research? The answer is: more than ever! Our existing qubits are great, but it is difficult to see them carrying us beyond the modern era of noisy intermediate-scale quantum processors, and ushering in the truly scalable, fault tolerant and commercially viable computers. Most likely, a qubit and its couplings, gates will once more need to be re-thought from the ground up - from the atomic first-principles level to the device architecture level. At the same time, the progress we made in quantum control and engineering is so huge that these methods have themselves become mature tools of science. They can be applied beyond quantum computing - to understand the basic questions of quantum dynamics and to probe exotic matter.

One solution may come from studying topological superconductivity. This is an interesting state most notable for spurring Majorana zero modes. Those modes can be both investigated as some of the first non-Abelian excitations, and they can be used in quantum circuits for storing and protecting quantum information. Majorana research has posed many basic materials questions of how to engineer the precise structures that possess intrinsic fault tolerance through topological protection. In my work I have developed new materials and devices for unambiguous observation of robust Majorana modes. I have worked on establishing clear criteria (signatures) of Majorana modes and how to tell them apart from other excitations that may be present in the same devices. It is exciting to apply modern quantum probes, developed based on transmon and spin qubits, to these systems in search for new exotic quantum matter. And perhaps solving some of the big challenges facing quantum computing at the same time.

Superconductor–semiconductor heterostructures represent a versatile platform for realizing Majorana zero-energy modes. In this talk, I will explain how to engineer topological superconductivity at the interface of a conventional (s-wave) superconductor and a semiconductor with spin-orbit interaction. I will discuss state-of-the-art numerical approaches for modeling realistic devices which take into account proximity-induced superconductivity, orbital and Zeeman effect of an applied magnetic field, spin-orbit coupling as well as the electrostatic environment on equal footing. Finally, I will review recent materials science progress in growing superconductor–semiconductor heterostructures and discuss promising new directions.

Nanoscale topological materials can enhance the contributions of the topologically protected surface states to transport signals by increasing their surface to volume ratios. For potential applications in quantum computing, and low-power electronics that exploit the topological properties, fundamental studies of electronic phases of nanoscale topological materials are critical. Recently, molybdenum phosphide (MoP) was discovered as a triple-point topological semi-metal with excellent transport properties of bulk resistivity of 8.2 μΩ*cm and carrier density of 1.1 × 10²³ cm^-3 at room temperature (Kumar et al., Nat.comm., 2019). Due to the topological protection predicted to suppress electron scattering at the surface, MoP may preserve its low resistivity values at the nanoscale, which may rival the line resistance of current Cu interconnects.
Here, we report the synthesis and transport properties of MoP nanostructures, which were obtained by the direct conversion of MoO₃ nanostructures. We observe that the initial size of the MoO₃ nanostructure critically determines the crystalline quality of the converted MoP nanostructures. Large MoO₃ flakes lead to porous MoP flakes, while narrow MoO₃ nanowires lead to MoP nanowires without pores. The size-dependent porosity observed in the MoP nanostructures is attributed to volume changes during the conversion reaction and nanoscale confinement effects. Notably, MoO₃ nanowires with diameters less than 10 nm were converted to single-crystalline MoP nanowires. Because of their porous nature, the resistivity values of the MoP nanoflake were higher than the reported values of MoP bulk crystals. However, despite the high porosity, the residual resistance ratio was ~ 2, and the temperature-dependent resistivity curves did not show any strong surface or grain-boundary scattering. The simple synthesis strategy opens opportunities to study the nature of topological surface states in MoP, which could be sensitive to the overall morphology and dimensions of the nanostructure, and to explore their feasibility as nanoscale interconnects.

III-V semiconductor nanowires (NWs) have been the building blocks of many different research areas including photonics, solar cells, energy storage, and recently, quantum computing.^1,2 In particular, the fast-growing field of topological quantum computing has brought great attention to In-based NWs, thanks to the core properties of these materials including high electron mobility, large Landé g-factor, and high spin-orbit coupling. The most common growth technique for these NWs results in the formation of free-standing, vertical structures which require transfer onto a host substrate for further characterization, and this hinders their scalability for industrial applications. To overcome this limitation and to achieve high-quality NW junctions, selected area epitaxy (SAE) of horizontal NW networks has been proposed as a more scalable approach. Recently, SAE of InAs NWs has been demonstrated on GaAs substrates.^3,4 To decrease the number of detrimental defects, nanomembranes or buffer layers have been used as templates. Thanks to the defect-free nature of these GaAs nanomembranes, NWs with high crystal-quality have been obtained. However, the attempt to grow pure InAs NWs on (111)B GaAs membranes resulted in the formation of intermixed InGaAs NWs with low In content (~20%).⁴ Even though these NWs were shown to have quasi-1D transport, their electrical and transport properties were insufficient for quantum applications, leaving much room for improvement.
Herein, we present remotely doped InGaAs NW branches to enhance the properties of these structures.⁵ Remote doping, or so-called modulation doping, has been commonly used in different material platforms to enhance mobility by suppressing ionized impurity scattering. In our growths, silicon dopants are introduced close to the interface during the membrane growth stage. The location of these dopants was later confirmed by atom-probe tomography. By using this information in our finite element analysis, we demonstrate that the electrons are localized in the wire segment thanks to the band alignment of GaAs and InGaAs. Moreover, by investigating the InAs growth window while changing In and As flux, we achieved In concentrations of up to 50% in our NWs. Finally, transport and electrical properties of these Y-junctions are characterized under magnetic field at low temperatures. Together with reduced donor scattering, the increase in the In content results in enhanced mobility and large electron mean free paths. Additionally, these NWs exhibit weak anti-localization, which is the hallmark of strong spin-orbit interaction. All these improvements make these NWs promising for future applications in topological quantum computing.


References
¹E. Garnett et al, Chem. Rev. 119 (2019)
²H.Zhang et al, Nat. Commun. 10 (2019)
³F. Krizek et al, Phys. Rev. Mat. 2 (2018)
⁴M. Friedl et al, Nano Lett. 18 (2018)
⁵M. Friedl et al, Nano Lett. 20 (2020)

Acknowledgements: Funding from Swiss National Science Foundation, NCCR QSIT

Two dimensional magnetic topological materials, such as magnetic topological insulators and magnetic Weyl semimetals, represent a novel platform exhibiting the quantum anomalous Hall effect (QAHE). To realize the quantum anomalous Hall state, the material system needs to break time reversal symmetry while maintaining a nontrivial 2D bulk gap protected by a nonzero Chern number. One pathway is to realize hybrid-interfaces between different quantum materials to achieve this synergy. The scientific objective is the rational design of quantum materials and control over their properties by exploiting low dimensionality at heterointerfaces.

