In superconducting quantum interference devices (SQUIDs), the superconducting diode effect may be generated by interference of multiple harmonic components in the current-phase relationships (CPRs) of different branches forming SQUID loops. Through the inclusion of two gate-tunable Josephson junctions in series in each interference branch of a double-loop SQUID, we demonstrate independent control over both the harmonic content and the amplitude of three interfering CPRs, facilitating significant improvement in the maximum diode efficiency. Through optimized gate-controlled tuning of individual Josephson energies, diode efficiency exceeding 50% is demonstrated. Flux-dependent oscillations show quantitative agreement with a simple model of SQUID operation.

The quantum Hopfield model is widely used to describe ultrastrong light–matter coupling between cavity photons and collective bosonic excitations in solids, where the diamagnetic interaction is conventionally assumed to be a constant. We experimentally demonstrate that the diamagnetic response of Landau polaritons is reduced under strong terahertz field excitation. We show that this behavior originates from field-driven redistribution of electrons into the nonparabolic regime of the conduction band of GaAs, which reduces the plasma frequency and consequently the diamagnetic interaction strength. A microscopic hot-electron model reproduces the observed nonlinear response. Motivated by this microscopic picture, we propose a nonlinear extension of the Hopfield model with a Kerr-like interaction. Our results establish a route toward nonlinear cavity quantum electrodynamics and driven ultrastrong light–matter coupling beyond the conventional linear Hopfield description, which is capable of creating uniquely quantum optical effects such as squeezed light generation.

Recent experiments have demonstrated that measurements of the entropy change associated with the addition of electrons to semiconductor- and graphene-based quantum dots accurately quantify the spin and orbital degeneracy of the states into which they are added. However, measuring more exotic entropies requires probing the entropy change of an entire system in response to an added particle. Here, we demonstrate that Maxwell relation-based measurements probe not only the entropy change associated with the added electron but also that of the surrounding system as it responds to that electron. Using a pair of capacitively coupled GaAs quantum dots, we show that charge measurements on one dot reveal entropy changes associated with the entire two-dot system, both at weak dot–reservoir coupling where microstate counting applies and at stronger coupling where numerical renormalization group calculations are required.

Hybrid superconductor-semiconductor nanostructures are a central component for research spanning condensed matter physics and quantum information processing. Continued progress relies critically on the ability to characterize, control, and optimize several intrinsic material properties including spin-orbit coupling, band offsets, and disorder in a device-relevant stack that necessarily couples the electronic states of a superconducting metal film and a semiconductor. Here we report a new method to extract fundamental material parameters utilizing simple Shubnikov-de Haas (SdH) oscillation measurements in heterostructures in which metallic electronic states are coupled to a two-dimensional electron gas (2DEG) residing in an InAs quantum well beneath an aluminum thin film. Proper analysis of the full magnetoresistance data facilitates extraction of the quantum well carrier density, spin-orbit coupling strength, and both transport and quantum scattering times. Most importantly, the extracted scattering times in the 2DEG are impacted by the metal-semiconductor coupling strength allowing us to quickly gain information on proximity-induced superconducting gap without any fabrication or mK measurements. The wealth of information that is accessed with these simple measurements positions this methodology as an important tool for hybrid materials optimization.

The coherence of superconductivity and its suppression near a quantum phase transition is governed by the interplay between local pairing and macroscopic phase coherence. Using scanning SQUID, we image the local susceptibility in a hybrid Josephson junction array. On a square lattice of narrow islands, we simultaneously access both the amplitude and spatial phase structure of sensitive superconducting states. We observe periodic phase patterns at commensurate magnetic fillings. At zero field the long-range phase coherence is strongest. At a finite field, smaller than one percent of flux quantum per unit cell, the system fragments into large regions of constant superconducting phase, as a function of the applied field. Our results provide the first direct measurement of long-range phase coherence in a Josephson junction array.

Arrays of hybrid metal-semiconductor quantum dots offer a new approach to quantum simulation, with key advantages over arrays of conventional quantum dots. Because the metallic component of these hybrid dots has a quasi-continuous level spectrum, each site in an array can be effectively electronically identical; in contrast, each conventional semiconductor quantum dot has its own spectral fingerprint. Meanwhile, the semiconductor component retains gate-tunability of intersite coupling. This combination creates a scalable platform for simulating correlated ground states driven by Coulomb interactions. We report the fabrication and characterization of hybrid metal-semiconductor dots, featuring a submicron metal island transparently contacting a gate-confined region of an InAs quantum well with tunable couplings to macroscopic leads. Tuning the dot-lead coupling to the weak-coupling limit yields highly-uniform Coulomb peaks, with no resolvable excitation spectrum in the Coulomb diamonds. We propose a realistic device design for a hybrid dot-based linear array quantum simulator and outline the correlated many-body physics accessible in this architecture.

