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We report on fabrication and performance of submicron Ni/Au/Ge contacts to a two-dimensional electron gas in an AlGaAs/GaAs heterostructure. Utilizing scanning transmission electron microscopy, energy dispersive x-ray spectroscopy, and low temperature electrical measurements we investigate the relationship between contact performance and the mechanical and chemical properties of the annealed metal stack. Contact geometry and crystallographic orientation significantly impact performance. Our results indicate that the spatial distribution of germanium in the annealed contact plays a central role in the creation of high transmission contacts. We characterize the transmission of our contacts at high magnetic fields in the quantum Hall regime. Our work establishes that contacts with area 0.5 square microns and resistance less than 400 Ohms can be fabricated with high yield.

We demonstrate transport in 2D arrays of multiterminal $\varphi_0$-junctions. When applying an in-plane magnetic field we observe nonreciprocal vortex depinning currents, induced by a ratchet-like pinning potential. The ratchet effect is explained as a consequence of spontaneous supercurrents that arise in the presence of next-nearest neighbor Josephson couplings. Tuning the density of vortices to commensurate values of the frustration parameter results in an enhancement of the ratchet effect. In addition, we find a surprising sign reversal of the ratchet effect near frustration 1/3., Comment: 15 pages, 11 Figures

Using comprehensive x-ray reciprocal space mapping, we establish the precise lattice-matching composition for wurtzite $Sc_xAl_{1-x}N$ layers on (0001) GaN to be x = 0.14+/-0.01. 100-nm thick $Sc_xAl_{1-x}N$ films (x = 0.09 - 0.19) were grown in small composition increments on c-plane GaN templates by plasma-assisted molecular beam epitaxy. The alloy composition was estimated from the fit of the (0002) x-ray peak positions assuming the c-lattice parameter of ScAlN films coherently-strained on GaN increases linearly with Sc-content determined independently by Rutherford Backscattering Spectrometry. Reciprocal space maps obtained from high-resolution x-ray diffraction measurements of the (10-15) reflection reveal that $Sc_xAl_{1-x}N$ films with x = 0.14+/-0.01 are coherently strained with the GaN substrate while the other compositions show evidence of relaxation. The in-plane lattice-matching with GaN is further confirmed for a 300-nm thick $Sc_{0.14}Al_{0.86}N$ layer. The full-width-at-half-maximum of the (0002) reflection rocking curve for this $Sc_{0.14}Al_{0.86}N$ film is 106 arcseconds and corresponds to the lowest value reported in the literature for wurtzite ScAlN films.

Environmental fluctuations degrade the performance of solid-state qubits but can in principle be mitigated by real-time Hamiltonian estimation down to time scales set by the estimation efficiency. We implement a physics-informed and an adaptive Bayesian estimation strategy and apply them in real time to a semiconductor spin qubit. The physics-informed strategy propagates a probability distribution inside the quantum controller according to the Fokker-Planck equation, appropriate for describing the effects of nuclear spin diffusion in gallium-arsenide. Evaluating and narrowing the anticipated distribution by a predetermined qubit probe sequence enables improved dynamical tracking of the uncontrolled magnetic field gradient within the singlet-triplet qubit. The adaptive strategy replaces the probe sequence by a small number of qubit probe cycles, with each probe time conditioned on the previous measurement outcomes, thereby further increasing the estimation efficiency. The combined real-time estimation strategy efficiently tracks low-frequency nuclear spin fluctuations in solid-state qubits, and can be applied to other qubit platforms by tailoring the appropriate update equation to capture their distinct noise sources., Comment: 21 pages, 10 figures, including 11-page 6-figure Supplemental Material

Artificial Kitaev chains can be used to engineer Majorana bound states (MBSs) in superconductor-semiconductor hybrids. In this work, we realize a two-site Kitaev chain in a two-dimensional electrongas by coupling two quantum dots through a region proximitized by a superconductor. We demonstrate systematic control over inter-dot couplings through in-plane rotations of the magnetic field and via electrostatic gating of the proximitized region. This allows us to tune the system to sweet spots in parameter space, where robust correlated zero bias conductance peaks are observed in tunnelling spectroscopy. To study the extent of hybridization between localized MBSs, we probe the evolution of the energy spectrum with magnetic field and estimate the Majorana polarization, an important metric for Majorana-based qubits. The implementation of a Kitaev chain on a scalable and flexible 2D platform provides a realistic path towards more advanced experiments that require manipulation and readout of multiple MBSs., Comment: 20 pages, 14 figures. Final version after publication. New title compared to previous versions due to character constraints