In this talk, I will focus on our recent works[1,2,3], employing a combination of density functional
Theory (DFT) and k.p model Hamiltonian calculations, to design synthesizable hybrid-interfaces where QAHE can potentially be realized. By systematically investigating such magnetically doped topological insulating materials, we reveal that interfaces between Cr-doped Sb₂Te₃ and the anitferromagnetic insulator Cr₂O₃ are ideal for realizing functional QAHE since the system consists of a large bulk insulating gap with the Fermi level pinned inside a bulk gap due to a type-I band offset, while the dopants at the interface of the TI layer exhibit robust ferromagnetism. We also screened viable vdW-bonded hybrid-interfaces, and discovered that exploiting the unique ‘buried’ topology in CrI₃ is a good strategy, which can be achieve by interfacing it with elemental layers of antimonene, so as to induce a semimetallic transition in CrI₃ through a charge transfer process. Progress in our ongoing effort in simulating topological heterogeneities, such as step-edges, at interfaces of intrinsic magnetic topological materials, such as MnBi₂Te₄, using real-space DFT-methods that allow few-thousand electrons, will also be presented.
[1] “Realizing gapped surface states in magnetic topological insulator MnBi_2−xSb_xTe₄”, Wonhee Ko et. al., arXiv:2003.00180 (2020)
[2] “Designing Magnetic Topological van der Waals Heterostructure”, Anh Pham and P. Ganesh,
arXiv:2003.05840 (2020)
[3] “Quantum material topology via defect engineering”, Anh Pham and P. Ganesh, Phys. Rev. B 100, 241110(R) (2020)

The emergence of quantum information systems necessitates the development of the tools and workflows capable of probing quantum functionalities on the atomic scale and connecting them to the local structures. Recent advances in Scanning Transmission Electron Microscopy have allowed the direct visualization of materials structures with picometer-scale precision, as well as collection of high-veracity information on local electronic, plasmonic, and vibrational properties via Electron Energy Loss Spectroscopy (EELS). However, the interpretation of these data sets even on the exploratory level of identification of the repeatable units, site symmetries, and outliers remains largely driven by human operator and limited to a priori defined models. Notably, the nature of the STEM data sets, as affected by observational biases and limited to small number of materials, limits the applicability of the classical correlation-based machine learning methods. However, the analysis of such data can be performed assuming the parsimony of structural descriptors and generative mechanisms and adopting the Bayesian analysis frameworks. In this presentation, I will illustrate several applications of the machine learning based workflows based on rotationally invariant and conditional variational autoencoders for the elucidation of the local structures and structure-breaking distortions in 2D materials and ferroelectric oxides. The Bayesian nature of these methods can also be used to extend these to the analysis of the causal relationships between the physical and chemical degrees of freedom. Ultimately, we seek to answer the questions such as whether electronic instability due to the average Fermi level guides the development of the local atomic structure, or frozen atomic disorder drives the emergence of the local structural distortions, whether the nucleation spot of phase transition can be predicted based on observations before the transition, and what is the driving forces controlling the emergence of unique functionalities of morphotropic materials and ferroelectric relaxors. The unique aspect of Bayesian methods is their potential to quantify uncertainty, and harnessing this for automated experimentation is discussed on example of ferroelectric domain patterning and atomic fabrication via electron beams.

We discuss computational strategies to predict materials with desired characteristics for quantum sensing and quantum computations, and the impact that quantum computers may have in revolutionizing materials simulations [1] and hence materials design and innovation.

[1] Quantum simulations of materials on near-term quantum computers, He Ma, Marco Govoni, Giulia Galli, npj, Comp. Mat. 2020 (in press)

As a pathway to quantum computing, spins in multiferroic materials offer novel routes of manipulation due to their coupled electrical and magnetic properties. In order to control individual spins as in defect-based qubits, we explore ferroelectric materials with magnetic dopants. Using Fe³⁺-doped PbTiO₃ as a model system, we present a combined first-principles and electron paramagnetic resonance study of the spin-orbit coupling effect in dilute magnetic ferroelectrics. We show that the spins in the Fe³⁺ ion are strongly aligned perpendicular to the ferroelectric polar axis and follow manipulation by an electric field. First principles modelling indicates the atomic scale effects of lattice modification on qubit switching parameters by way of strain and defect complexes, in excellent agreement with experimental observations. These results demonstrate the manipulation of spins by electric field and the possibility to tailor spin control by the atomic environment.

The rutile crystal structure is a classic crystal structure type that is an important platform for potential device application, including solar cells (TiO₂) and ultrafast optical switches (VO₂). Open d-shell systems such as VO₂ usually undergo a structural distortion that is strongly coupled to changes in the electronic properties. Using single crystal total x-ray scattering and the 3D-ΔPDF method, we discovered a new manifestation of structural order in rutile characterized by a loss of displacement coherence in one dimension but not the other two, resulting in the formation of nanoscopically ordered domains of size ~50x50x5 Å. The loss of dimensionality is attributed to geometric frustration of strongly interacting atomic displacements, since the long- and short-range axes are crystallographically equivalent.

Tungsten ditelluride (WTe2) is a layered semimetal with many unique features. It is a Type-II Weyl semimetal candidate(1), exhibits large magnetoresistance(2), and superconductivity under strain(3). To gain insight into the electron-electron and electron-phonon interactions in WTe2, we study the spatial structure of current flowing through a WTe2 flake. Significant electron-electron scattering can lead to hydrodynamic effects in electron flow(4), which have been imaged in graphene(5,6). We adapt these techniques to study the nature of interactions in WTe2.

We study the current profile in WTe2 by imaging the local magnetic field above it using a nitrogen-vacancy (NV) center in a diamond scanning tip. Using coherent quantum sensing, we obtain magnetic field resolution of ~10nT and spatial resolution of ~100nm. The current pattern we observe differs substantially from the flat profile of a normal metal, and indicates correlated flow through the semimetal. The pattern also shows significant temperature dependence, with hydrodynamic effects increasing as the system is cooled and peaking at ~20 K. At lower temperatures the current profile becomes flatter, likely due to a competition between hydrodynamic and ballistic effects.

We compare our results to an ab initio calculation of electron scattering mechanisms, which are used to extract a current profile using the electronic Boltzman transport equation. These calculations show quantitative agreement with our measurement, capturing the non-monotonic temperature dependence. This comparison allows us to quantitatively extract the strength of electron-electron interactions in our material. Furthermore, this strong interaction cannot be explained by a direct Coulomb effect, and is mostly phonon-mediated electron-electron interaction. This surprising result opens a path for hydrodynamic flow and strong interactions even in high-density materials with significant screening of Coulomb interactions.

