Symposium MQ02—Materials for Quantum Computing Applications

Nanoscale hybrid III-V semiconductor/superconductor heterostructures have demonstrated potential to advanced quantum transport physics, in particular to host the elusive Majorana quasiparticles. Virtually all progress on the materials side in the last 6 years has relied on high quality vapor liquid solid growth of III-V free-standing nanowires functionalized at a later stage by ex-situ deposited Al. Despite these early successes a more robust, scalable materials platform is still missing to enable the future topological quantum computer. Here I will report on our progress in using molecular beam epitaxy (MBE) to grow both III-V semiconductor nanowire networks and a high quality epitaxial s-wave metal superconductor, using selective area epitaxy. I will first introduce the current state of the art, advantages and challenges of the selective area approach and MBE with respect to other growth techniques and geometries. Fundamental knowledge gained by growth studies will be applied to obtain reproducible, high yield advanced high spin-orbit III-V nanowire/superconductor networks. I will present low temperature transport experiments that characterize the quality of the materials.

Amorphous aluminum oxide-based tunnel junctions have been widely adopted for fabricating superconducting qubit devices, but further improvement is hindered by potentially fundamen- tal limits of uniformity and interactions with intrinsic tunneling two-level-systems (tTLS) origi- nating from amorphous AlOx and disordered interfaces. The amorphization and disordering are inevitable due to large lattice mismatch between crystalline aluminum oxide and the aluminum substrate. The Re/Al2O3/Re material stack has well-matched lattice parameters, and prior work has demonstrated reduced TLS density. We aim to understand the materials property of these in- terfaces, detailing their thermodynamic property such as interfacial energy and work of adhesion; and structure-related electronic properties such as fundamental energy gap, ionic charge transfer, and local dipole formation. The results show how the lattice-matching strategy improved sev- eral microscopic properties of the tunnel barrier junction, laying out a strategy to develop more coherent superconducting quantum computing components. (Prepared by LLNL under Contract DE-AC52-07NA27344.)

Donor-based qubits in silicon have attracted broad interest as a promising candidate for scalable solid-state quantum computing. Central to the scalability and performance of this quantum system is atomically precise control of dopant confinement and single dopant placement in silicon. Atomically precise fabrication in silicon has enabled atom-by-atom control and characterization of quantum materials properties for quantum information processing and quantum simulation applications, such as high-fidelity spin readout/initialization and tunable exchange coupling in multi-qubit gate operations.

Success in atomically precise fabrication is a direct consequence of controlling materials properties at the atomic scale. This presentation will present our recent advances to address critical challenges in atomically precise fabrication and characterization of donor-based quantum devices in silicon by using enhanced STM-based hydrogen lithography to define atomically precise dopant-based devices, and by suppressing atomic-scale dopant movement during device encapsulation overgrowth. We will report on STM-defined single electron transistors that can be used as charge sensors, where the source/island/drain tunnel coupling can be controlled with atomic-precision for the optimization of charge sensing performance. We will present resonant tunneling spectroscopy analysis of STM-defined single and few donor quantum dots with atomically defined tunnel gaps. Finally, we combine single/few donor quantum dots with atomically defined single electron transistors as charge sensors and report our recent results in atomic-scale control of tunnel coupling between few-donor quantum dots and single electron charge sensors to illustrate the impact of atomically defined tunnel coupling on single electron charge sensing and spin selective-readout.

Low microwave loss superconducting circuit components are a necessity of fabricating high-fidelity superconducting qubits. Accordingly, significant research has focused on making high-quality planar resonators from elemental and nitride superconductors. Josephson junctions are the nonlinear component of superconducting qubits, that also need to be high performance. Interestingly, superconducting qubits all use Josephson junctions fabricated from aluminum and aluminum oxide using the double angle evaporation process. Details of this alternative design will be presented.
Plasma assisted Molecular beam epitaxy (PAMBE) is used to grow niobium titanium nitride alloys (Nb_xTi_1-xN) and wide bandgap nitride (AlN) superconductors directly on sapphire wafers. This combination of nitride materials provides sufficient degrees of freedom that synthesis of an epitaxial Josephson junction may be possible. Growth results of Nb_xTi_1-xN films on c-plane sapphire substrates, and initial trilayer NbTiN/AlN/NbTiN (superconductor-insulator-superconductor) Josephson junction structures on sapphire will be presented along with structural analysis and results from superconducting IV and microwave-loss measurements.

For quantum computing to become fault tolerant, the underlying quantum bits must be effectively isolated from the noisy environment. It is well known that including an electromagnetic bandgap around the qubit operating frequency improves coherence for superconducting circuits. However, investigations of bandgaps to other environmental coupling mechanisms remain largely unexplored. We present a method to enhance the coherence of superconducting circuits by introducing a phononic bandgap around the device operating frequency. The phononic bandgap blocks resonant decay of defect states within the gapped frequency range, removing the electromagnetic coupling to phonons at the gap frequencies. We constructed a multi-scale model that derives the decrease in the density of states due to the bandgap and the resulting increase in defect state T₁ times. We demonstrated that emission rates from in-plane defect states can be suppressed by up to two orders of magnitude. We combine these simulations with full master equation simulations to demonstrate the suppression of qubit energy relaxation even when interacting with 200 defects states.

As the field of superconducting qubits progresses, it will become increasingly important to use 3D integration to address large arrays of qubits. Traditional 3D integration methods pose challenges for superconducting qubits, because these qubits are vulnerable to losses introduced by bulk dielectrics and at interfaces. To overcome these challenges, we are developing a novel three-tier stack where an interposer with superconducting through-silicon vias (TSVs) isolates sensitive qubits from a potentially-lossy readout and interconnect chip. This isolation prevents degradation of the qubit coherence time, and the high-aspect ratio superconducting TSVs enable dense circuitry to control and read out the qubits. The components of the three-tier stack are bonded using indium bump bonds, providing both structural support and a galvanic connection between the three chips. I will present our work developing and characterizing 3D integration components including indium bumps and TSVs, and I will discuss recent demonstrations incorporating superconducting TSVs in qubit control and readout circuitry.

This research was funded by the Office of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA) and by the Assistant Secretary of Defense for Research & Engineering under Air Force Contract No. FA8721-05-C-0002. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of ODNI, IARPA, or the US Government.

Trapped ion systems have many advantages that make them promising quantum computing platforms. They are well isolated from the environment, feature high-fidelity quantum operations, and exhibit basically no idle errors. Microfabrication enables the realization of complex traps that allow one to precisely control large ion chains, move qubits through the trap, and reconfigure the trapped ion quantum processor, which are necessary features for building large scale systems capable of performing complex algorithms. However, microfabricated surface traps bring unique challenges. Proximity of ions to trap structures can lead to electric field noise which induces anomalous heating of ions in the trap. Routing through traps to make complex electrode patterns leads to higher capacitances and thus higher thermal dissipation on the device. Also, complex RF geometry can lead to excessive micromotion and AC Stark shifts, which can vary or broaden the energy levels used to perform gates. Reducing these sources of error is essential to further improve the fidelity of trapped ion quantum processors.