Designing novel photonic materials relies on the ability to modulate semiconductor band structure with monolayer precision. We employ conduction band engineering and modifications of the molecular-beam epitaxy (MBE) process to tune the optical transitions of Sc0.14Al0.86N/GaN/Al(Sc)N/GaN double quantum well (DQW) structures above 800 meV. The incorporation of rare-earth scandium into traditional nitride semiconductors opens new design space by enabling low-defect nitride heterostructures lattice-matched to GaN. However, recent studies found that significant intermixing at interfaces considerably distorts the band profile of ultra-thin GaN/ScAlN quantum wells. To overcome these challenges, the structure–property relationships of Sc-containing DQWs were assessed by correlating infrared intersubband absorption with high-resolution x-ray diffraction and electron microscopy techniques. Computational simulations of quantum confinement and carrier distribution explain the observed experimental trends of intersubband absorption associated with variation of layer widths, middle barrier composition, and doping density. MBE growth in a metal-accumulation mode is found to be crucial for improving structural conformity to design by reducing interface roughness and thickness and for reaching the telecom-relevant spectral range.

Measurement perturbs a quantum system by coupling it to external degrees of freedom, but detector backaction depends on the physical mechanism of measurement itself. In solid-state devices, detectors driven far from equilibrium to enable faster measurements produce backaction that can often be understood as classical noise. However, a strong measurement can also induce backaction from quantum many-body correlations in the detector that are intrinsic to the measurement, even without shot noise. Here, we probe this near-equilibrium backaction through the effect of a quantum-dot charge sensor on tunnelling between a second quantum dot and its reservoirs. The measurement realizes the Anderson Orthogonality Catastrophe (AOC): electrons in the detector leads reorganize in response to an abrupt change in local scattering potential, suppressing resonant tunnelling while enabling inelastic processes that exchange energy with the detector. Changing the detector energy level tunes the AOC backaction from negligible to dominant in the tunnelling dynamics. More broadly, these results establish detector-induced many-body correlations as a controllable influence on quantum dynamics.

We propose a novel strategy and a new class of detectors for the direct detection of axion dark matter in the meV mass range, based on resonantly enhanced axion-photon conversion through the inverse Primakoff effect in engineered radiometers composed of quantum semiconductor heterostructures. Semiconductor-Quantum-Well Axion Radiometer Experiments (SQWAREs) are multiple quantum well structures forming magnetoplasmonic cavities, containing high-mobility two-dimensional electron gases, realizing tunable epsilon-near-zero resonances in the terahertz frequency range. By controlling the orientation of the cavity within a strong external magnetic field, both the resonance frequency and the axion-induced current are optimized in situ, enabling efficient scanning across a broad mass range without the need for complex mechanical adjustments. The axion-induced electromagnetic signal radiatively emitted from the cavity is then detected by a photodetector. We present the theoretical basis for resonant enhancement, detail the experimental design and benchmarks through extensive simulations, project the sensitivity of an example SQWARE for several realistic configurations, and demonstrate the modularity and flexibility of the design to fit reasonably with any lab's existing capabilities and target unique axion mass ranges. Our results demonstrate that the SQWAREs can probe the well-motivated quantum chromodynamics axion parameter space and close a critical gap in direct searches at meV masses.

Scandium aluminum nitride (ScxAl1-xN) is attractive for novel quantum photonic applications due to the prospect of growing arbitrarily thick low-defect ScxAl1-xN/GaN heterostructures on c-plane GaN. While high-quality GaN growth by molecular-beam epitaxy (MBE) is performed under metal-rich conditions at substrate temperatures >700 degrees C, ScxAl1-xN undergoes phase segregation and roughening under these conditions. For this reason, MBE of Sc-containing nitrides has been typically done under nitrogen-rich conditions at relatively low substrate temperatures (<600 degrees C). We demonstrate a metal-modulated epitaxy method for ScxAl1-xN growth under metal-rich conditions that produces smooth ScxAl1-xN layers and ScxAl1-xN/GaN superlattices (SLs) with enhanced structural quality. In our approach, metal and nitrogen shutters are precisely timed to maintain approximately one monolayer (ML) of metal adlayer during the majority of ScxAl1-xN growth while preventing excessive metal buildup on the surface in each similar to 1 nm cycle. This approach is especially beneficial for ScxAl1-xN/GaN SLs with ultrathin GaN quantum wells that require near-ML control of interface abruptness to limit intermixing and roughness that undermine optical or electronic properties. Using this metal-modulated approach at 600 degrees C, interface root mean square roughness in a 6 nm Sc0.14Al0.86N/2 nm GaN SL is kept below 0.1 nm, and interface widths are improved substantially relative to SLs grown under nitrogen-rich conditions at the same temperature.