1 Soluyanov, A. et al. Nature 527 (2015)
2 Ali, M. et al. Nature 514 (2014)
3 Kang, D. et al. Nat. Comm. 6 (2015)
4 Levitov, L. and Falkovich. G., Nat. Phys 12 (2016)
5 Sulpizio, J. et al. Nature 576(2019)
6 Ku, M. et al. arXiv:1905.10791 (2019)

Harnessing rare-earth (RE) ions outstanding optical coherent properties for quantum information technologies has attracted a lot of attention recently, in the race for exploiting emerging solid-state quantum-grade systems. Indeed, REs offer a wide tunability of their ultra-narrow optical transitions, including the useful telecom-band wavelength. While macroscopic bulk oxide crystals (such as Y₂SiO₅) are usually the preferred host material for RE ions, the development of a silicon-compatible thin film platform would greatly facilitate post-processing, up-scalability as well as interfacing with other systems in a hybrid design. In this work, we focus on the synthesis of nanoscale Eu-doped Y₂O₃ thin films on silicon wafers using a modified version of Chemical Vapour Deposition (CVD) based on direct liquid injection (DLI-CVD) of the precursors. This approach improves the control of the composition and purity of the films by alleviating the issue of low volatility of heavy RE elements. We successfully obtained films in the full Eu composition range, with a strong-(111) texture that crystallized in the bixbyite cubic phase. We probed the ⁵D₀ - ⁷F₂ transition lifetime following resonant excitation of the ⁷F₀ - ⁵D₂ level and found values of the order of 1 ms for the lowest Eu concentrations. Inhomogeneous linewidths as narrow as 50 GHz were also measured. Finally, using spectral hole burning (SHB) spectroscopy we successfully burned a spectral hole in a 200 nm-thin film with a 2 % Eu doping with an estimated homogeneous linewidth of 11 MHz. These values are still at least an order of magnitude higher than those reported for bulk single crystals indicating that additional decoherence mechanisms exist in our nanometric films. Further improvements of the crystalline quality and purity of the films is thus desired so that the full potential of such CVD-grown thin films is unleashed for integrated quantum information processing devices.

In the present work, we show that the in situ growth of highly oriented cadmium selenide quantum dots can be explored to alter and tune the emission intensity of the particles. As such, the impact of particle size and distribution on the band emission is studied. Monodispersed cadmium selenide quantum dots (CdSe QDs) were prepared through non-coordinating solvent route, using cadmium oxide (CdO) and selenium (Se) as precursor materials and oleic acid as a capping molecule. The as-synthesized particles were found to be highly stable, with narrow size distribution and good homogeneity. No precipitate was observed for several months. X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements confirm the formation of (Wurtzite structure) phase-pure CdSe QDs with a diameter tunable between 4 and 6 nm. The particle size is controlled by varying the precursor concentration, reaction time and temperature, without the need for post-synthesis heat treatment techniques. This work was performed without the use of pyrophoric precursors and the reported method represents a basic concept for the preparation of small nanoparticles in general [1]. The smaller sized quantum dots exhibit a strong band emission and are more easily tuned because of their larger surface-area-to-volume ratio [2]. Results demonstrate that the emission wavelength of CdSe QDs vary as a function of particle size, exhibiting high photoluminescence quantum yield (QY) (≈ 62%.) This finding reveals the fluorescence property of CdSe QDs that offers new opportunities for future applications of quantum sources as emerging light emitting materials [3].

References:

[1] Pu, Y., Cai, F., Wang, D., Wang, J.X. and Chen, J.F., 2018. Colloidal synthesis of semiconductor quantum dots toward large-scale production: a review. Industrial & Engineering Chemistry Research, 57(6), pp.1790-1802.

[2] Shang, Y. and Ning, Z., 2017. Colloidal quantum-dots surface and device structure engineering for high-performance light-emitting diodes. National Science Review, 4(2), pp.170-183.

[3] Jiang, Y., Cho, S.Y. and Shim, M., 2018. Light-emitting diodes of colloidal quantum dots and nanorod heterostructures for future emissive displays. Journal of Materials Chemistry C, 6(11), pp.2618-2634.

Electronic and spin states in single coordination complexes containing a paramagnetic center are a promising material platform for use in Quantum Information Science (QIS).^1,2 Such molecules can display a high degree of molecular anisotropy that can be tuned synthetically either by the choice of the metal ion center or through the ligand field of the complex, making them attractive as potential qubits. However, metal-containing complexes have recently been observed to fragment on gold surfaces, impeding the use of these molecules for QIS applications.³ Here, we demonstrate the in situ assembly of coordination complexes between the tip and substrate of our scanning tunneling microscope break junction (STMBJ) setup. Our measurements and density functional theory (DFT) calculations point to the formation of an open-shell structure where a single gold atom coordinates to the nitrogen atoms of imidazolate ligands; this structure bridges the metal electrodes with at least ~1.5 eV of binding energy per bond. We measure a reproducible conductance signature of this coordination complex, which suggests that a selective binding mechanism results in repeatable junction geometries.⁴ This in situ formation of stable spin ½ single molecule junctions with a robust electronic signature at room temperature points to potential applications of these nanoscale materials in QIS.
(1) Wasielewski, M. R.; Forbes, M. D. E.; Frank, N. L.; Kowalski, K.; Scholes, G. D.; Yuen-Zhou, J.; Baldo, M. A.; Freedman, D. E.; Goldsmith, R. H.; Goodson, T.; et al. Exploiting Chemistry and Molecular Systems for Quantum Information Science. Nat. Rev. Chem. 2020, 1–15. https://doi.org/10.1038/s41570-020-0200-5.
(2) Atzori, M.; Sessoli, R. The Second Quantum Revolution: Role and Challenges of Molecular Chemistry. Journal of the American Chemical Society. American Chemical Society July 24, 2019, pp 11339–11352. https://doi.org/10.1021/jacs.9b00984.
(3) Knaak, T.; González, C.; Dappe, Y. J.; Harzmann, G. D.; Brandl, T.; Mayor, M.; Berndt, R.; Gruber, M. Fragmentation and Distortion of Terpyridine-Based Spin-Crossover Complexes on Au(111). J. Phys. Chem. C 2019, 123 (7), 4178–4185. https://doi.org/10.1021/acs.jpcc.8b11242.
(4) Pan, X.; Lawson, B.; Rustad, A. M.; Kamenetska, M. PH-Activated Single Molecule Conductance and Binding Mechanism of Imidazole on Gold. Nano Lett. 2020, 20 (6), 4687–4692. https://doi.org/10.1021/acs.nanolett.0c01710.

Quantum computation has captivated the minds of many for almost two decades. For much of that time, it was seen mostly as an extremely interesting scientific problem. In the last few years, we have entered a new phase as the belief has grown that a large-scale quantum computer can actually be built. Quantum bits encoded in the spin state of individual electrons in silicon quantum dot arrays, have emerged as a highly promising direction [1]. In this talk, I will present our vision of a large-scale spin-based quantum processor, and ongoing work to realize this vision.

First, we created local registers of spin qubits with sufficient control that we can program arbitrary sequences of operations. We show the creation of each of the Bell states with fidelities of 85-90% and the implementation of the four instances of the Deutsch-Jozsa and the Grover algorithms on two qubits [2].