We will present Sandia’s ion-trap microfabrication capabilities and introduce ion trap designs that are being used in the leading trapped-ion groups, as well as our characterization and mitigations for these sources of error. We show exquisite classical control of ions as well as high-fidelity quantum operations, demonstrating the viability of microfabricated surface electrode ion traps for quantum information processors.

SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525. SAND2019-6322 A

The neutral divacancy (VV⁰) in silicon carbide (SiC) is a candidate solid state qubit which promises a combination of long spin coherence and near-infrared emission in a material which is already optimized for state-of-the-art scalable semiconductor manufacturing. Here, we use this mature host material to integrate quantum states into classical electronic devices.

First, we show that we can create and isolate single divacancy in p-i-n diodes. We then perform spin manipulation and readout of these single defects inside the junction and demonstrate high fidelity Rabi oscillations (with >98% contrast). This allows us to measure a long spin coherence time (T₂) which persist inside the device.

Second, we use the diode to confine the electric field in the i-layer which allows us to Stark tune the single photon emission frequency by almost a terahertz. Furthermore, since this field is applied along the defect axis, the tuning does not change the symmetry of the orbitals which minimizes unwanted spin flip rate.

Finally, we show how we can use charge depletion in the junction to drastically reduce electric field noise at the defect. This allows us to effectively eliminate spectral diffusion and obtain an optical linewidth approaching the lifetime limit of the emitter (a 50x enhancement). Moreover, we show that this optical line is very stable and fluctuates by less than a linewidth over many hours.

Overall, these results demonstrate the power in integrating quantum states into engineered classical electronics devices.

Related publication:
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, arXiv:1906.08328 (2019)

Rare earth ions (REI) have a spin-optical interface that enables storage of optical quantum information in the long-lived nuclear spin levels. Further, a microwave resonator can couple to the spin transitions inductively, making REI suitable for quantum memory and transduction applications. In the case of Er3+, the optical excitation matches the telecom transmission wavelength (~1.55 um), making it technologically attractive. When embedded in a crystalline host, the full 5s and 5p orbitals of the REI shield the intra 4f electrons from the crystal field effects from the host. This results in very narrow transition linewidths and high spectral stability, making it an ideal candidate for solid state quantum memory. Long coherence times of 1.3s in Er:YSO [1] bulk single crystals have been demonstrated. However, very little has been done in this regard when it comes to demonstrating thin films that could be integrated into scalable devices.

Using molecular beam epitaxy (MBE), we demonstrate the growth and characterization of high quality single crystal thin films of Er:Y₂O₃ on Si 111. Using optical and electron paramagnetic resonance spectroscopy, we confirm that Er substitutes for Y at both C2 and C3 sites in the bixbyite structure of Y₂O₃. We obtain a photoluminescence linewidth of 7.9 GHz and 5.1 GHz for the 1536 nm transition (4I13/2 → 4I15/2) at 4 K and 7 mK, respectively. We will discuss the role of the microstructure (carried out using extensive electron microscopy), the use of buffer layers, the Y₂O₃/Si and the Y₂O₃/air interface, and the optimization of these parameters that led to the demonstration of narrow optical linewidth of Er³⁺. We also discuss the gains resulting from using enriched ¹⁶⁷Er (91.5%) as the dopant.

In order to transfer the growth processes for compatibility with compact, optically modulated solid state devices, it is necessary to develop these heterostructures on silicon-on-Insulator (SOI) wafers—preferably (but not necessarily) oriented along <100> —so that they may be integrated with silicon photonics based optical waveguides. We demonstrate the growth of Y₂O₃ on Si 100 with a Y₂O₃(110)//Si(100) epitaxial relationship. We will also report on the demonstration of full SOI/Er:Y₂O₃/Si overlayer heterostructures and the quality of optical waveguides fabricated using this platform.

[1] Rancic et al. Nature Physics, 14, 50–54 (2018)

Trapped-ion quantum systems are a promising platform for large-scale quantum information processing, with all relevant operations having been demonstrated with the highest fidelities in any qubits to date [1,2], and fully connected systems with of order 10 qubits [3]. Given this established set of basic operations, methods to allow their robust implementation at a large scale, while also significantly reducing error rates, will play an increasingly central role in work towards realizing useful quantum processors.

A major element of this in the context of trapped-ion qubits will be the integration of optics used to control many ions in parallel into the ion traps themselves [4]. This enables stable beam delivery, tight focusing to µm-scale spots for individual ion-addressing and efficient power usage, and straightforward parallelization. I will describe our recent work at ETH developing on these ideas [5], in devices with efficient fiber coupling on multiple channels directly to waveguides on trap die (2 dB coupling loss measured at 729 nm wavelengths), and low-loss routing and emission to ions trapped 50 µm above the surface of the chip. I will present our work towards realizing multi-qubit quantum logic in these devices, a few novel beam arrangements utilizing such optics, and observations on relevant noise sources present owing to the various materials near the ions.

Our current experiments are based on integrated devices for light at red and near-IR wavelengths, and use modulators off-chip. In longer-term systems there will be considerable interest in compact integrated modulators, including for blue and near-UV wavelengths, able to be implemented within the trap chip. This will motivate research on thin-film material systems suitable for integrated electro- and acousto-optic devices at short wavelengths; these would additionally have broad applicability beyond quantum information.

[1] T.P. Harty, et al. “High-Fidelity Preparation, Gates, Memory, and Readout of a Trapped-Ion Quantum Bit.” Physical Review Letters 113, 220501 (2014).
[2] J.P. Gaebler, et al. “High-fidelity universal gate set for 9Be+ qubits.” Physical Review Letters 117, 060505 (2016).
[3] K. Wright, et al. “Benchmarking an 11-qubit quantum computer.” arXiv preprint, arXiv:1903.08181 (2019).
[4] K.K. Mehta, et al. “Integrated optical addressing of an ion qubit.” Nature Nanotechnology 11, 1066-1070 (2016).
[5] K.K. Mehta, et al. “Towards fast and scalable trapped-ion quantum logic with integrated photonics.” Proc. SPIE 10933 (2019).

An on-chip approach to optical delivery for portable quantum sensors is described. The optical design, epitaxy, nanofabrication and measurement of polarisation-maintaining, deep-etched aluminium gallium arsenide (Al_xGa_(1-x)As) waveguides for near-infra-red 780 nm light was achieved, in an analogous approach to earlier work at (1300-1550) nm wavelengths [1]. Material characterisation techniques were developed to infer improved optical quality and reduced optical loss intrinsic to the material, grown by Molecular Beam Epitaxy (MBE). High, continuous-wave optical power densities were guided with low polarisation noise and photonic waveguide performance was stable over time. The optical loss was measured to be below 4.3(±0.4) dB cm^-1 corresponding to an attenuation coefficient, α, of 1.0 (±0.08) cm^-1 for single mode waveguides and demonstrating improved performance data [2]. The polarisation extinction ratio was better than −19 (±1) dB for orthogonal polarisations.
These novel components illustrate the feasibility of passive photonic integrated circuits for the 780 nm wavelength. Future work will investigate the optical switching properties of these structures. The ultimate aim of the work is the creation of waveguide-based photonic circuits for compact cold atom sensors based on the D₂ hyperfine transition of ⁸⁷Rb at 780.24 nm [3]. Further applications include free-space short-wave communications, hybrid quantum systems [4] and epitaxial structures transparent to shorter wavelengths for other cold atom species and trapped ions. To this end, recent experiments examined device functionality as a function of wavelength.