Recent theoretical studies have highlighted the role of spatially varying cavity electromagnetic fields in exploring novel cavity quantum electrodynamics (cQED) phenomena, such as the potential realization of the elusive Dicke superradiant phase transition. One-dimensional photonic-crystal cavities (PCCs), widely used for studying solid-state cQED systems, have uniform spatial profiles in the lateral plane. Three-dimensional (3D) PCCs, which exhibit discrete in-plane translational symmetry, overcome this limitation, but fabrication challenges have hindered the achievement of strong coupling in 3D-PCCs. Here, we report the realization of multimode ultrastrong coupling in a 3D-PCC at terahertz frequencies. The multimode coupling between the 3D-PCC's cavity modes and the cyclotron resonance of a Landau-quantized two-dimensional electron gas in GaAs is significantly influenced by the spatial profiles of the cavity modes, leading to distinct coupling scenarios depending on the probe polarization. Our experimental results are in excellent agreement with a multimode extended Hopfield model that accounts for the spatial inhomogeneity of the cavity field. Guided by the model, we discuss the possible strong ground-state correlations between different cavity modes and introduce relevant figures of merit for the multimode ultrastrong coupling regime. Our findings emphasize the importance of spatially nonuniform cavity mode profiles in probing nonintuitive quantum phenomena expected for the ground states of cQED systems in the ultrastrong coupling regime., Comment: 36 pages, 13 figures

Optimal control of qubits requires the ability to adapt continuously to their ever-changing environment. We demonstrate a real-time control protocol for a two-electron singlet-triplet qubit with two fluctuating Hamiltonian parameters. Our approach leverages single-shot readout classification and dynamic waveform generation, allowing full Hamiltonian estimation to dynamically stabilize and optimize the qubit performance. Powered by a field-programmable gate array (FPGA), the quantum control electronics estimates the Overhauser field gradient between the two electrons in real time, enabling controlled Overhauser-driven spin rotations and thus bypassing the need for micromagnets or nuclear polarization protocols. It also estimates the exchange interaction between the two electrons and adjusts their detuning, resulting in extended coherence of Hadamard rotations when correcting for fluctuations of both qubit axes. Our study emphasizes the critical role of feedback in enhancing the performance and stability of quantum devices affected by quasistatic noise. Feedback will play an essential role in improving performance in various qubit implementations that go beyond spin qubits, helping realize the full potential of quantum devices for quantum technology applications.

In Josephson diodes the asymmetry between positive and negative current branch of the current-phase relation leads to a polarity-dependent critical current and Josephson inductance. The supercurrent nonreciprocity can be described as a consequence of the anomalous Josephson effect -- a $\varphi_0$-shift of the current-phase relation -- in multichannel ballistic junctions with strong spin-orbit interaction. In this work, we simultaneously investigate $\varphi_0$-shift and supercurrent diode efficiency on the same Josephson junction by means of a superconducting quantum interferometer. By electrostatic gating, we reveal a direct link between $\varphi_0$-shift and diode effect. Our findings show that the supercurrent diode effect mainly results from magnetochiral anisotropy induced by spin-orbit interaction in combination with a Zeeman field., Comment: 15 pages, 8 figures

Superconducting qubits with intrinsic noise protection offer a promising approach to improve the coherence of quantum information. Crucial to such protected qubits is the encoding of the logical quantum states into wavefunctions with disjoint support. Such encoding can be achieved by a Josephson element with an unusual charge-4e supercurrent emerging from the coherent transfer of pairs of Cooper-pairs. In this work, we demonstrate the controlled conversion of a conventional charge-2e dominated to a charge-4e dominated supercurrent in a superconducting quantum interference device (SQUID) consisting of gate-tunable planar Josephson junctions (JJs). We investigate the ac Josephson effect of the SQUID and measure a dominant photon emission at twice the fundamental Josephson frequency together with a doubling of the number of Shapiro steps, both consistent with the appearance of charge-4e supercurrent. Our results present a step towards novel protected superconducting qubits based on superconductor-semiconductor hybrid materials.

Superconducting devices with broken time-reversal and spatial-inversion symmetries can exhibit novel superconducting phenomena. The observation of superconducting diode effects, which is applicable for dissipationless rectification, provides information on the breaking of such symmetries. We experimentally study a Josephson junction (JJ) coupled to another adjacent JJ as a new system exhibiting the superconducting diode effect. We demonstrate that the observed superconducting diode effect can be controlled non-locally based on the phase difference with the adjacent JJ. These results indicate that the time-reversal and spatial-inversion symmetries of a JJ are broken by the coherent coupling to an adjacent JJ, and this enables the engineering of novel superconducting phenomena mediated by coherent coupling among JJs and development of their applications for superconducting diode devices.

A Josephson junction (JJ) is a key device in the development of superconducting circuits, wherein a supercurrent in the JJ is controlled by the phase difference between the two superconducting electrodes. Recently, it has been shown that the JJ current is nonlocally controlled by the phase difference of another nearby JJ via coherent coupling. Here, we use the nonlocal control to engineer the anomalous Josephson effect. We observe that a supercurrent is produced by the nonlocal phase control even without any local phase difference, using a quantum interference device. The nonlocal phase control simultaneously generates an offset of a local phase difference giving the JJ ground state. These results provide novel concepts for engineering superconducting devices such as phase batteries and dissipationless rectifiers., Comment: 15 pages and 4 figures for the main manuscript, 8 pages and 6 figures for the supplementary information