Second, we have explored coherent coupling of spin qubits at a distance via two routes. In the first approach, the electron spins remain in place and our coupled via an intermediary degree of freedom. After showing this principle with an ancillary quantum dot as a coupler [3], we recently observed strong coupling of a single spin to a single microwave photon in a superconducting resonator [4]. In the second approach, spins are shuttled along a quantum dot array, preserving both the spin projection [5] and spin phase [6].

Third, in close collaboration with Intel, we have fabricated and measured quantum dots using all-optical lithography on 300 mm wafer, using industry-standard processing [7]. Qubit measurements are now underway. We expect that this industrial approach to nanofabrication will be critical for achieving the extremely high yield necessary for devices containing hundreds or thousands of qubits. Furthermore, we anticipate that the device stability and charge noise levels will eventually outperform those of devices made in university-style cleanrooms.

When combined, the progress along these various fronts can lead the way to scalable networks of high-fidelity spin qubit registers for computation and simulation.

[1] L.M.K. Vandersypen, H. Bluhm, J.S. Clarke, A.S. Dsurak, R. Ishihara, A. Morello, D.J. Reilly, L.R. Schreiber, M. Veldhorst, Interfacing spin qubits in quantum dots and donors – hot, dense and coherent, npj Quantum Information 3, 34 (2017).
[2] T. F. Watson, S. G. J. Philips, E. Kawakami, D. R. Ward, P. Scarlino, M. Veldhorst, D. E. Savage, M. G. Lagally, Mark Friesen, S. N. Coppersmith, M. A. Eriksson, L. M. K. Van- dersypen, A programmable two-qubit quantum processor in silicon, Nature 555, 633-637 (2018)
[3] T.A. Baart, T. Fujita, C. Reichl, W. Wegscheider, L.M.K. Vandersypen, Coherent spin-exchange via a quantum mediator, Nature Nanotechnology, 12, 26 (2017)
[4] N. Samkharadze, G. Zheng, N. Kalhor, D. Brousse, A. Sammak, U. C. Mendes, A. Blais, G. Scappucci, L. M. K. Vandersypen, Strong spin-photon coupling in silicon, Science 359, 1123-1127 (2018)
[5] T. A. Baart, M. Shafiei, T. Fujita, C. Reichl, W. Wegscheider, L. M. K. Vandersypen, Single-Spin CCD, Nature Nanotechnology 11, 330 (2016)
[6] T. Fujita, T. A. Baart, C. Reichl, W. Wegscheider, L. M. K. Vandersypen, Coherent shuttle of electron-spin states, npj Quantum Information 3, 22 (2017)
[7] R. Pillarisetty, H.C. George, T.F. Watson, L. Lampert, N. Thomas, S. Bojarski, P. Amin, R. Caudillo, E. Henry, N. Kashani, P. Keys, R. Kotlyar, F. Luthi, D. Michalak, K. Millard, J. Roberts, J. Torres, O. Zietz, T. Krähenmann, A.-M. Zwerver, M. Veldhorst, G. Scappucci, L.M.K. Vandersypen, and J.S. Clarke, 2019 IEEE IEDM San Francisco, pp. 31.5.1-31.5.4.

Quantum information and computing are at the forefront of computer science, but their implementation relies on the identification and development of a suitable materials system. One such candidate is Si/SiGe. Much progress has been made in Si/SiGe heterostructures grown on metamorphic SiGe substrates as hosts to lithographically defined quantum dots. In this presentation we focus on the general structure and requirements of SiGe quantum dot heterostructures, the demands imposed by necessary dot performance, and advances in characterization methods for these materials.

Silicon is an attractive materials platform for developing large-scale quantum computers because of its compatibility with classical silicon electronics and its potential for scalability. Recent progress towards quantum computing in silicon has been impressive, but better understanding of the materials will enable improved controllability and predictability of qubit devices.

This talk will discuss some of the fundamental physics and materials science challenges that arise, focussing on the nontrivial interplay between Coulomb interactions and the valley degree of freedom in silicon quantum dots containing more than one electron. Prospects for further development will also be discussed.

High-fidelity quantum operations of single and two spins as well as readout and initialization have recently been demonstrated for Si quantum dots. Among them exchange control between spins is an ingredient of quantum entanglement for implementing quantum algorithms, but the demonstration of entanglement has been limited to two-qubit entanglement mainly due to difficulties in controlling exchange couplings in a multi-qubit array. We use a triple quantum dot equipped with a micro-magnet to control exchange couplings in two or three spin systems. We first measure the single qubit fidelity for three spins in the (1,1,1) charge state and obtain the values of 99.4 to 99.9 % for three spins. Then we use two spins to study exchange control of two qubits or singlet-triplet qubit drive. We use a resonant drive technique of the exchange interaction at the qubit frequency and obtained the fidelity of 99.6% thanks to the symmetric operation and a large micro-magnet Zeeman field gradient. Finally we use three spins to implement three-qubit entanglement. We precisely control exchange couplings between three spins and generate GHZ state. The GHZ state generation protocol will be useful to encode and decode a logical state for the bit-flip quantum error correction code.

Donor-based quantum devices in silicon, manufactured with atomic precision, are a feasible alternative for the realization of universal quantum computing and analog quantum simulation. Protocols for operation, readout, and active control of these devices require a complete understanding of the quantum confinement effects, many-body interactions, transport characteristics, and response of the system to external fields. We report theoretical calculations on Si:P nanoscale transistors consisting of a single and multiple quantum dots, based on solutions of the Schrödinger-Poisson equations. In particular, we investigate the role of dopant location, dopant-dopant distance, and array orientation in the shape of the potential in different dopant arrays including 2 × n and n × n arrays. We adopt a combined tight-binding description of the electronic properties of the Si:P substrate, and exact diagonalization techniques for electrons in the dopant array. The effect of external gates and source/drain potentials is accounted for by solving the Poisson equation with the finite element method under appropriate boundary conditions. Most importantly, we develop a way to describe transport through multi-electron, multidot quantum devices. Our simulations allow us to rationalize recent experimental measurements in these devices in terms of their stability diagrams, thus demonstrating that our approach provides an accurate description of single-electron transistors with multiple quantum dots.

Quantum dots (QD) in the silicon MOS system benefit from the use of palladium or platinum for gate materials because the small grain size of these metals compared to aluminum aids in shrinking the gate and dot dimensions. However, it is not clear what differences arise with respect to process-induced defect density and inhomogeneous strain. Here, we present measurements of oxide defect densities (fixed charge and interface trap density) as a function of forming gas anneal temperature for three different gate metals: Al, Ti/Pd, and Ti/Pt. We also investigate the concomitant effect of these anneals on the mechanical properties of the gate material, such as the intrinsic film stress (σ₀) and coefficient of thermal expansion (α). Our results show that Ti/Pd and Ti/Pt result in much higher fixed charge density and higher, though comparable, interface trap densities when compared to Al with all three metals showing a minimum in interface trap density for a 30 minute anneal at 350C. Moreover, we find that α is larger than the bulk value and that both α and σ₀ increase with temperature with σ₀ ≈ 1 GPa at 350 C for Pd. These results demonstrate that the forming gas anneal which minimizes electrical defects (inhomogeneous strain) strongly increases the effects of inhomogeneous strain (electrical defects).