[1] Heaton, J.M., Bourke, M.M., Jones, S.B., Smith, B.H., Hilton, K.P., Smith, G.W., Birbeck, J.C.H., Berry, G., Dewar, S.V. and Wight, D.R., “Optimization of Deep-etched, Single-mode GaAs / AlGaAs Optical Waveguides using Controlled Leakage into the Substrate”, Journal of Lightwave Technology, 17(2), 267-281 (1999).
[2] Maclean, J.O., Greenaway, M.T., Campion, R.P., Pyragius, T., Fromhold, T.M., Kent, A.J., Mellor, C.J., “III-V semiconductor waveguides for photonic functionality at 780 nm” Proc. SPIE 8988, Integrated Optics: Devices, Materials, and Technologies XVIII, 898805 (2014)
[3] Bongs, K., Malcolm, J., Ramelloo, C., Zhu, L., Boyer, V., Valenzuela, T., Maclean, J., Piccardo-Selg, A., Mellor, C., Fernholz, T., Fromhold et al., “iSense: A Technology Platform for Cold Atom Based Quantum Technologies” Optics InfoBase Conference Papers [1-55752-995-7] (2014)
[4] Gleyzes, S., El Amili, A., Cornelussen, R.A., Lalanne, P., Westbrook, C.I., Aspect, A., Esteve, J., Moreau, G., Martinez, A., Lafosse, X., Ferlazzo, L., Harmand, J. C., Mailly, D. and Ramdane, A., "Towards a monolithic optical cavity for atom detection and manipulation," European Physical Journal D, 53(1), 107-111 (2009).

Acknowledgements: We should like to thank Dr M.C. Rosamond and Prof. E.H. Linfield for expertise in Electron Beam Lithography at the School of Electronic and Electrical Engineering, University of Leeds, UK. This work was supported by the Engineering and Physical Sciences Research Council [grant number SM-30535 EP/M013294/1] through the EPSRC UK Quantum Technologies Hub for Sensors and Metrology.

High quality factor (Q), small mode volume (V) optical microcavities are coveted for a variety of applications, including low-energy nonlinear optics and precision sensing. Recent theory and fabrication advances have led to the experimental realization of resonators featuring subwavelength mode confinement and quality factors exceeding 10 million. These continued improvements lend a natural question – is there any limit to the highly sought-after Q/V ratio? Here, we show that the presence of thermo-refractive noise (TRN) – stochastic perturbations of the cavity material’s index of refraction due to fundamental temperature fluctuations – imposes a fundamental bound of the achievable cavity performance. Specifically, our results reveal a mode volume-dependent limit to an “effective quality factor,” which accounts for both intrinsic decay and resonant frequency noise. We compare this performance bound in various common integrated photonics materials systems, and explore the implications for proposed all-optical quantum information schemes. These results thus illustrate the importance of considering thermal noise in state-of-the-art microcavities, but also inform the choice of materials and processes which minimize its performance impact.

Atom-like emitters in diamond are among the promising qubits for quantum computing and information processing. In particular, the silicon vacancy (SiV) and germanium vacancy (GeV) centers are outstanding emitters due to their long spin coherence times and narrow optical transitions. Here, we describe the design and fabrication of diamond waveguide arrays containing single SiV and GeV centers coupled to aluminum nitride photonic circuits. A high-yield qubit creation process along with deterministic photonic nanofabrication produces unity coupling of emitters to single-mode diamond waveguide arrays. These arrays are subsequently assembled on aluminum nitride photonic waveguide arrays for on-chip routing and manipulation of photons. The combination of these advances allows the initial demonstration of a 72-channel solid-state spin qubit microphotonic chip for large-scale manipulation of quantum information.

Erbium ions are a promising resource for long-range quantum communication applications. Optical transitions operating at 1.5um minimize propagation loss in optical fibers, enabling communication on the 100 km scale, while electron and nuclear spin degrees of freedom can be used as a local quantum memory. However, currently known hosts for Er³⁺ ions contain atoms with nonzero nuclear spin, which leads to magnetic noise that dephases the Er³⁺ electron spin.
We present a new approach to incorporating Er³⁺ into nuclear spin free materials via ion implantation while achieving narrow (<4GHz) inhomogeneous optical linewidths. By implanting Er³ into crystalline hosts of appropriate symmetry, we are able to significantly reduce the effect of inhomogeneous broadening on the optical linewidths, observing few GHz linewidths prior to thermal processing. We use a hybrid optical-ESR technique to unambiguously relate optical and spin degrees of freedom in the system. These results present a general route forward for incorporating Er³⁺ and other ions into non-standard crystalline hosts.

Color centers in diamond are promising candidates for quantum networks, as they can serve as solid state quantum memories with efficient optical transitions. Prior work has focused on the NV- center in diamond, which exhibits long spin coherence times and has narrow, spin-conserving optical transitions. However, the NV- center is prone to spectral diffusion, and over 97% of emission is in an incoherent phonon side band, severely limiting scalability. Alternatively, SiV- exhibits excellent optical properties, with 70% of its emission in the zero phonon line and a narrow inhomogeneous linewidth. However, SiV- suffers from significant interaction with phonons, with spin coherence times limited by an orbital relaxation time (T1) of around 40 ns at 5 K.

Informed by the limitations of NV- and SiV-, we have developed new methods to control the diamond Fermi level to stabilize the neutral charge state of SiV, thus accessing a new spin configuration. SiV0 exhibits a spin T1 of around 1 minute at 4 K, coherence time (T2) approaching 1 second, over 90% of emission in the zero phonon line, and near-transform limited optical linewidths, making it a promising candidate for applications in quantum networks.

The neutral silicon-vacancy color center has S=1 spin with long longitudonal spin relaxation time, close-to-unity Debye-Waller factor, and its optical transition is intrinsically resistent against stray electric fields. These properties are attractive in quantum communication applications. Unlike the negatively charged silicon-vacancy color centers, the magneto-optical properties of the neutral silicon-vacancy center is not well understood. Furthermore, the other neutral XV split-vacancy complexes in diamond, where X and V labels a group-IV impurity atom of X = Si, Ge, Sn, Pb and the vacancy, are practically unknown, although the negatively charged counterparts can be fabricated in diamond.
Our first principles calculations reveal the charge state stability of the neutral XV split-vacany complexes in diamond which may guide the experiments to create these centers. In addition, we develop and apply ab initio theory to quantify the strength of electron–phonon coupling for neutral XV complexes in diamond, and find a significant impact on the corresponding optical properties of these centers. Our results show good agreement with recent experimental data on the prospective neutral SiV quantum bit, and reveals the complex nature of the excited states of neutral XV color centers in diamond. We predict the zero-phonon-line of neutral XV color centers in diamond and other magneto-optical properties.
Support from UNKP-17-3-III New National Excellence Program of the Ministry of Human Capacities of Hungary, the National Research Development and Innovation Office of Hungary within the Quantum Technology National Excellence Program (Project Contract No. 2017-1.2.1-NKP-2017-00001), and the European Commission of H2020 ASTERIQS project (Grant No. 820394) is acknowledged. We thank the National Information Infrastructure Development Program for the high-performance computing resources in Hungary.