Electronic Fabry-P{\'e}rot interferometry is a powerful method to probe quasiparticle charge and anyonic braiding statistics in the fractional quantum Hall regime. We extend this technique to the hierarchy $\nu = 2/5$ fractional quantum Hall state, possessing two edge modes that in our device can be interfered independently. The outer edge mode exhibits interference similar to the behavior observed at the $\nu = 1/3$ state, indicating that the outer edge mode at $\nu = 2/5$ has properties similar to the single mode at $\nu = 1/3$. The inner mode shows an oscillation pattern with a series of discrete phase jumps indicative of distinct anyonic braiding statistics. After taking into account the impact of bulk-edge coupling, we extract an interfering quasiparticle charge ${e^*} = 0.17 \pm 0.02$ and anyonic braiding phase $\theta _a = (-0.43 \pm 0.05)\times 2\pi$, which serve as experimental verification of the theoretically predicted values of $e^* = \frac{1}{5}$ and $\theta _a = -\frac{4\pi}{5}$., Comment: 10+12 pages, 6+12 figures

The Josephson diode (JD) is a non-reciprocal circuit element that supports a larger critical current in one direction compared to the other. This effect has gained a growing interest because of promising applications in superconducting electronic circuits with low power consumption. Some implementations of a JD rely on breaking the inversion symmetry in the material used to realize Josephson junctions (JJs), but two recent theoretical proposals have suggested that the effect can also be engineered by combining two JJs hosting highly transmitting Andreev bound states in a Superconducting Quantum Interference Device (SQUID) at a small, but finite flux bias [1, 2]. We have realized a SQUID with two JJs fabricated in a proximitized InAs two-dimensional electron gas (2DEG). We demonstrate gate control of the diode efficiency from zero up to around 30% at specific flux bias values which comes close to the maximum of approxiomately 40% predicated in Ref. [1]. The key ingredients to the JD effect in the SQUID arrangement is the presence of highly transmitting channels in the JJs, a flux bias and an asymmetry between the two SQUID arms., Comment: 14 pages, 9 figures (main text + appendix all in one file)

The Josephson junction (JJ) is an essential element of superconducting (SC) devices for both fundamental and applied physics. The short-range coherent coupling of two adjacent JJs forms the Andreev molecule states (AMSs), which will provide a new ingredient to engineer the SC transport in JJs and control the Andreev qubits. However, no experimental evidence of the AMSs in the coupled JJs has been reported. Here we provide the tunnel spectroscopic results of electrically controllable two planar JJs sharing one SC electrode. We discover that the coupled JJ results are highly modulated from the single JJ results, due to formation of the phase-dependent AMSs, meaning that the two JJs are coherently coupled. In addition, the superconducting gap closing due to the AMS formation is observed. Our results would help in understanding the microscopic mechanism of the coherent coupling and promoting the AMS physics to apply for research of the topological superconductivity and quantum information technology.

A single electron shared between two levels threaded by a magnetic flux is an irreducibly simple quantum system in which interference is predicted to occur. We demonstrate tuning of the tunnel coupling between two such electronic levels with flux, implemented in a loop comprising two quantum dots. Using radio-frequency reflectometry of the dots' gate electrodes we extract the inter-dot coupling, which exhibits oscillations with a periodicity of one flux quantum. In different tunneling regimes we benchmark the oscillations' contrast, and find that their amplitude varies with the levels involved, while tunneling is generically not suppressed at oscillation minima. These results establish the feasibility and limitations of parity readout of qubits with tunnel couplings tuned by flux., Comment: 15 pages, 10 figures. Version as submitted to a scientific journal with updated author list, corrected figure S5, and minor changes to data analysis

The two-dimensional electron gas residing in shallow InAs quantum wells coupled to epitaxial aluminum is a widely utilized platform for exploration of topological superconductivity. Strong spin-orbit coupling, large effective $g$-factor, and control over proximity-induced superconductivity are important attributes. Disorder in shallow semiconductor structures plays a crucial role for the stability of putative topological phases in hybrid structures. We report on the transport properties of 2DEGs residing 10nm below the surface in shallow InAs quantum wells in which mobility may exceed 100,000 cm$^2$/Vs at 2DEG density n$_{2DEG}$$\leq$1$\times$10$^{12}$cm$^{-2}$ at low temperature., Comment: 11 pages, 10 figures

Carrier transport in materials is often diffusive due to momentum-relaxing scattering with phonons and defects. Suppression of momentum-relaxing scattering can lead to the ballistic and hydrodynamic transport regimes, wherein complex non-Ohmic current flow patterns, including current vortices, can emerge. In the ballistic regime addressed here, transverse magnetic focusing is habitually understood in a familiar single-particle picture of carriers injected from a source, following ballistic cyclotron orbits and reaching a detector. We report on a distinctive nonlocal magnetoresistance phenomenon exclusive to fermions, in an enclosed mesoscopic geometry wherein transverse focusing magnetoresistance peaks also occur at values of the cyclotron diameter that are incommensurate with the distance between the source and detector. In low-temperature experiments and simulations using GaAs/AlGaAs heterostructures with high electron mobility, we show that the peaks occur independently of the location of the detector, and only depend on the source-drain separation. We reproduce the experimental findings using simulations of ballistic transport in both semiclassical and quantum-coherent transport models. The periodicity of magnetic field at which the peaks occur is matched to the lithographically defined device scale. It is found that, unlike in transverse magnetic focusing, the magnetoresistance structure cannot be attributed to any set of ordered single-particle trajectories but instead requires accounting for the collective dynamics of the fermion distribution and of all particle trajectories. The magnetoresistance is further associated with current flow vorticity, a collective phenomenon., Comment: 11 figures