Nano-scale quantum dot (QD) qubits in SiGe are exhibiting competitive quantum characteristics, e.g. long coherence times, required of future materials platforms for quantum information technologies. Qubits are intimately nested at semiconductor interfaces between Si and SiGe in layered heterostructures. A major challenge in developing the SiGe platform lies in understanding relationships between interface structure and quantum electronic properties. Specifically, atomic steps at Si/SiGe interfaces is theoretically predicted to strongly modulate a critical quantum property, the conduction band valley spitting. This atomic-scale interface structure-to-electronic-function relationship may be a determining factor in manufacturability of SiGe qubits. However, it is difficult to examine the relationship since it’s exceedingly challenging to probe atomic structure at buried interfaces.

We characterize atomic-scale structural disorder at Si/SiGe qubit interfaces by using scanning tunneling microscopy in parallel with molecular beam epitaxial growth. We report a counterintuitive evolution of roughness and step spatial correlations during growth. We find interfaces are completely uncorrelated with Si sample miscut and take-on unique distributions at each interface, stemming from growth kinetics and strain effects. In addition, at the Si qubit interface, there are widely-ranging atomic step arrangements, at all length scales relevant for QDs reported in literature. This indicates that one should expect widely varying valley splittings and suppressed values measured in both QDs, and mesoscale electronic transport devices. Further, our measurements make it clear that sparse transmission electron and atomic force microscopy (AFM) measurements (the standard approach) will not suffice to evaluate these interfaces. For example, the Si qubit interface and other nearby interfaces have completely different structure and can’t serve as proxies for one another to allow top-surface AFM to estimate the buried qubit interface structure. These results are meaningful toward elucidating structure-function relationships in SiGe QDs and a pathway to understanding the complex engineering required to leap beyond current device performance.

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 multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the United States DOE’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the article do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

Motivation

Spin qubits of electrons in Si are a leading route to implement quantum computing, due to silicon’s weak spin-orbit coupling and the existence of stable zero nuclear spin isotopes. However, a fundamental challenge is the degeneracy of the conduction band minima, which is a decoherence source.
For electron qubits, a thin Si layer is typically grown on a Si_0.7Ge_0.3 relaxed buffer for a tensile strain of 1.2%, which will break the cubic symmetry of Si and split its 6-fold degenerate valleys into four in-plane △₄ high-energy valleys and two out-of-plane △₂ low-energy valleys. This strain-induced valley splitting is a very large ~200 meV. The two degenerate Si_0.7Ge_0.3 △₂ valleys are further split by the quantum well confinement potential, estimated to be 1 meV [1], sufficient for QC. However, experimental measurements are 10 times lower than this expectation [2], which seriously hinders the further development of Si-based QC. It is believed that a roughness as small as one atomic step at the SiGe/Si interface could cause this reduction in valley splitting, so that an atomically flat interface is required [1]. The typical roughnesses of relaxed SiGe layers in the literature are in the nm range or more, at least 10X too high. In this paper, we demonstrate the growth of an atomically flat relaxed Si_0.7Ge_0.3 buffer layer, which could be the key to enhance the splitting of the two △₂ valleys in a thin Si layer grown on top of the SiGe. We also outline the fundamental mechanisms leading to SiGe roughness that must be suppressed.

Methods

Conventional thick (3-micron) graded relaxed Si_0.7Ge_0.3 relaxed buffers are first grown by CVD on Si (100) substrates. They are rough, so they are then chemically and mechanically polished to a surface roughness less than 0.2 nm. After wet acid cleaning, SiGe and Si layers are grown in a custom-built UHV-CVD system using silane and germane sources. The typical growth rates for Si and SiGe at 600°C are ~1-2 nm/min respectively. Oxygen and carbon contamination in Si and SiGe layers are all below 5×10¹⁷ cm^-3, indicating clean layers. The roughness is characterized by a Bruker Dimension ICON3 Atomic Force Microscope (AFM) in tapping mode with a 5µm×5µm scan size. The noise error is ~0.10 nm RMS.

Results

Important parameters affecting the growth of the SiGe layer are pre-growth “baking” of the polished relaxed SiGe in the UHV CVD chamber before growth and the growth temperature of the SiGe. Unlike the growth of Si on Si in UHV-CVD, we find that for growth on SiGe, baking is necessary to remove surface contamination, and thus layers grown after baking are initially rough. The surface roughness is smoother as the regrowth interface contamination is reduced, from 3.4 nm RMS for 750°C baking to 0.46 nm RMS for 800°C baking, indicating that the cleanliness of the regrowth interface is important for layer roughness. The smoothest SiGe surface morphology occurs when the growth temperature is around 650°C, and the morphology gets rougher if the growth temperature is either lower or higher than 650°C. Growing SiGe at lower temperatures, adatoms from the gas source may not find minimum energy locations for step flow growth, possibly due to low adatom surface mobility, leading to roughness. At higher temperatures, multiple mechanisms may to contribute to roughness, such as the re-evaporation process and the role of entropy. Silicon layers are consistently smoother than SiGe layers, and a nearly atomically flat relaxed SiGe (roughness = 0.17 nm RMS) is achieved by inserting a thin Si layer into the relaxed SiGe layer.

[1] Friesen, Mark, et al. Physical Review B 75.11 (2007): 115318.
[2] Borselli, Matthew G., et al. Applied Physics Letters 98.12 (2011): 123118.

Plasma oxidation and ultra-high vacuum handling enable the formation of ultra-stable aluminum oxides (AlO_x), demonstrated here using single-electron transistor (SET) measurements and improving prospects for broader implementation in quantum information applications. These devices exhibit remarkably mitigated charge offset drift (best ΔQ₀ = 0.13 e ± 0.01 e over ≈ 7.6 days) in our Al/AlO_x/Al SETs, compared to previous SET results [1-4] with conventional, thermally oxidized tunnel barriers (best ΔQ₀ = 0.43 e ± 0.007 e over ≈ 9 days and most ΔQ₀ ≥ 1 e within one day). Slow, linear drift (< 0.025 e/day) accounts for most of the charge offset drift in our devices and ΔQ₀^corr = 0.07 e ± 0.01 e over ≈ 7.6 days when the linear background is subtracted. Slow drift, combined with a control voltage feedback loop, allows good charge sensing as long as the control voltage activity does not obscure “real” charge transition events. The charge stability demonstrated here enables these Al/AlO_x/Al SETs to be integrated as persistent charge sensors for projective spin-to-charge readout in qubit devices. For example, a signal-to-noise > 2 can be achieved when the coupling capacitance is > 3.5 aF between an AlO_x SET and a Si MOS quantum dot with a self-capacitance ≈ 50 aF.