The Kondo singlet—a many-body state formed by entanglement between a localized spin and the Fermi sea—has been studied extensively through its transport signatures in quantum dots. Here we report a thermodynamic measurement of the entropy suppression associated with the formation of the Kondo singlet, using temperature-dependent charge sensing and a Maxwell relation to track the suppression of spin entropy as the first electron is added to a strongly-coupled GaAs quantum dot. Plotting dN/dT against the simultaneously measured occupation N reveals an asymmetric lineshape with its peak shifted to N>1/2—a hallmark of Kondo screening—that weakens with increasing temperature and is qualitatively reproduced by numerical renormalization group (NRG) calculations, with a small but persistent offset to lower occupation relative to the theory. An independent measurement of conductance versus occupation on the same device provides a test of these quantities through the mixed-valence crossover and matches NRG within experimental uncertainty.

We report on the near-infrared intersubband (ISB) absorption properties of strain- free Sc0.14Al0.86N/GaN multiple quantum wells (MQWs) grown on c-plane GaN substrates by molecular beam epitaxy. These MQWs exhibit strong, sharp, and tunable absorption energies between 515 meV and 709 meV, for well widths ranging from 7 nm to 1.5 nm, respectively. Observation of ISB absorption in ultra-thin Sc0.14Al0.86N/GaN MQWs not only extends the near-infrared range accessible with Sc-containing nitrides but also highlights the challenges of growing nanometer-thick GaN quantum wells. We explore the effects of growth temperature on absorption characteristics and find that substrate temperatures above 600 degrees C significantly enhance ISB absorption intensity but also introduce an energy redshift for the narrowest wells. The redshift is attributed to increased interface roughness due to ScAlN surface morphology degradation at higher temperatures. Additionally, a comparison of experimental results with simulated band-structures indicates that the magnitude of net polarization rises faster with Sc-composition than previously suggested by theoretical calculations. This study advances the prospects of ScAlN/GaN heterostructures for novel photonic devices in the technologically important near-infrared range.(c) 2025 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

In optical diffraction, the phase difference between sources in a grating or multi-slit mask is determined by the angle to the imaging screen, yielding the familiar multi-lobed diffraction image. Here, we realize a similar phenomenon in a superconductor-semiconductor hybrid circuit configured to allow Andreev scattering from multiple parallel scatterers. Phase differences between scatterers are set by tapping off of a remote superconducting meander. We investigate arrays with two, three, four, and ten Andreev scatterers, examining local and nonlocal diffraction patterns, finding good agreement with a theory of multiple Andreev scattering, not to be confused with multiple Andreev reflection. Adding current-carrying taps to the meander allows individual phase control.

Controlling the current-phase relation (CPR) of Josephson elements is essential for tailoring the eigenstates of superconducting qubits, tuning the properties of parametric amplifiers, and designing nonreciprocal superconducting devices. Here, we introduce the hybrid Josephson rhombus, a highly tunable superconducting circuit containing four semiconductor-superconductor hybrid Josephson junctions embedded in a loop. Combining magnetic frustration with gate-voltage-controlled tuning of individual Josephson couplings provides deterministic control of the harmonic content of the rhombus CPR. We show that, for balanced Josephson couplings at full frustration, the hybrid rhombus displays a π-periodic cos(2φ) potential, indicating coherent charge-4e transport. Tuning away from the balanced configuration, we observe a superconducting diode effect with efficiency exceeding 25%. These results showcase the potential of hybrid Josephson rhombi as fundamental building blocks for noise-resilient qubits and quantum devices with custom transport properties. Published by the American Physical Society 2025

The multimode ultrastrong coupling (USC) regime has emerged as a novel platform for accessing previously inaccessible phenomena in cavity quantum electrodynamics. Of particular interest are cavity-mediated correlations between local and nonlocal excitations, or equivalently, between modes at zero and finite in-plane momentum, which offer new opportunities for controlling light–matter interactions across space. However, direct experimental evidence of such interactions has remained elusive. Here, we demonstrate nonlocal multimode coupling in a Landau polariton system, where cavity photons simultaneously interact with the zero-momentum cyclotron resonance and finite-momentum magnetoplasmons of GaAs two-dimensional electron gas. Our slot cavities, with their subwavelength mode volumes, supply in-plane momentum components that enable the excitation of finite-momentum matter modes. Terahertz time-domain magnetospectroscopy measurements reveal a clear splitting of the upper-polariton branch, arising from hybridization between magnetoplasmon modes and the cavity–cyclotron-resonance hybrids. Extracted coupling strengths confirm USC of the cyclotron resonance and strong coupling of the magnetoplasmon modes to the cavity field, respectively. The experimental results are well captured by the multimode Hopfield model and finite-element simulations. These findings establish a pathway for engineering multimode light–matter interactions involving zero- and finite-momentum matter modes in the USC regime.