In analogy to conventional semiconductor diodes, the Josephson diode exhibits superconducting properties that are asymmetric in applied bias. The effect has been investigated in number of systems recently, and requires a combination of broken time-reversal and inversion symmetries. We demonstrate a dual of the usual Josephson diode effect, a nonreciprocal response of Andreev bound states to a superconducting phase difference across the normal region of a superconductor-normal-superconductor Josephson junction, fabricated using an epitaxial InAs/Al heterostructure. Phase asymmetry of the subgap Andreev spectrum is absent in the absence of in-plane magnetic field and reaches a maximum at 0.15 T applied in the plane of the junction transverse to the current direction. We interpret the phase diode effect in this system as resulting from finite-momentum Cooper pairing due to orbital coupling to the in-plane magnetic field, without invoking Zeeman or spin-orbit coupling.

The recent discovery of intrinsic supercurrent diode effect, and its prompt observation in a rich variety of systems, has shown that nonreciprocal supercurrents naturally emerge when both space- and time-inversion symmetries are broken. In Josephson junctions, nonreciprocal supercurrent can be conveniently described in terms of spin-split Andreev states. Here, we demonstrate a sign reversal of the supercurrent diode effect, in both its AC and DC manifestations. In particular, the AC diode effect -- i.e., the asymmetry of the Josephson inductance as a function of the supercurrent -- allows us to probe the current-phase relation near equilibrium. Using a minimal theoretical model, we can then link the sign reversal of the AC diode effect to the so-called 0-$\pi$-like transition, a predicted, but still elusive feature of multi-channel junctions. Our results demonstrate the potential of inductance measurements as sensitive probes of the fundamental properties of unconventional Josephson junctions., Comment: 13 pages, 6 figures

We realize a device, based on InAs two-dimensional electron gas proximitized by superconducting Al, which allows a single quantum dot level to be used as a spectrometer of the density of states in a nanowire. Applying a magnetic field parallel to the plane of the device causes the levels of the dot to split, enabling spin resolved spectroscopy. Using this method, we are able to study the spin and charge polarization of the allowed transport through sub-gap states which form in the nanowire and evolve with varying magnetic field and gate voltage.

The $\nu = 2/3$ fractional quantum Hall state is the hole-conjugate state to the primary Laughlin $\nu = 1/3$ state. We investigate transmission of edge states through quantum point contacts fabricated on a GaAs/AlGaAs heterostructure designed to have a sharp confining potential. When a small but finite bias is applied, we observe an intermediate conductance plateau with $G = 0.5 \frac{e^2}{h}$. This plateau is observed in multiple QPCs, and persists over a significant range of magnetic field, gate voltage, and source-drain bias, making it a robust feature. Using a simple model which considers scattering and equilibration between counterflowing charged edge modes, we find this half-integer quantized plateau to be consistent with full reflection of an inner counterpropagating -1/3 edge mode while the outer integer mode is fully transmitted. In a QPC fabricated on a different heterostructure which has a softer confining potential, we instead observe an intermediate conductance plateau at $G = \frac{1}{3} \frac{e^2}{h}$. These results provide support for a model at $\nu = 2/3$ in which the edge transitions from a structure having an inner upstream -1/3 charge mode and outer downstream integer mode to a structure with two downstream 1/3 charge modes when the confining potential is tuned from sharp to soft and disorder prevails., Comment: 6+4 pages, 4+4 figures

Cooper pairs occupy the ground state of superconductors and are typically composed of maximally entangled electrons with opposite spin. In order to study the spin and entanglement properties of these electrons, one must separate them spatially via a process known as Cooper pair splitting (CPS). Here we provide the first demonstration of CPS in a semiconductor two-dimensional electron gas (2DEG). By coupling two quantum dots to a superconductor-semiconductor hybrid region we achieve efficient Cooper pair splitting, and clearly distinguish it from other local and non-local processes. When the spin degeneracy of the dots is lifted, they can be operated as spin-filters to obtain information about the spin of the electrons forming the Cooper pair. Not only do we observe a near perfect splitting of Cooper pairs into opposite-spin electrons (i.e. conventional singlet pairing), but also into equal-spin electrons, thus achieving triplet correlations between the quantum dots. Importantly, the exceptionally large spin-orbit interaction in our 2DEGs results in a strong triplet component, comparable in amplitude to the singlet pairing. The demonstration of CPS in a scalable and flexible platform provides a credible route to study on-chip entanglement and topological superconductivity in the form of artificial Kitaev chains., Comment: Published version