Optically active spins coupled to single-mode diamond optical waveguides are efficient spin-photon interfaces for quantum networks. In a previous abstract, we described the unity creation, coupling and integration of single silicon vacancy (SiV) and germanium vacancy (GeV) in diamond waveguide arrays with a large-scale photonic integrated circuit (PIC) in aluminum nitride. Here, we present the detailed characterization of our 72-qubit quantum memory chip. Single-photon emission from 72 distinct optical channels are routed in the PIC and coupled to optical fibers. We performed low-temperature laser spectroscopy of the emitters, confirming that lifetime-limited optical transitions from both SiV and GeV centers are preserved following nanofabrication, micromanipulation and heterogeneous integration. With qubit energy and spin control, this fiber-coupled quantum photonic circuit establishes the prospects for high-efficiency, multiplexed quantum repeaters on a chip.

Entanglement is a core component in quantum information science including quantum computing and quantum repeater. Indistinguishability of two sub-systems is a prerequisite for measurement-based entanglement, and thus, its creation process is very susceptible to any kinds of local noise. In this work, we propose a noise resilient entanglement protocol based on detuned Rayleigh scattering of photons from atomic memories. In principle, near unity fidelity is possible in the presence of noise, and only limited by the energy splitting of state selective transitions. The protocol make use of a cyclic transition and is compatible with ancillary memory spins such as nuclear spins. The combination of these properties enables fault-tolerant quantum repeater with quantum error correcting nodes.

Point defects in hexagonal boron nitride (hBN) have recently emerged as potential single- photon emitters due to their high brightness, strong zero-phonon line and room temperature stability. However, the atomic structures of the point defects to which light emission in hBN has been attributed are still unknown. Several candidate defect structures have been proposed using density-functional theory (DFT) calculations, which have so far focused on the defect formation energy, structure, symmetry, and electronic energy levels; yet, direct comparisons with experiments remain inconclusive. In this talk, we present first-principles calculations of the optical excitation energy and radiative lifetimes of various defects in monolayer hBN; our results are obtained from DFT plus the GW-Bethe-Salpeter Equation (BSE) method, which properly accounts for dielectric screening and excitonic effects, and can provide accurate radiative lifetimes. We find that
the radiative lifetimes of various defect structures can differ by over an order of
magnitude due to microscopic details such as the defect optical transition dipoles and spin polarization. We identify several defects with optical transitions close to the 2 eV value found in experiments, including an NV-like defect with a two-level structure with 2.4 eV optical transition and a 72 ns radiative lifetime, as well as defects with ~20 ns radiative lifetimes. Our results provide a new approach for identifying promising defect structures for single photon emitters and quantum technology applications.

Current interest in quantum information processing provides a strong motivation for exploring hybrid quantum materials that interface topological phases of matter with magnetism and superconductivity [1]. This nexus can potentially lead to diverse emergent phenomena including topological superconductivity, axion electrodynamics, Majorana modes, and monopole superconductivity. We provide an overview of the colorful palette of quantum materials and epitaxial synthesis techniques that can yield quantum engineered hybrid materials platforms of interest in this context. Examples include magnetically-doped topological insulators that host robust quantum anomalous Hall insulator or axion insulator phases [2] and oxide-topological insulator interfaces that allow erasable spatial patterning of magnetic and chemical potential landscapes at the mesoscale [3]. Finally, we provide a perspective on the potential opportunities and the daunting challenges for experimentally realizing topological quantum computing platforms using these hybrid quantum materials [4].

Supported by the Penn State 2DCC-MIP under NSF grant (DMR-1539916).

[1] N. Samarth, "Quantum materials discovery from a synthesis perspective," Nature Materials 16, 1068-1076 (2017).
[2] Di Xiao et al., “Realization of the Axion Insulator State in Quantum Anomalous Hall Sandwich Heterostructures,” Phys. Rev. Lett. 120, 056801 (2018).
[3] A. Yeats et al., "Local optical control of ferromagnetism and chemical potential in a topological insulator," PNAS 114, 10379-10383 (2017).
[4] M. Kayyalha et al., "Non-Majorana Origin of the Half-Quantized Conductance Plateau in Quantum Anomalous Hall Insulator and Superconductor Hybrid Structures," arXiv:1904.06463.

One-dimensional hybrid superconductor/semiconductor (SU/SE) nanowires are promising material platforms for gatable superconducting device measurements.^1,2 In this research we grow stacking-faults-free InAs, InSb and InAs_0.3Sb_0.7 nanowire networks with epitaxial Al shadowed junctions in single-step growth process using molecular beam epitaxy (MBE). We compare different material systems and discuss associated pros and cons. We also study the formation of superconductor/semiconductor junctions and characterize the structural and electrical properties of the junctions. We show optimized junctions provide enhanced transport properties exhibiting repeatable quantized conductance for all three type of wires and potential host for Majorana bound states. Furthermore, we characterize both in-situ and post-growth processed junctions in a single nanowire and demonstrate in-situ SU-SE junctions exhibit clean surface morphology and enhanced electrical transport.

References
¹ P. Krogstrup et al. Nature Materials 14, (2015) 400-406.
² F. Krizek et al. Physical Review Materials vol. 2, (2018) 0934011-0934018.

The project was supported by the European Union Horizon 2020 research and innovation program under the Marie-Curie grant agreement No 722176, Microsoft Station Q, the European Research Council (ERC) under the grant agreement No.716655 (HEMs-DAM), and the Microsoft Quantum project.

Combining topology and superconductivity provides a powerful tool for investigating fundamental physics as well as a route to fault-tolerant quantum computing. There is mounting evidence that the Fe-Based Superconductor FeTe_0.55Se_0.45 (FTS) may also be topologically non-trivial. Should the superconducting order be s^±, then FTS could be a higher order topological superconductor with Helical Hinge Zero Modes (HHZM). To test the presence of these modes we’ve fabricated normal-metal/superconductor junctions on different surfaces via 2D atomic crystal heterostructures. As expected, junctions in contact with the hinge reveal a sharp zero-bias anomaly that is absent when tunneling purely into the c-axis. Additionally, the shape and suppression with temperature are consistent with highly coherent modes along the hinge, and are incongruous with other origins of zero bias anomalies. Furthermore, additional measurements with soft-point contacts in bulk samples with various Fe interstitial contents demonstrate the intrinsic nature of the observed mode. Thus we provide evidence that FTS is indeed a higher order topological superconductor.