References
1. 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.
2. Zimmerman, N.M. and W.H. Huber, Microscope of glassy relaxation in femtogram samples: Charge offset drift in the single electron transistor. Physical Review B, 2009. 80(19): p. 195304.
3. Stewart, M.D. and N.M. Zimmerman, Stability of Single Electron Devices: Charge Offset Drift. Applied Sciences, 2016. 6(7): p. 187.
4. Huber, W.H., S.B. Martin, and N.M. Zimmerman, Long-term charge offset noise in Coulomb-Blockade devices. Experimental Implementation of Quantum Computation; Clark, R., Ed.; Rinton Press, INC: Princeton, NJ, USA, 2001: p. 176-182.

In my talk, I will discuss low-dimensional condensed matter systems, in which topological properties could be engineered per demand. Majorana fermions can emerge in hybrid systems with proximity-induced superconducting pairing. I will present our numerical and analytical studies of such geometries with proximity effects [1-3]. In the second part of the talk, I will discuss an analytical model of a Rashba nanowire that is partially covered by and coupled to a thin superconducting layer, where the uncovered region of the nanowire forms a quantum dot [4,5]. Even if there is no topological superconducting phase possible, there is a trivial Andreev bound state that becomes pinned exponentially close to zero energy as a function of magnetic field strength when the length of the quantum dot is tuned with respect to its spin-orbit length such that a resonance condition of Fabry-Perot type is satisfied. In this case, the Andreev bound state remains pinned near zero energy for Zeeman energies that exceed the characteristic spacing between Andreev bound state levels but that are smaller than the spin-orbit energy of the quantum dot. Importantly, as the pinning of the Andreev bound state depends only on properties of the quantum dot, this behavior is unrelated to topological superconductivity.

References
[1] C. Reeg, D. Loss, and J. Klinovaja, Phys. Rev. B 97, 165425 (2018).
[2] C. Reeg, D. Loss, and J. Klinovaja, Phys. Rev. B 96, 125426 (2017).
[3] C. Reeg, J. Klinovaja, and D. Loss, Phys. Rev. B 96, 081301 (2017).
[4] C. Reeg, O. Dmytruk, D. Chevallier, D. Loss, and J. Klinovaja, Phys. Rev. B 98, 245407 (2018).
[5] O. Dmytruk, D. Chevallier, D. Loss, and J. Klinovaja, Phys. Rev. B 98, 165403 (2018).

Majorana modes are one of the most promising candidates for the creation of topological qubits in the long term. While evidence for Majorana modes has been obtained for Majorana modes in several systems such as semiconductor nanowires, semiconductor 2DEGs, iron-based superconductors, topological insulators as well as iron atom chains to name a few, evidence for a qubit degree of freedom is elusive. I will begin by reviewing what it means experimentally to observe a qubit. Following this we will discuss the central device challenge in this regard i.e. quasiparticle poisoning. I will then describe potential material dependence of this phenomena as well as how quasiparticle poisoning can manifest in various experiments related to topological superconductivity. Specifically, we will discuss how the observation of quantized conductance into a zero-bias conductance peak associated with a Majorana over a range of tunneling provides some bounds on quasiparticle poisoning times. The measurement of Coulomb blockade transport, which has been observed only for a limited number of superconductors can provide more precise information on such bounds.

Superconductor–semiconductor heterostructures represent a versatile platform for realizing Majorana zero-energy modes. In this talk, I will explain how to engineer topological superconductivity at the interface of a conventional (s-wave) superconductor and a semiconductor with spin-orbit interaction. I will discuss state-of-the-art numerical approaches for modeling realistic devices which take into account proximity-induced superconductivity, orbital and Zeeman effect of an applied magnetic field, spin-orbit coupling as well as the electrostatic environment on equal footing. Finally, I will review recent materials science progress in growing superconductor–semiconductor heterostructures and discuss promising new directions.

With Quantum Supremacy upon us, and with major tech companies investing in quantum computing, one may wonder how much room is left there for academic research? The answer is: more than ever! Our existing qubits are great, but it is difficult to see them carrying us beyond the modern era of noisy intermediate-scale quantum processors, and ushering in the truly scalable, fault tolerant and commercially viable computers. Most likely, a qubit and its couplings, gates will once more need to be re-thought from the ground up - from the atomic first-principles level to the device architecture level. At the same time, the progress we made in quantum control and engineering is so huge that these methods have themselves become mature tools of science. They can be applied beyond quantum computing - to understand the basic questions of quantum dynamics and to probe exotic matter.

One solution may come from studying topological superconductivity. This is an interesting state most notable for spurring Majorana zero modes. Those modes can be both investigated as some of the first non-Abelian excitations, and they can be used in quantum circuits for storing and protecting quantum information. Majorana research has posed many basic materials questions of how to engineer the precise structures that possess intrinsic fault tolerance through topological protection. In my work I have developed new materials and devices for unambiguous observation of robust Majorana modes. I have worked on establishing clear criteria (signatures) of Majorana modes and how to tell them apart from other excitations that may be present in the same devices. It is exciting to apply modern quantum probes, developed based on transmon and spin qubits, to these systems in search for new exotic quantum matter. And perhaps solving some of the big challenges facing quantum computing at the same time.

Atom qubits in silicon have demonstrated extremely long electron and nuclear spin coherence times [1,2] making silicon a promising semiconductor material for spin-based quantum information. The two-level spin state of single electrons bound to shallow phosphorus donors in silicon provide well defined, reproducible qubits [3] with low gate densities. An important challenge in these systems is the realisation of an architecture, where we can position donors within a crystalline environment with <20nm separation, individually address each donor, manipulate the electron spins using ESR techniques, read-out their spin states and perform both single and two qubit gates.

We have pioneered a unique strategy to build a scalable quantum computer in silicon atom by atom. We use scanning tunneling microscope lithography and ultra-high vacuum crystal growth to precisely position individual P donors in Si [4] aligned with nanoscale precision to local control gates [5] necessary to initialize, manipulate, and read-out the spin states [6-8].

During this talk I will focus on demonstrating fast, high fidelity single-shot spin read-out [9], ESR control of precisely-positioned P donors in Si [10] and our recent results of demonstrating a fast (~ns) two-qubit SWAP gate [11,12]. We have also published a 3D architecture for implementation of error corrrection using the surface code [13]. With important advances in control at the atomic-scale, I will attempt to highlight the benefits of single atom qubits in silicon.

References
[1] K. Saeedi et al., Science 342, 130 (2013).
[2] J. T. Muhonen et al., Nature Nanotechnology 9, 986 (2014).
[3] B.E. Kane, Nature 393, 133 (1998).
[4] M. Fuechsle et al., Nature Nanotechnology 7, 242 (2012).
[5] B. Weber et al., Science 335, 6064 (2012).
[6] H. Buch et al., Nature Communications 4, 2017 (2013).
[7] B. Weber et al., Nature Nanotechnology 9, 430 (2014).
[8] T. F. Watson et al., Science Advances 3, e1602811 (2017).
[9] D. Keith et al., Physical Review X 9, 041003 (2019)
[10] S. Hile et al., Science Advances 4, eaaq1459 (2018).
[11] M.A. Broome et al., Nature Communications 9, 980 (2018).
[12] S. Gorman, Y. He et al., Nature 571, 371 (2019).
[13] C. Hill et al., Science Advances 1, e1500707 (2015).