Tunneling spectroscopy measurements are often used to probe the energy spectrum of Andreev bound states (ABSs) in semiconductor-superconductor hybrids. Recently, this spectroscopy technique has been incorporated into planar Josephson junctions (JJs) formed in two-dimensional electron gases, a potential platform to engineer phase-controlled topological superconductivity. Here, we perform ABS spectroscopy at the two ends of planar JJs and study the effects of the magnetic vector potential on the ABS spectrum. We show that the local superconducting phase difference arising from the vector potential is equal in magnitude and opposite in sign at the two ends, in agreement with a model that assumes localized ABSs near the tunnel barriers. Complemented with microscopic simulations, our experiments demonstrate that the local phase difference can be used to estimate the relative position of localized ABSs separated by a few hundred nanometers.

Andreev bound states with opposite phase-inversion asymmetries are observed in local and non-local tunneling spectra at the two ends of a superconductor-semiconductor-superconductor planar Josephson junction in the presence of a perpendicular magnetic field. Spectral signatures agree with a theoretical model, yielding a physical picture in which phase textures in superconducting leads localize and control the position of Andreev bound states in the junction, demonstrating a simple means of controlling the position and size of Andreev states within a planar junction.

The charge character of Andreev bound states (ABSs) in a three-terminal semiconductor-superconductor hybrid nanowire was measured using local and nonlocal tunneling spectroscopy. The device is fabricated using an epitaxial InAs/Al two-dimensional heterostructure with several gate-defined side probes. ABSs are found to oscillate around zero as a function of gate voltage, with modifications of their charge consistent with theoretical expectations for the total Bardeen-Cooper- Schrieffer (BCS) charge of ABSs.

Strong spin-orbit coupling, the resulting large $g$ factor, and small effective mass make InAs an attractive material platform for inducing topological superconductivity. The surface Fermi level pinning in the conduction band enables highly transparent ohmic contact without excessive doping. We investigate electrostatically defined quantum point contacts (QPCs) in a deep-well InAs two-dimensional electron gas. Despite the 3.3% lattice mismatch between the InAs quantum well and the InP substrate, we report clean QPCs with up to eight pronounced quantized conductance plateaus at zero magnetic field. Source-drain dc bias spectroscopy reveals a harmonic confinement potential with a nearly $5$ meV subband spacing. We find a many-body exchange interaction enhancement for the out-of-plane $g$ factor $|g_{\perp}^*| = 27 \pm 1$, whereas the in-plane $g$ factor is isotropic $|g^*_{x}| = |g^*_{y}| = 12 \pm 2$, close to the bulk value for InAs., Comment: Main text (8 pages, 5 figures), Supplemental Material (10 pages, 10 figures)

We investigate local and nonlocal signatures of hybridization between a quantum dot state and an extended Andreev bound state (ABS) in a gate-defined InAs nanowire with multiple side probes. When a quantum dot in one of the side probes was hybridized with an ABS in the nanowire, a characteristic spectroscopic pattern was observed both locally, i.e., in the probe with the quantum dot, and nonlocally, in the tunnel conductance of a remote probe. Nonlocal signatures of hybridization reveal the extended nature of the ABS.

Impacts of domain textures on low-lying neutral excitations in the bulk of fractional quantum Hall effect (FQHE) systems are probed by resonant inelastic light scattering. We demonstrate that large domains of quantum fluids support long-wavelength neutral collective excitations with well-defined wave vector (momentum) dispersion that could be interpreted by theories for uniform phases. Access to dispersive low-lying neutral collective modes in large domains of FQHE fluids such as long wavelength magnetorotons at filling factor v=1/3 offer significant experimental access to strong electron correlation physics in the FQHE., Comment: 11 pages, 4 figures, and Supplemental Material

Most of 2D superconductors are of type II, i.e., they are penetrated by quantized vortices when exposed to out-of-plane magnetic fields. In presence of a supercurrent, a Lorentz-like force acts on the vortices, leading to drift and dissipation. The current-induced vortex motion is impeded by pinning at defects, enabling the use of superconductors to generate high magnetic fields without dissipation. Usually, the pinning strength decreases upon any type of pair-breaking. Here we show that in Rashba superconductors the application of an in-plane field leads, instead, to an unexpected enhancement of pinning. When rotating the in-plane component of the field with respect to the current direction, the vortex inductance turns out to be highly anisotropic. We explain this phenomenon as a manifestation of Lifshitz invariant terms in the Ginzburg-Landau free energy, which are enabled by inversion and time-reversal symmetry breaking and lead to an elliptic squeezing of vortex cores. Our experiment provides access to a fundamental property of Rashba superconductors and offers an entirely new approach to vortex manipulation., Comment: 23 pages, 9 figures