Landau polaritons arise when cavity photons strongly couple, or hybridize, with the cyclotron resonance of two-dimensional electrons in a strong perpendicular magnetic field. These polaritons provide a powerful platform for studying quantum electrodynamical phenomena in the ultrastrong coupling regime, where the Rabi frequency becomes a significant fraction of the bare frequencies of light and matter. Theories of Landau polaritons are well-developed under the electric-dipole approximation, including the selection rules for inter-Landau-level transitions, i.e., from the n -th to the ( n +1)-th Landau level with frequency ω c = eB / m *, where e , B , and m * are the elementary charge, magnetic field strength, and electron effective mass, respectively.

The superconducting diode effect (SDE) is a phenomenon that has attracted significant attention for its potential to establish new Josephson junction (JJ) physics and its application as a rectifier in Josephson circuits. While many experimental platforms for SDE have been reported, only a few demonstrate high efficiency. One promising system is a JJ coherently coupled with a second JJ embedded in a superconducting loop, where SDE is controllable in direction via the phase difference. Here we study SDE in such devices under microwave irradiation. With the appearance of Shapiro steps, SDE efficiency increases as the microwave power is raised. Additionally, a finite voltage difference appears without any direct bias current at high microwave powers. These results are explained by the dynamics of the JJ's washboard potential, which are common to all SDE systems. Our findings enhance the understanding of SDE dynamics and expand their potential applications.

A time-periodic drive enables the engineering of non-equilibrium quantum systems by hybridizing Floquet sidebands. We investigated DC voltage-biased planar Josephson junctions built upon epitaxial Al/InAs heterostructures in which the intrinsic AC Josephson effect is theoretically expected to provide a time-periodic drive leading to Floquet hybridization. Tunneling spectroscopy is performed using probes positioned at the ends of the junction to study the evolution of the local density of states. With applied drive, we observe multiple coherence peaks which are studied as a function of DC voltage bias and in-plane magnetic field. Our analysis suggests that these spectral gaps arise from a direct mesoscopic coupling between the tunneling probe and the superconducting leads rather than from a Floquet-driven gap opening. Our numerical simulations indicate that an increase in the ratio of junction width to coherence length will enhance the contribution of Floquet hybridization. This work lays a foundation for the exploration of Floquet physics utilizing voltage-biased hybrid superconductor-semiconductor Josephson junctions and provides means for distinguishing direct couplings from genuine Floquet effects.

The fusion of non-Abelian anyons is a fundamental operation in measurement-only topological quantum computation1. In one-dimensional topological superconductors (1DTSs)2-4, fusion amounts to a determination of the shared fermion parity of Majorana zero modes (MZMs). Here we introduce a device architecture5 that is compatible with future tests of fusion rules. We implement a single-shot interferometric measurement of fermion parity6-11 in indium arsenide-aluminium heterostructures with a gate-defined superconducting nanowire12-14. The interferometer is formed by tunnel-coupling the proximitized nanowire to quantum dots. The nanowire causes a state-dependent shift of the quantum capacitance of these quantum dots of up to 1 fF. Our quantum-capacitance measurements show flux h/2e-periodic bimodality with a signal-to-noise ratio (SNR) of 1 in 3.6 μs at optimal flux values. From the time traces of the quantum-capacitance measurements, we extract a dwell time in the two associated states that is longer than 1 ms at in-plane magnetic fields of approximately 2 T. We discuss the interpretation of our measurements in terms of both topologically trivial and non-trivial origins. The large capacitance shift and long poisoning time enable a parity measurement with an assignment error probability of 1%.

We discuss the interfacial and structural properties of Sc0.14Al0.86N/GaN multi-quantum well structures grown using plasma-assisted molecular beam epitaxy. Using high-angle annular dark-field scanning transmission electron microscopy and high-resolution x-ray diffraction, the effects of growth conditions on interface quality are investigated. The results reveal that higher growth temperatures improve structural uniformity but can induce interface grading and interdiffusion, leading to deviations from targeted layer thicknesses and compositions. These findings provide critical insights into optimizing ScAlN/GaN heterostructures for enhanced performance in advanced optoelectronic devices.