The Majorana fermion based topologial quantum computing (QC) has immense potential for decreasing decoherence, which is one of greatest challenges for conventional QC. However, very few topological materials are known to date. To accelarte the identification of topological materials, we present a novel methodology to identify topologically non-trivial materials based on band inversion induced by spin-orbit coupling (SOC) effect. Specifically, we compare the density functional theory (DFT) based wavefunctions with and without spin-orbit coupling and compute the ‘spin-orbit-spillage’ as a measure of band-inversion. Due to its ease of calculation, without any need for symmetry analysis or dense k-point interpolation, the spillage is an excellent tool for identifying topologically non-trivial materials. Out of 30000 materials available in the JARVIS-DFT database, we applied this methodology to more than 4835 non-magnetic materials consisting of heavy atoms and low bandgaps. We found 1868 candidate materials with high-spillage (using 0.5 as a threshold). We validated our methodology by carrying out conventional Wannier-interpolation calculations for 289 candidate materials. We demonstrate that in addition to Z₂ topological insulators, this screening method successfully identified many semimetals and topological crystalline insulators. Importantly, our approach is applicable to the investigation of disordered or distorted as well as magnetic materials, because it is not based on symmetry considerations. We discuss some individual example materials, as well as trends throughout our dataset, which is available at the websites: https://www.ctcms.nist.gov/~knc6/JVASP.html and https://jarvis.nist.gov/.

Low loss microwave circuits are an essential ingredient that enables superconducting quantum processors. Most of the present day superconducting quantum computers rely on qubits and resonators with long life times to demonstrate basic quantum computing applications. To reach the goal of a fault tolerant universal quantum computer with superconducting circuits, coherence times still need to improve several orders of magnitude. Some improvements will likely come from better designs, fabrication processes, and materials used to build quantum processors. In this talk I will discuss the history of coherence improvements in qubits and current challenges in achieving low loss quantum circuits.

Rapid progress in being made towards optimizing materials to increase coherence superconducting quantum circuits. In particular, both intrinsic and extrinsic materials and effects have been studied. In the former, new techniques to study the effects of dielectric loss and the role of defects in and around junctions have been devised as well as new ideas for mitigating these effects and designing new types of qubits. For the latter, extrinsic causes of decoherence including the effects of radiation, quasiparticles, piezoelectricity, trapped flux, and excess photons. In this talk a general review of coherence and fabrication techniques for this circuits will be given with a few examples from the general literature. So of our work on intrinsic effects will be discussed, including studies of long-term measurements of highly coherent, non-tunable superconducting transmon qubits. Measurements of relaxation and dephasing are used to understand the low-frequency burst noise in coherence times and qubit transition frequency, leading to information about the microscopic origin of the intrinsic decoherence mechanisms in Josephson qubits. Progress towards a test-bed for studying loss in superconducting resonators and the associated analysis and interpretations will be presented. Interlaboratory comparisons of these analysis will be presented.

In recent years, silicon-based quantum dots have been shown to be a promising avenue for quantum computing. However, the near-degeneracy of the two low-lying valley states in silicon quantum wells can inhibit the formation of distinct quantum states. Motivated by the desire to increase the magnitude and tunability of this valley splitting, a Si/SiGe heterostructure is grown with a thin layer of SiGe embedded within the silicon quantum well. The Si/SiGe heterostructure is grown via UHV-CVD on a linearly graded SiGe alloy with a final germanium concentration of 29%. STEM measurements reveal the quantum well structure to consist of a ~10 nm silicon layer, followed by a thin ~1 nm SiGe layer, and subsequent ~2 nm layer of pure silicon. Above this quantum well, a ~35 nm layer of SiGe with 29% germanium is grown to separate the quantum well from the surface. The intent of this ~1 nm layer of SiGe, positioned just below the upper interface of the quantum well, is to modify the valley splitting of electrons in a 2-dimensional electron gas (2DEG) that reside near this interface. By varying an external vertical electric field, the electron wavefunction can be moved on and off this spike in germanium concentration, modifying the magnitude of the valley splitting.
We report electronic measurements of both Hall bars and electrically gated quantum dot devices that are fabricated on this heterostructure. Shubnikov-de Haas (SdH) and quantum Hall (QH) measurements reveal a peak transport mobility in excess of 100,000 cm²/(V s) at 6 x 10¹¹ cm^-2 carrier density. We report SdH and QH measurements over a wide range of carrier density and magnetic field in the form of a fan diagram. Valley splitting values are measured in the quantum dot device by magneto-spectroscopy, in which a few-electron dot transition is measured while an in-plane magnetic field is swept. Measuring at the second, third, and fourth electron transition in the quantum dot, we find valley splittings of 29, 48, and 65 μeV, respectively. To measure tunability of valley splitting, nearby gate voltages are changed to vary the vertical electric field at constant charge occupation. We find that both the lowest lying valley splitting and the valley splitting in the first excited orbital can be tuned over a factor of 2 by means of such changes in gate voltage. Additionally, we report modeling of the applied electric field at the dot in support of experimental results.

A series of recent high-impact researches on silicon-based quantum computers [1] has proven that it is crucial to prepare high quality silicon matrixes that are free of background nuclear spins. For example, electron spin qubits placed in a natural silicon substrate decoher quickly due to their interaction with background ²⁹Si nuclear spins that exist with the natural abundance of 4.7%. On the other hand, electron spin qubits placed in zero-nuclear-spin ²⁸Si stable isotope matrixes show much extended coherence time leading to increase in the possible quantum gate operation number and gate fidelity [1].
This talk will review the state-of-the-art in silicon isotope materials engineering [2] followed by in-depth discussions on how not only the background ²⁹Si nuclear spins but also the mass fluctuation caused by co-existence of ²⁸Si, ²⁹Si, and ³⁰Si isotopes can affect the properties of both electron and nuclear spin qubits placed in silicon. While majority of the results that will be presented are obtained in the present author’s group, some significant and relevant results achieved under the projects supported by the MEXT - Quantum Leap Flagship Program in Japan are also introduced.
This work has been supported by the Center for Spintronics Research Network, Keio University. The author acknowledges fruitful collaborations with Satoru Miyamoto, Noritaka Usami, Yusuke Hoshi, Seigo Tarucha’s group, Mark Eriksson’s group, Andrew Dzurak’s group and Andrea Morello’s group.

[1] W. Huang et al, “Fidelity benchmarks for two-qubit gates in silicon,” Nature, vol. 569, 7757 (2019), S. Asaad et al, “Coherent electrical control of a single high-spin nucleus in silicon,” arXiv:1906.01086, R. Zhao et al, “Coherent single-spin control with high-fidelity singlet-triplet readout in silicon,” arXiv:1812.08347, J. Yoneda et al, “A quantum-dot spin qubit with coherence limited by charge noise and fidelity higher than 99.9%,”Nature Nanotech. vol. 13, 102 (2018), etc.
[2] K. M. Itoh and H. Watanabe, "Isotope engineering of silicon and diamond for quantum computing and sensing applications," MRS Communications vol. 4, 143 (2014).