Simultaneous breaking of inversion- and time-reversal symmetry in Josephson junction leads to a possible violation of the $I(\varphi)=-I(-\varphi)$ equality for the current-phase relation. This is known as anomalous Josephson effect and it produces a phase shift $\varphi_0$ in sinusoidal current-phase relations. In ballistic Josephson junctions with non-sinusoidal current phase relation the observed phenomenology is much richer, including the supercurrent diode effect and the magnetochiral anisotropy of Josephson inductance. In this work, we present measurements of both effects on arrays of Josephson junctions defined on epitaxial Al/InAs heterostructures. We show that the orientation of the current with respect to the lattice affects the magnetochiral anisotropy, possibly as the result of a finite Dresselhaus component. In addition, we show that the two-fold symmetry of the Josephson inductance reflects in the activation energy for phase slips., Comment: 9 pages, 4 figures

Quantum Hall interferometers have been used to probe fractional charge, and more recently, fractional statistics of quasiparticles. Theoretical predictions have been made regarding the effect of electrostatic coupling on interferometer behavior and observation of anyonic phases. Here we present measurements of a small Fabry-Perot interferometer in which these electrostatic coupling constants can be determined experimentally, facilitating quantitative comparison with theory. At the $\nu = 1/3$ fractional quantum Hall state, this device exhibits Aharonov-Bohm interference near the center of the conductance plateau interrupted by a few discrete phase jumps, and $\Phi_0$ oscillations at higher and lower magnetic fields, consistent with theoretical predictions for detection of anyonic statistics. We estimate the electrostatic parameters $K_I$ and $K_{IL}$ by two methods: by the ratio of oscillation periods in compressible versus incompressible regions, and from finite-bias conductance measurements, and these two methods yield consistent results. We find that the extracted $K_I$ and $K_{IL}$ can account for the deviation of the values of the discrete phase jumps from the theoretically predicted anyonic phase $\theta _a = 2\pi /3$. In the integer quantum Hall regime, we find that the experimental values of $K_I$ and $K_{IL}$ can account for the the observed Aharonov-Bohm and Coulomb dominated behavior of different edge states., Comment: 11 pages, 6 figures

Topological superconductivity can be engineered in semiconductors with strong spin-orbit interaction coupled to a superconductor. Experimental advances in this field have often been triggered by the development of new hybrid material systems. Among these, two-dimensional electron gases (2DEGs) are of particular interest due to their inherent design flexibility and scalability. Here we discuss results on a 2D platform based on a ternary 2DEG (InSbAs) coupled to in-situ grown Aluminum. The spin-orbit coupling in these 2DEGs can be tuned with the As concentration, reaching values up to 400 meV$\unicode{xC5}$, thus exceeding typical values measured in its binary constituents. In addition to a large Land\'e g-factor $\sim$ 55 (comparable to InSb), we show that the clean superconductor-semiconductor interface leads to a hard induced superconducting gap. Using this new platform we demonstrate the basic operation of phase-controllable Josephson junctions, superconducting islands and quasi-1D systems, prototypical device geometries used to study Majorana zero modes.

In many physical approaches to quantum computation, error-correction schemes assume the ability to form two-dimensional qubit arrays with nearest-neighbor couplings and parallel operations at multiple qubit sites. While semiconductor spin qubits exhibit long coherence times relative to their operation speed and single-qubit fidelities above error correction thresholds, multiqubit operations in two-dimensional arrays have been limited by fabrication, operation, and readout challenges. We present a two-by-two array of four singlet-triplet qubits in gallium arsenide and show simultaneous coherent operations and four-qubit measurements via exchange oscillations and frequency-multiplexed single-shot measurements. A larger multielectron quantum dot is fabricated in the center of the array as a tunable interqubit link, which we utilize to demonstrate coherent spin exchange with selected qubits. Our techniques are extensible to other materials, indicating a path towards quantum processors with gate-controlled spin qubits., Comment: Revised version after peer review, with additional supporting data. 13 pages, 9 figures (including 4-page 4-figure supplement)

Flat bands near M points in the Brillouin zone are key features of honeycomb symmetry in artificial graphene (AG) where electrons may condense into novel correlated phases. Here we report the observation of van Hove singularity doublet of AG in GaAs quantum well transistors, which presents the evidence of flat bands in semiconductor AG. Two emerging peaks in photoluminescence spectra tuned by backgate voltages probe the singularity doublet of AG flat bands, and demonstrate their accessibility to the Fermi level. As the Fermi level crosses the doublet, the spectra display dramatic stability against electron density, indicating interplays between electron-electron interactions and honeycomb symmetry. Our results provide a new flexible platform to explore intriguing flat band physics., Comment: 11 pages, 4 figures, and Supplementary Material

Transport is called nonreciprocal when not only the sign, but also the absolute value of the current, depends on the polarity of the applied voltage. It requires simultaneously broken inversion and time-reversal symmetries, e.g., by the interplay of spin-orbit coupling and magnetic field. So far, observation of nonreciprocity was always tied to resistivity, and dissipationless nonreciprocal circuit elements were elusive. Here, we engineer fully superconducting nonreciprocal devices based on highly-transparent Josephson junctions fabricated on InAs quantum wells. We demonstrate supercurrent rectification far below the transition temperature. By measuring Josephson inductance, we can link nonreciprocal supercurrent to the asymmetry of the current-phase relation, and directly derive the supercurrent magnetochiral anisotropy coefficient for the first time. A semi-quantitative model well explains the main features of our experimental data. Nonreciprocal Josephson junctions have the potential to become for superconducting circuits what $pn$-junctions are for traditional electronics, opening the way to novel nondissipative circuit elements., Comment: 16 pages, 11 figures