Silicon is an attractive materials platform for developing large-scale quantum computers because of its compatibility with classical silicon electronics and its potential for scalability. Recent progress towards quantum computing in silicon has been impressive, but better understanding of the materials will enable improved controllability and predictability of qubit devices.

This talk will discuss some of the fundamental physics and materials science challenges that arise, focussing on the nontrivial interplay between Coulomb interactions and the valley degree of freedom in silicon quantum dots containing more than one electron. Prospects for further development will also be discussed.

High-fidelity quantum operations of single and two spins as well as readout and initialization have recently been demonstrated for Si quantum dots. Among them exchange control between spins is an ingredient of quantum entanglement for implementing quantum algorithms, but the demonstration of entanglement has been limited to two-qubit entanglement mainly due to difficulties in controlling exchange couplings in a multi-qubit array. We use a triple quantum dot equipped with a micro-magnet to control exchange couplings in two or three spin systems. We first measure the single qubit fidelity for three spins in the (1,1,1) charge state and obtain the values of 99.4 to 99.9 % for three spins. Then we use two spins to study exchange control of two qubits or singlet-triplet qubit drive. We use a resonant drive technique of the exchange interaction at the qubit frequency and obtained the fidelity of 99.6% thanks to the symmetric operation and a large micro-magnet Zeeman field gradient. Finally we use three spins to implement three-qubit entanglement. We precisely control exchange couplings between three spins and generate GHZ state. The GHZ state generation protocol will be useful to encode and decode a logical state for the bit-flip quantum error correction code.

Quantum computation has captivated the minds of many for almost two decades. For much of that time, it was seen mostly as an extremely interesting scientific problem. In the last few years, we have entered a new phase as the belief has grown that a large-scale quantum computer can actually be built. Quantum bits encoded in the spin state of individual electrons in silicon quantum dot arrays, have emerged as a highly promising direction [1]. In this talk, I will present our vision of a large-scale spin-based quantum processor, and ongoing work to realize this vision.

First, we created local registers of spin qubits with sufficient control that we can program arbitrary sequences of operations. We show the creation of each of the Bell states with fidelities of 85-90% and the implementation of the four instances of the Deutsch-Jozsa and the Grover algorithms on two qubits [2].

Second, we have explored coherent coupling of spin qubits at a distance via two routes. In the first approach, the electron spins remain in place and our coupled via an intermediary degree of freedom. After showing this principle with an ancillary quantum dot as a coupler [3], we recently observed strong coupling of a single spin to a single microwave photon in a superconducting resonator [4]. In the second approach, spins are shuttled along a quantum dot array, preserving both the spin projection [5] and spin phase [6].

Third, in close collaboration with Intel, we have fabricated and measured quantum dots using all-optical lithography on 300 mm wafer, using industry-standard processing [7]. Qubit measurements are now underway. We expect that this industrial approach to nanofabrication will be critical for achieving the extremely high yield necessary for devices containing hundreds or thousands of qubits. Furthermore, we anticipate that the device stability and charge noise levels will eventually outperform those of devices made in university-style cleanrooms.

When combined, the progress along these various fronts can lead the way to scalable networks of high-fidelity spin qubit registers for computation and simulation.

[1] L.M.K. Vandersypen, H. Bluhm, J.S. Clarke, A.S. Dsurak, R. Ishihara, A. Morello, D.J. Reilly, L.R. Schreiber, M. Veldhorst, Interfacing spin qubits in quantum dots and donors – hot, dense and coherent, npj Quantum Information 3, 34 (2017).
[2] T. F. Watson, S. G. J. Philips, E. Kawakami, D. R. Ward, P. Scarlino, M. Veldhorst, D. E. Savage, M. G. Lagally, Mark Friesen, S. N. Coppersmith, M. A. Eriksson, L. M. K. Van- dersypen, A programmable two-qubit quantum processor in silicon, Nature 555, 633-637 (2018)
[3] T.A. Baart, T. Fujita, C. Reichl, W. Wegscheider, L.M.K. Vandersypen, Coherent spin-exchange via a quantum mediator, Nature Nanotechnology, 12, 26 (2017)
[4] N. Samkharadze, G. Zheng, N. Kalhor, D. Brousse, A. Sammak, U. C. Mendes, A. Blais, G. Scappucci, L. M. K. Vandersypen, Strong spin-photon coupling in silicon, Science 359, 1123-1127 (2018)
[5] T. A. Baart, M. Shafiei, T. Fujita, C. Reichl, W. Wegscheider, L. M. K. Vandersypen, Single-Spin CCD, Nature Nanotechnology 11, 330 (2016)
[6] T. Fujita, T. A. Baart, C. Reichl, W. Wegscheider, L. M. K. Vandersypen, Coherent shuttle of electron-spin states, npj Quantum Information 3, 22 (2017)
[7] R. Pillarisetty, H.C. George, T.F. Watson, L. Lampert, N. Thomas, S. Bojarski, P. Amin, R. Caudillo, E. Henry, N. Kashani, P. Keys, R. Kotlyar, F. Luthi, D. Michalak, K. Millard, J. Roberts, J. Torres, O. Zietz, T. Krähenmann, A.-M. Zwerver, M. Veldhorst, G. Scappucci, L.M.K. Vandersypen, and J.S. Clarke, 2019 IEEE IEDM San Francisco, pp. 31.5.1-31.5.4.

Quantum information and computing are at the forefront of computer science, but their implementation relies on the identification and development of a suitable materials system. One such candidate is Si/SiGe. Much progress has been made in Si/SiGe heterostructures grown on metamorphic SiGe substrates as hosts to lithographically defined quantum dots. In this presentation we focus on the general structure and requirements of SiGe quantum dot heterostructures, the demands imposed by necessary dot performance, and advances in characterization methods for these materials.

Silicon carbide (SiC) is a technologically mature semiconductor material that is host to a variety of different solid-state spin defects. These spin qubits not only boast long coherences but also a natural spin-photon interface that can mediate entanglement at a distance. However, as for any solid-state quantum technology, issues of charge noise and magnetic fluctuations are prevalent. In this talk, we present recent advances in minimizing and controlling these sources of noise.

We first discuss our ability to mitigate charge noise in commercial SiC electronic devices [1]. Surprisingly, the integration of single spins into these devices results in THz-scale tunability of a single divacancy spin’s optical interface while simultaneously eliminating spectral diffusion and blinking. This results in an electrically-tunable single photon source that can be tuned over 40,000 linewidths, constituting a near-ideal spin-photon interface at wafers scales near the telecommunications band.