Semiconductor quantum dots constitute a promising platform for quantum computation. More than two decades of intensive research has led to impressive demonstrations of single-shot initialization and readout, coherent control, and coupling of single electron spins. Group IV materials have emerged as leading platforms, since they can be isotopically enriched to remove nuclear spin decoherence to provide excellent quantum coherence.

I will present our efforts on silicon and germanium quantum devices and discuss the opportunities in these systems. In silicon, these include operation at elevated temperatures for hot-qubit operation, enabling to integrate classical electronics on the same chip for scalability and superior control. In germanium, I will show fast single and two qubit logic, and semiconductor-superconductor devices for novel hybrid quantum hardware. Moving forward, I will discuss our vision to increase the number of qubits toward practical quantum information.

Metal-semiconductor-metal heterostructures are attractive for both fundamental studies of low-dimensional nanostructures as well as future high-performance low power dissipating nanoelectronic and quantum devices. Most notably, they bear enormous potential for a vast array of key components for quantum computing such as SQUIDs, oscillators, mixers and amplifiers. In this context, combining high carrier mobilities and leveraging strong quantum confinement effects due to a more than five times larger exciton Bohr radius compared to Si, Ge occupies an exceptional position for the development of novel quantum devices in the post Si era.
Here, we demonstrate that utilizing a thermally induced exchange reaction between single-crystalline Ge nanowires and Al pads, monolithic Al-Ge-Al nanowire heterostructures with ultra-small Ge segments contacted by self-aligned, quasi-1D, crystalline Al leads can be fabricated without lithographic constraints. High-resolution transmission electron microscopy and energy dispersive X-ray spectroscopy proved the composition and perfect crystallinity of these metal−semiconductor nanowire heterostructures. Integrating such nanowire heterostructures as active channels in electrostatically gated field-effect transistor devices, provides a platform for the systematic investigation of electrical transport mechanisms in ultra-scaled Ge nanowires. In contrast to common short channel devices, the 1D monolithic metal-semiconductor-metal architecture effectively prevents screening of the gate electric field by lithographically defined contacts and thus enables perfect electrostatic control of field effect devices.
We will demonstrate ballistic transport as well as quantum ballistic transport phenomena, which were systematically investigated depending on the Ge channel length and nanowire diameter. Temperature depending measurements in the range between 5 K and 300 K of the resistivity and gate-dependent conductance measurements including a detailed bias spectroscopy study revealed transport through spin-degenerate 1D sub-bands in ultra-scaled Ge channels up to room temperature.
Finally, we will present low temperature (400 mK) DC spectroscopy measurements, revealing the high gate-tunability of hole transport in ultra-scaled Ge channels monolithically integrated in Al-Ge-Al nanowire heterostructures. In particular, we will show the ability to tune the Ge segment from a completely insulating regime, through a low conductive regime, that exhibits properties of a single-hole transistor, to a superconducting regime, resembling a Josephson field effect transistor with a maximum critical current of 15 nA. The experimental proof of exchanging cooper-pairs between superconducting Al leads through a gate-tunable Ge channel, mediated by the superconducting proximity effect is the first demonstration of superconductivity induced in a pure Ge channel.

For quantum dots (QD) in the silicon MOS system, the local potential is a defining factor in device performance. Silicon-based devices experience a local potential modulation at the edge of lithographically defined gates due to mechanical strain. The strain originates in the mismatch of the coefficient of thermal expansion (CTE) between silicon and the gate material as well as the intrinsic strain from the deposition process. The resulting potential modulation is large enough to alter the tunnel coupling or induce unintentional QDs at low temperatures. Here, we present measurements of the barrier height in tunnel junction devices defined by aluminum and titanium gates. These measurements reveal the difference in barrier height between the two materials due to strain. Our results agree with COMSOL simulations which use the CTE value achieved in the deposition and strongly disagree with simulations which use the bulk CTE value. These results show that using simulations alone as a guide towards improving devices performance is inadequate thus highlighting the need for more experimental work.

Highly tensile strained Ge quantum well (QW) is a long-sought-for low-dimensional system to investigate and develop hole spin-based qubits and high hole mobility electronics. Under tensile in-plane biaxial strain, light-hole (LH) occupies the top of the valence band and the LH-HH splitting can exceed 100s meV. Moreover, when the tensile in-plane biaxial strain exceeds 1.5% only LH is confined in the QW thus minimizing LH-HH interactions. With this perspective, this work provides the first experimental demonstration of the epitaxial growth of tensile strained Ge QW on Si substrates. Sn-rich GeSn layers are used as barriers to achieve high tensile in-plane biaxial strain and engineering the band structure for LH confinement. We will discuss the epitaxial growth of this tensile-strained GeSn/Ge/GeSn heterostructure exhibiting atomically-sharp interfaces and in-plane biaxial strain above 1.5 % grown on a lattice-mismatched Ge_0.86Sn_0.14 substrate. The sharpness of the interfaces across the Ge/GeSn heterostructure will be addressed using EELS and atom probe tomography (APT) measurements. A tunable s-Ge layer thickness ranging from ~13 nm down to ~1.2 nm is achieved. The pseudomorphic nature of the s-Ge layer will be discussed based on TEM/STEM and XRD-RSM measurements, while dislocations are mainly observed in the low Sn content buffer layers.[1,2] Ongoing work focuses on band structure calculations and low-temperature magneto-electric transport measurements to probe the confined LH states in the grown s-Ge QWs.[3]
References
[1] S. Assali, et al., “Atomically uniform Sn-rich GeSn semiconductors with 3.0-3.5 μm room-temperature optical emission”, Appl. Phys. Lett. 112, 251903 (2018).
[2] S. Assali, et al., “Enhanced Sn incorporation in GeSn epitaxial semiconductors via strain relaxation”, J. Appl. Phys. 125, 025304 (2019).
[3] S. Assali, et al., “Highly tensile strained Ge light-hole quantum well on Si”, submitted.

To date various materials have been used to fabricate spin qubit devices for quantum computation. Some devices are advantageous for fast qubit operations and others for low noise operations or weak decoherence. Among them isotopically purified silicon is one of the appropriate choices to achieve high-fidelity in single and two-qubit gates. So far we have used quantum dots (QDs) made in GaAs, natural Si/SiGe, and isotopically purified 28Si/SiGe materials to implement high-fidelity spin qubits and study the decoherence mechanisms. Decoherene generally arises from coupling of spin to the environment, which is a source of magnetic and charge noise. In this talk I will first discuss the noise source to the electron spin in the GaAs, natural Si, and 28Si QDs equipped with a miro-magnet. We measure the noise dynamics of the environment to fluctuate the Zeeman field or Larmor precession on the spin, and assign the fidelity limiting factor to nuclear spin noise in GaAs and natural Si, while 1/f-like charge noise in 28Si. For the 28Si/SiGe the highest fidelity of 99.93% has been achieved, and to further raise the fidelity it is crucial to eliminate the charge noise. We have developed a feedback technique to suppress the low frequency magnetic noise in GaAs QDs, and then observed enhancement of the decoherence time but limited by the 1/f-like charge noise same as in 28Si/SiGe QDs. Secondly I will discuss cross-correlated noise in Si QD spin qubits. We used the Ramsey interference protocol to evaluate the power spectral densities of correlated and anti-correlated noise between two Rabi frequencies. We observed positively cross-correlated noise in the low-frequency range. Such noise correlation between qubits may influence two-spin entanglement control and quantum error correction performance.