We use a superconducting microresonator as a cavity to sense absorption of microwaves by a superconducting quantum point contact defined by surface gates over a proximitized two-dimensional electron gas. Renormalization of the cavity frequency with phase difference across the point contact is consistent with adiabatic coupling to Andreev bound states. Near $\pi$ phase difference, we observe random fluctuations in absorption with gate voltage, related to quantum interference-induced modulations in the electron transmission. We identify features consistent with the presence of single Andreev bound states and describe the Andreev-cavity interaction using a dispersive Jaynes-Cummings model. By fitting the weak Andreev-cavity coupling, we extract ~GHz decoherence consistent with charge noise and the transmission dispersion associated with a localized state.

Most proof-of-principle experiments for spin qubits have been performed using GaAs-based quantum dots because of the excellent control they offer over tunneling barriers and the orbital and spin degrees of freedom. Here, we present the first realization of high-quality single and double quantum dots hosted in an InAs two-dimensional electron gas (2DEG), demonstrating accurate control down to the few-electron regime, where we observe a clear Kondo effect and singlet-triplet spin blockade. We measure an electronic $g$-factor of $16$ and a typical magnitude of the random hyperfine fields on the dots of $\sim 0.6\, \mathrm{mT}$. We estimate the spin-orbit length in the system to be $\sim 5-10\, \mu \mathrm{m}$, which is almost two orders of magnitude longer than typically measured in InAs nanostructures, achieved by a very symmetric design of the quantum well. These favorable properties put the InAs 2DEG on the map as a compelling host for studying fundamental aspects of spin qubits. Furthermore, having weak spin-orbit coupling in a material with a large Rashba coefficient potentially opens up avenues for engineering structures with spin-orbit coupling that can be controlled locally in space and/or time.

We demonstrate a new fabrication process for hybrid semiconductor-superconductor heterostructures based on anodic oxidation (AO), allowing controlled thinning of epitaxial Al films. Structural and transport studies of oxidized epitaxial Al films grown on insulating GaAs substrates reveal spatial non-uniformity and enhanced critical temperature and magnetic fields. Oxidation of epitaxial Al on hybrid InAs heterostructures with a conducting quantum well show similarly enhanced superconducting properties transferred to the two-dimensional electron gas (2DEG) by proximity effect, with critical perpendicular magnetic fields up to 3.5 T. An insulating AlOx film, that passivates the heterostructure from exposure to air, is obtained by complete oxidation of the Al. It simultaneously removes the need to strip Al which damages the underlying semiconductor. AO passivation yielded 2DEG mobilities two times higher than similar devices with Al removed by wet etching. An AO-passivated Hall bar showed quantum Hall features emerging at a transverse field of 2.5 T, below the critical transverse field of thinned films, eventually allowing transparent coupling of quantum Hall effect and superconductivity. AO thinning and passivation are compatible with standard lithographic techniques, giving lateral resolution below <50 nm. We demonstrate local patterning of AO by realizing a semiconductor-based Josephson junction operating up to 0.3 T perpendicular.

Because of their long coherence times and potential for scalability, semiconductor quantum-dot spin qubits hold great promise for quantum information processing. However, maintaining high connectivity between quantum-dot spin qubits, which favor linear arrays with nearest neighbor coupling, presents a challenge for large-scale quantum computing. In this work, we present evidence for long-distance spin-chain-mediated superexchange coupling between electron spin qubits in semiconductor quantum dots. We weakly couple two electron spins to the ends of a two-site spin chain. Depending on the spin state of the chain, we observe oscillations between the distant end spins. We resolve the dynamics of both the end spins and the chain itself, and our measurements agree with simulations. Superexchange is a promising technique to create long-distance coupling between quantum-dot spin qubits., Comment: 6+9 pages, 4+7 figures

We present simultaneous measurements of Josephson inductance and DC transport characteristics of ballistic Josephson junctions based upon an epitaxial Al-InAs heterostructure. The Josephson inductance at finite current bias directly reveals the current-phase relation. The proximity-induced gap, the critical current and the average value of the transparency $\bar{\tau}$ are extracted without need for phase bias, demonstrating, e.g.,~a near-unity value of $\bar{\tau}=0.94$. Our method allows us to probe the devices deeply in the non-dissipative regime, where ordinary transport measurements are featureless. In perpendicular magnetic field the junctions show a nearly perfect Fraunhofer pattern of the critical current, which is insensitive to the value of $\bar{\tau}$. In contrast, the signature of supercurrent interference in the inductance turns out to be extremely sensitive to $\bar{\tau}$., Comment: 13 pages, 13 figures