We then describe the control and optimization of the nuclear spin environment for these spin qubits [2]. In general, nuclear spins are both a cause of decoherence and a valuable resource for electron spins in the solid-state. While isotopic purification removes noise, it also reduces the availability of nuclear spin quantum memories that are vital to many quantum technologies. Guided by ab-initio theory, we strike this balance and find an optimal isotopic concentration that maximizes the number of available quantum memories. Experimentally, we demonstrate extended coherences and the first entanglement and control of single nuclear spins in silicon carbide through isotopic engineering.

The elimination of electrical noise combined with finely tuning the nuclear environment results in a material platform for quantum technology that combines a stable spin-photon interface with an electron spin that can leverage entanglement with local nuclear spin qubit clusters. We show that this electron spin state has long coherence times (T₂>2.3 ms, T₂*>370 μs), state-of-the-art control fidelities (>99.984%), and displays readout and initialization contrast exceeding 99%, providing an exciting pathway toward scalable quantum information technologies in the solid-state.

This work was done in collaboration with Mykyta Onizhuk, He Ma, Gary Wolfowicz, Peter Mintun, Alex Crook, Hiroshi Abe, Jawad Ul Hassan, Nguyen Son, Takeshi Ohshima and Giulia Galli.

[1] C. P. Anderson*, A. Bourassa*, K. C. Miao, G. Wolfowicz, P. J. Mintun, A. L. Crook, H. Abe, J. U. Hassan, N. T. Son, T. Oshima, D. D. Awschalom, “Electrical and optical control of single spins integrated in scalable semiconductor devices,” Science 336, 6470, 1225-1230 (2019)

[2] A. Bourassa*, C. P. Anderson*, K. C. Miao, M. Onizhuk, H. Ma, A. Crook, H. Abe, J. Ul-Hassan, T. Ohshima, N. T. Son, G. Galli, D. D. Awschalom, “Entanglement and control of single quantum memories in isotopically engineered silicon carbide,” arXiv 2005.07602 (2020)

The central challenge in building quantum computers and long-range quantum networks is distributing entanglement across multiple individually-controllable quantum memories. While proof-of-concept experiments are possible with today’s optical components, scaling these systems to tens, hundreds, or thousands of individually controllable quantum memories requires very-large-scale integration of electronic and photonic circuits.
Diamond color centers (CC) have emerged as leading solid state “artificial atom” qubits, having recently surpassed the entanglement-to-decoherence threshold for deterministic delivery of quantum entanglement[1] as well as the first demonstration of “useful” quantum-memory-enhanced communication beating any direct-transmission optical channel[2]. A central challenge now lies in the development of quantum information processing architectures to improve entanglement rates, reduce decoherence, and to scale systems for applications ranging from quantum repeaters to general-purpose quantum computers.
To answer this challenge, we are developing photonic integrated circuits (PICs) with integrated quantum memories based on diamond CCs. In particular, this talk discusses our recent demonstration of a PIC with 128 waveguide-integrated silicon vacancy (SiV) and germanium vacancy (GeV) color centers[3].
Secondly, this talk will review recent advances on spin and optical properties of recently discovered tin-vacancy (SnV) CCs which have significantly larger ground state orbital splitting than the lighter Group IV-vacancy complexes. We observe that even at modest temperatures near ~ 4K, the SnV possesses the essential attributes for a spin-photon interface: optical transitions approaching the lifetime limit and a long-lived electronic spin state[4].
The talk will finish with an outlook on resource-performance analyses for large-scale quantum repeaters [3,5,6], modular quantum computers[7,8], and coherent interfaces to phonons and microwave photons[9].
[1] P. C. Humphreys, N. Kalb, J. P. J. Morits, R. N. Schouten, R. F. L. Vermeulen, D. J. Twitchen, M. Markham, and R. Hanson, Nature 558, 268 (2018).
[2] M. K. Bhaskar, R. Riedinger, B. Machielse, D. S. Levonian, C. T. Nguyen, E. N. Knall, H. Park, D. Englund, M. Lončar, D. D. Sukachev, and M. D. Lukin, arXiv [quant-Ph] (2019).
[3] N. H. Wan, T.-J. Lu, K. C. Chen, M. P. Walsh, M. E. Trusheim, L. De Santis, E. A. Bersin, I. B. Harris, S. L. Mouradian, I. R. Christen, E. S. Bielejec, and D. Englund, arXiv [quant-Ph] (2019); to appear in Nature (2020)
[4] M. E. Trusheim, B. Pingault, N. H. Wan, M. Gündoğan, L. De Santis, R. Debroux, D. Gangloff, C. Purser, K. C. Chen, M. Walsh, J. J. Rose, J. N. Becker, B. Lienhard, E. Bersin, I. Paradeisanos, G. Wang, D. Lyzwa, A. R.-P. Montblanch, G. Malladi, H. Bakhru, A. C. Ferrari, I. A. Walmsley, M. Atatüre, and D. Englund, Phys. Rev. Lett. 124, 023602 (2020).
[5] Y. Lee, E. Bersin, A. Dahlberg, S. Wehner, and D. Englund, arXiv [quant-Ph] (2020).
[6] K. C. Chen, E. Bersin, and D. Englund, arXiv [quant-Ph] (2020).
[7] H. Choi, D. Zhu, Y. Yoon, and D. Englund, Phys. Rev. Lett. 122, 183602 (2019).
[8] H. Choi, M. Pant, S. Guha, and D. Englund, Npj Quantum Information 5, 104 (2019).
[9] T. Neuman, M. Eichenfield, M. Trusheim, L. Hackett, P. Narang, and D. Englund, arXiv [quant-Ph] (2020).

In this talk, we present the demonstration of a superconducting transmon qubit realized using a graphene-based weak-link junction. The graphene is encapsulated by hexagonal Boron Nitride (hBN), forming a van der Waals heterostructure. Applying a voltage to a backgate in proximity to the weak-link junction enables voltage tunability of the qubit frequency. We present the coherent control of this qubit, and discuss the promise and the challenges if building superconducting qubits with such van der Waals heterostructures.

Quantum computing has made tremendous progress in recent years, including both experimental advances and hardware developments. Quantum processors have scaled significantly in size, as measured by the number of quantum bits (qubits) connected on a chip, with devices incorporating 10's of qubits available today. An example of a publicly available quantum system is the IBM Q Experience (www.research.ibm.com/ibm-q). While the goal of fault-tolerant universal quantum computing is still some time away, early applications and demonstrations can be implemented on smaller scale near-term quantum systems. Among the different hardware implementations, Josephson-junction-based superconducting quantum circuits are a promising technology to scale quantum processors. Together with increasing the number of qubits, other metrics need to evolve and improve as well, such as qubit quality, hardware integration, and system level performance. In this talk, I will review the current status and discuss some of the remaining challenges.