For decades, Si-compatible low-dimensional systems and quantum devices have been exploiting either tensile strained Si or compressively strained Germanium (Ge) quantum wells (QWs), which are the only group IV systems that can currently be routinely obtained using SiGe as growth template and barrier layers. For quantum information, the former has been used as the building block for electron spin qubits, whereas the latter has been explored in new schemes for hole spin qubits. Herein, we present a third low-dimension system consisting of highly tensile strained Ge QW integrated on an optically active platform and discuss its basic properties experimentally and theoretically. The growth of tensile strained Ge QW is achieved using direct bandgap GeSn as barrier layers grown on silicon wafers. This heterostructure yields high tensile strain in Ge QW and band structure corresponding to a sizable light hole-heavy hole (LH-HH) splitting exceeding 100 meV. Unlike compressively strained Ge, the top of the valence band is occupied by LH in tensile strained Ge. Interestingly, by tuning the tensile strain above 1.5%, only LH can be confined in the QW and LH-HH mixing can be strongly reduced. We also found a high LH g-factor anisotropy in the GeSn/Ge/GeSn QW, with g = 21.8 for in-plane B-field and g = 0.69 for perpendicular-to-plane field.

These properties lay the groundwork to implement LH spin qubits with potentially easier manipulation due to the combined effects of the large Rashba-type spin-orbit interaction (SOI), the absence of Dresselhaus SOI, and the spin ½ of LH. Indeed, for quantum computing, tensile strained Ge QW combines all the sought-for advantages of Ge in qubits including: (i) high hole mobility; (ii) holes have much stronger spin-orbit coupling, which can be exploited to achieve much faster spin manipulations; (iii) the fourfold degeneracy of the valence band, which is not compatible with the two-level system at the core of quantum system, is eliminated by introducing strain and quantum confinement in the QW. This leads to a reduced hole effective mass and a large intrinsic splitting between LH and HH bands, which can enable high mobilities and tunnel rates in addition to eliminating the need for smaller quantum dot size; and (iv) a reduced hyperfine interaction with surrounding nuclear spins for improved quantum coherence as Ge can be purified to remove nuclear spin-full ⁷³Ge impurities.

Ongoing work includes transport measurements of the fabricated QWs, which will give a clear description of the hole mobility within the structure. Additionally, a detailed discussion of the mechanisms of spin manipulation within such heterostructures will be presented, including various spin manipulation schemes such as EDSR and g-tensor modulation resonance.

Material development holds promise as the basis of topological quantum computing with Majorana fermions. These quasiparticles have been predicted to be formed in semiconductor nanowires coupled to conventional superconductors.^1-2 This prediction was followed by a series of experiments providing strong evidence.^3-4 However, in the current system, an external magnetic field along the NW axis is always needed to realize Majorana states. Therefore, in order to integrate and scale up qubit devices, it is aimed to induce a self-sustaining parallel magnetic field on semiconductor–superconductor hybrid NWs. Composite materials using ferromagnetic insulators (FMIs) in close proximity to a semiconductor – superconductor structure have been proposed as a solution to reach a zero-field topological state⁵, where the effective Zeeman splitting is induced by an magnetic exchange coupling by the FMI. In this work, we grow epitaxial semiconductor – ferromagnetic insulator – superconductor InAs-EuS-Al hybrid nanowires in-situ in the molecular beam epitaxy system. The results show the superconducting hard gap, the hysteretic evolution of density of states as a function of magnetic field and the shape-defined magnetic single domain structures based on well-controlled epitaxy, which suggests that this highly ordered material system is a promising platform for scalable topological quantum computing.

References
1. R. M. Lutchyn et al, Phys. Rev. Lett. 105, 077001 (2010)
2. Y. Oreg et al, Phys. Rev. Lett. 105, 177002 (2010)
3. V. Mourik et al, Science 336, 1003 (2012)
4. A. Das et al, Nat. Phys. 8, 887 (2012)
5. J. D. Sau et al, Phys. Rev. Lett. 104, 040502 (2010)

We have measured the loss in a superconducting thin film titanium nitride microwave resonator from 20 mK to 1 K and at different stored microwave powers. The titanium nitride film had a superconducting transition temperature of Tc = 5.5 K and was grown in an MBE by reactively reacting evaporated titanium in a nitrogen plasma. The measured internal loss of the resonator is found to decrease by approximately a factor of 10 from Q^-1 = 2.5 x 10^-6 at low applied microwave powers and low temperatures to Q^-1 = 2.5 x 10^-7 at large applied powers and a temperature of 600 mK. We compare the measured Q data to a model based on loss from the interaction of the superconducting resonator with lossy two-level systems and separately to a model we have developed based on non-equilibrium quasiparticles accumulating in regions of the TiNx film with a lower superconducting gap (D). To distinguish between these competing models, we will also discuss results where we apply superconducting pair-breaking infrared light directly to the resonator device and measure the loss.

The development of predictable superconducting circuits incorporating Josephson Junctions (J-Junctions) is essential for developing scalable quantum computing devices. Here we report on atomic resolution analysis of J-Junctions consisting of two polycrystalline Al layers separated by a barrier layer of amorphous AlOx. Samples were also probed with analytical (S)TEM techniques such as EDS and EELS, and correlated with the performance of the device. Our results demonstrate that both the top-Al and the bottom-Al layers comprise columnar grains of around 30 nm with a AlOx barrier layer approximately 3-5 nm thick. Our analysis will discuss how the e-beam evaporation process effects the microstructure of the resulting layers. For instance, the roughness of the Al/AlOx interface seems to be related to grain boundaries in the Al layers. The effect of residual strain will also be discussed, correlated with device performance and analysis of the local strain field with scanning nanobeam diffraction (4D-STEM).

Majorana bound states (MBSs) are spatially-localized zero-energy fractional quasiparticles with non-Abelian braiding statistics that hold great promise for topological quantum computing. By using scanning tunneling microscopy/spectroscopy (STM/STS), here we report the observation of MBSs in a single material platform of high-T_c iron-based superconductors, FeTe_0.55Se_0.45, which combined advantages of simple material, high- T_c, and large ratio of Δ/E_F. We observed a sharp zero-bias peak inside a vortex core that does not split when moving away from the vortex center. The evolution of the peak under varying magnetic field, temperature, and tunneling barrier is consistent with the tunneling to a nearly pure MBS, separated from nontopological bound states [1]. We further investigated the MBS and observed a conductance plateau feature, which is protected by the Majorana particle-hole symmetry [2]. The observations not only prove a strong evidence of MBSs in this iron-based superconductor, but also offer a single-material platform for Majorana braiding at relatively high temperature.