Semiconductor quantum-dot spin qubits are a promising platform for quantum computation, because they are scalable and possess long coherence times. In order to realize this full potential, however, high-fidelity information transfer mechanisms are required for quantum error correction and efficient algorithms. Here, we present evidence of adiabatic quantum-state transfer in a chain of semiconductor quantum-dot electron spins. By adiabatically modifying exchange couplings, we transfer single- and two-spin states between distant electrons in less than 127 ns. We also show that this method can be cascaded for spin-state transfer in long spin chains. Based on simulations, we estimate that the probability to correctly transfer single-spin eigenstates and two-spin singlet states can exceed 0.95 for the experimental parameters studied here. In the future, state and process tomography will be required to verify the transfer of arbitrary single qubit states with a fidelity exceeding the classical bound. Adiabatic quantum-state transfer is robust to noise and pulse-timing errors. This method will be useful for initialization, state distribution, and readout in large spin-qubit arrays for gate-based quantum computing. It also opens up the possibility of universal adiabatic quantum computing in semiconductor quantum-dot spin qubits., Comment: 7+17 pages, 5+8 figures

Utilizing an electronic Fabry-Perot interferometer in which Coulomb charging effects are suppressed, we report experimental observation of anyonic braiding statistics for the $\nu=1/3$ fractional quantum Hall state. Strong Aharonov-Bohm interference of the $\nu=1/3$ edge mode is punctuated by discrete phase slips consistent with an anyonic phase of $\theta_{anyon}=\frac{2\pi}{3}$. Our results are consistent with a recent theory of a Fabry-Perot interferometer operated in a regime in which device charging energy is small compared to the energy of formation of charged quasiparticles. Close correspondence between device operation and theoretical predictions substantiates our claim of observation of anyonic braiding., Comment: 9 pages, 4 figures

The transfer of information between quantum systems is essential for quantum communication and computation. In quantum computers, high connectivity between qubits can improve the efficiency of algorithms, assist in error correction, and enable high-fidelity readout. However, as with all quantum gates, operations to transfer information between qubits can suffer from errors associated with spurious interactions and disorder between qubits, among other things. Here, we harness interactions and disorder between qubits to improve a swap operation for spin eigenstates in semiconductor gate-defined quantum-dot spins. We use a system of four electron spins, which we configure as two exchange-coupled singlet-triplet qubits. Our approach, which relies on the physics underlying discrete time crystals, enhances the quality factor of spin-eigenstate swaps by up to an order of magnitude. Our results show how interactions and disorder in multi-qubit systems can stabilize non-trivial quantum operations and suggest potential uses for non-equilibrium quantum phenomena, like time crystals, in quantum information processing applications. Our results also confirm the long-predicted emergence of effective Ising interactions between exchange-coupled singlet-triplet qubits., Comment: v2: 11+2 pages, 5+3 figures

Heisenberg exchange coupling between neighboring electron spins in semiconductor quantum dots provides a powerful tool for quantum information processing and simulation. Although so far unrealized, extended Heisenberg spin chains can enable long-distance quantum information transfer and the generation of non-equilibrium quantum states. In this work, we implement simultaneous, coherent exchange coupling between all nearest-neighbor pairs of spins in a quadruple quantum dot. The main challenge in implementing simultaneous exchange couplings is the nonlinear and nonlocal dependence of the exchange couplings on gate voltages. Through a combination of electrostatic simulation and theoretical modeling, we show that this challenge arises primarily due to lateral shifts of the quantum dots during gate pulses. Building on this insight, we develop two models, which can be used to predict the confinement gate voltages for a desired set of exchange couplings. Although the model parameters depend on the number of exchange couplings desired (suggesting that effects in addition to lateral wavefunction shifts are important), the models are sufficient to enable simultaneous and independent control of all three exchange couplings in a quadruple quantum dot. We demonstrate two-, three-, and four-spin exchange oscillations, and our data agree with simulations., Comment: 9+10 pages, 5+5 figures

Indium antimonide (InSb) two-dimensional electron gases (2DEGs) have a unique combination of material properties: high electron mobility, strong spin-orbit interaction, large Land\'{e} g-factor, and small effective mass. This makes them an attractive platform to explore a variety of mesoscopic phenomena ranging from spintronics to topological superconductivity. However, there exist limited studies of quantum confined systems in these 2DEGs, often attributed to charge instabilities and gate drifts. We overcome this by removing the $\delta$-doping layer from the heterostructure, and induce carriers electrostatically. This allows us to perform the first detailed study of stable gate-defined quantum dots in InSb 2DEGs. We demonstrate two distinct strategies for carrier confinement and study the charge stability of the dots. The small effective mass results in a relatively large single particle spacing, allowing for the observation of an even-odd variation in the addition energy. By tracking the Coulomb oscillations in a parallel magnetic field we determine the ground state spin configuration and show that the large g-factor ($\sim$30) results in a singlet-triplet transition at magnetic fields as low as 0.3 T., Comment: Includes supplementary information. Data files available at https://doi.org/10.4121/uuid:28a121af-1e08-429d-9d53-1cc53764e91e