[1] Science 362, 333 (2018)
[2] arXiv:1904.06124 (submitted to Science on February 15, 2019)

We have used Ramsey tomography to characterize charge noise in a weakly charge-sensitive superconducting qubit. We find a charge noise that scales with frequency as 1/f^α over 5 decades with α = 1.93 and a magnitude S_q(1 Hz)= 2.9 x 10^-4 e²/Hz. The noise exponent and magnitude of the low-frequency noise are much larger than those seen in prior work on single electron transistors, yet are consistent with reports of frequency noise in other superconducting qubits. Moreover, we observe frequent large-amplitude jumps in offset charge exceeding 0.1e; these large discrete charge jumps are incompatible with a picture of localized dipole-like two-level fluctuators. The data reveal an unexpected dependence of charge noise on device scale and suggest models involving either charge drift or fluctuating patch potentials. We describe follow-on experiments to systematically explore the dependence of charge noise on device geometry.

In spin-based quantum computing, the spin readout is achieved through spin-to-charge conversion and single-electron charge detection. Often, single-electron transistors (SETs) capacitively coupled to the qubit are used for sensing the charge transition. Since the SET control curve is sensitive to the electrostatic environment, the local charge stability is of great importance. Although metallic SETs are simple to fabricate, historically they were found to be very unstable, preventing use as charge sensors. We revisit this challenge by using plasma oxidized aluminum oxide (AlO_x) tunnel barrier based SETs. We have successfully overcome the charge offset drift issue previously seen in metallic Al/AlO_x/Al SETs. The long-term charge stability presented here are better than any other reported metallic SETs, with standard deviation of < 0.01 e over 7.5 days. To evaluate the charge sensing ability, we plan to couple the plasma oxidized Al/AlO_x/Al SET to a quantum dot and sense the charge of the quantum dot. By modeling in FastCap and using the data, a 1 e charge change on the quantum dot will cause a measurable current shift in the SET control curve. This charge sensing measurement will be a significant step for the application of ultra-stable Al/AlO_x/Al SETs in a quantum computing architecture.

Phosphorus dopants placed with atomic precision using scanning tunneling microscopy-based hydrogen depassivation lithography on silicon (Si:P) have long been considered a leading pathway to both high-performance conventional and quantum electronic devices. However, the realization of practical Si:P device architectures requires low processing temperatures to keep the carefully placed dopants from diffusing.

Among recent developments in the field are advanced molecular beam epitaxy (MBE) techniques and integration of microfabrication processes like ion implantation and surface gates. Much of this effort has focused on preserving the atomic fidelity of Si:P devices by minimizing P dopant diffusion and segregation. However, the incorporation of vacancies and contaminants into both the P dopant layer and Si capping layer also significantly impacts the performance of resulting devices and must often be balanced with the need for low process temperatures. An improved understanding of the whole material system is necessary for optimization of devices and systems, particularly for integration with conventional CMOS technology.

Here, we report on the presence and influence of atomic defects in both the dopant layer and Si overgrowth, the techniques available for characterization of these defects, and the influence of defects on the properties of the resulting Si:P devices. Using a combination of optical spectroscopy, transport measurements, and transmission electron microscopy, we relate the appearance of defects in the Si:P and Si films to parameters of the fabrication process and the electronic characteristics of the resulting devices. This is directly applicable to the next generation of digital electronics at the atomic limits of precision and has implications for quantum applications.

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 in this abstract do not necessary represent the views of the U.S. DOE or the United States Government.

Hybrid epitaxial semiconductor-superconductor materials have recently shown promise as a platform for topological quantum computing applications. Targeted engineering of their key ingredient, the electronic hybridization, requires detailed understanding of the electrostatic properties, in particular the alignment between the band edges of the semiconductor and the Fermi level, in the vicinity of the interface. In this work we use a combination of angle-resolved photoelectron spectroscopy (ARPES) measurements, Schroedinger-Poisson (SP) calculations and a core level fitting procedure to obtain the band offset and other fundamental electronic structure parameters of a promising materials platform for topological Majorana devices, the buried InAs(100)/Al interface. Furthermore, we demonstrate how starting from the investigation of the pristine InAs(100) surface, this combined approach makes it possible to access the electrostatic properties of any heterostructure with the same substrate on which a sequence of core level spectra can be obtained. To verify the validity of our approach we compare the result with the first direct observation of a buried InAs/Al electron accumulation layer.

Atomic ions arrayed and manipulated via electromagnetic fields form the basis for one of the leading candidate systems for quantum computing, with potential applications in quantum sensing and communications as well. While ion traps based on a combination of radio-frequency and static electric fields applied to microfabricated electrode structures have emerged as a powerful platform for this technology, electric-field noise originating from the electrode surfaces of small traps can limit quantum operation fidelity. Though the mechanisms behind this noise are not completely understood, recent experiments have uncovered evidence that is helping to constrain possible theories. I will describe our experimental work utilizing temperature variation and ion-milling treatment of microfabricated traps made from multiple materials in order to better understand and potentially mitigate ion heating due to electric-field noise.

Van der Waals (vdW) materials and their heterostructures have been extensively studied and broadly applied, owing to their extraordinary and versatile electronic properties in combination with epitaxially-precise interfaces. While vdW heterostructures have been well investigated via DC transport and optical techniques, experimental exploration in the microwave regime has heretofore been relatively limited. This has left an unexplored potential for incorporating the advantages of vdW materials into the circuit quantum electrodynamics (cQED) platform of relevance to advancing quantum technologies.

Two-level-systems (TLS) are identified as a major coherence-limiting factor that exist both within and at the interface of amorphous oxide found in current qubit devices. To investigate the concentration of TLSs in vdW dielectric materials, we recently studied the microwave loss of hexagonal boron nitride (hBN), a dielectric that can be used to both encapsulate qubits and to serve as the tunneling barrier of Josephson junctions. We were able to extract a lower bound for the hBN quality factor of ~3x10⁵, at least two orders of magnitude higher than that of amorphous dielectric materials such as AlOx or SiOx commonly used in current quantum devices.

Motivated by these promising results, in this work, we fabricate and characterize high-quality, all-vdW Josephson junctions, a key component in superconducting quantum circuits. VdW heterostructures consisting of 2D transition metal dichalcogenide (TMD) superconductors and thin layers of hBN are assembled together in an inert (argon-filled) environment, which promotes pristine materials with atomically flat interfaces. These procedures may significantly reduce the number of two-level-systems. We measure the vdW tunnel junction in temperature well below the superconducting gap and characterize its DC and AC Josephson response. We expect these efforts will lead to high-coherence qubit devices with small form factors.

This research was funded in part by the ARO grant No. W911NF-17-S-0001, and by the Department of Defense via MIT Lincoln Laboratory under AF Contract No. FA8721-05-C-0002. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the U.S. Government.