Cavity exciton–polaritons are quasiparticles that form when quantum well excitons hybridize with a cavity mode. Here, we carry out photon correlation measurements under continuous wave resonant laser excitation to demonstrate quantum correlations between cavity–polaritons. Our experiments reveal an unexpectedly strong dependence of polariton interactions on cavity–exciton detuning. When the polaritons are predominantly exciton-like, we observe a transition from photon antibunching to bunching as the laser is tuned across the polariton resonance, in agreement with a simple Kerr-nonlinearity model. When the lower-branch polariton energy is tuned to induce a two-polariton Feshbach resonance with the biexciton mode, the degree of polariton antibunching becomes independent of the laser detuning: we explain our finding by invoking a dissipative blockade mechanism arising from large biexciton broadening. Our experiments demonstrate that the strong polariton blockade regime would be achieved by reducing the polariton decay rate by a factor of 10.

Lithium niobate-on-insulator (LNOI) is an emerging photonic platform with high potential for scalable quantum information processing due to its strong second-order nonlinearity. However, little progress has been made in developing on-chip single-photon detectors on LNOI. Niobium titanium nitride (NbTiN) superconducting nanowire single-photon detectors (SNSPDs) are a promising candidate for this application. In this work, we use DC reactive magnetron sputtering to grow high-quality NbTiN thin films using an ultrahigh vacuum deposition system with a base pressure lower than 2 & times; 10-10 mbar. Enabled by the low concentration of background impurities in this system, we investigate the impact of substrate temperature during NbTiN growth. We achieve four nm thick superconducting films with a critical temperature (Tc) of 12.3 K grown at a substrate temperature of 825 K. We find that the NbTiN films grow in the (111) orientation and evolve from a porous pillar structure when grown at low temperatures to densely packed fibrous grains at higher temperatures. Furthermore, we demonstrate that the increased substrate temperature reduces the oxygen concentration in our films and improves the overall stoichiometry. In addition, we integrate these films with the LNOI platform and investigate the obtained interface quality. Lastly, we fabricate SNSPDs from the NbTiN film on LNOI and characterize the detector performance.

High-efficiency single-photon detection in the microwave domain is a key enabling technology for various quantum applications. However, the extremely low energy of microwave photons presents a fundamental challenge, preventing direct photon-to-charge conversion as achieved in optical systems using semiconductors. Here, we demonstrate continuous microwave photon detection with an efficiency approaching 70% in the single-photon regime. We use a hybrid system comprising a gate-defined double quantum dot (DQD) charge qubit in a gallium arsenide/aluminum gallium arsenide heterostructure, coupled to a high-impedance Josephson junction array cavity. We systematically optimize the hybrid architecture to maximize the detection efficiency by leveraging strong charge-photon coupling, tunable DQD tunnel rates, and the frequency tunability of both subsystems. The system efficiency is characterized over a frequency range of 3 to 5.2 gigahertz. Our results establish semiconductor-based cavity-quantum electrodynamics architectures as a scalable and versatile platform for efficient microwave photon detection, opening promising avenues for quantum microwave optics and quantum information technologies.

Non-reciprocal microwave components are indispensable in quantum information processing and cryogenic measurement. Conventional implementations, however, are bulky and incompatible with on-chip scalable integration. Recent efforts to develop compact on-chip alternatives often rely on active modulation or complex circuit architectures, which introduce additional losses and degrade performance. We demonstrate the realization of compact, self-impedance-matched gyrators based on edge magnetoplasmons in a two-dimensional electron gas. Gyrators can be used as building blocks for other non-reciprocal elements such as isolators and circulators. Our devices achieve gyration from 0.2 to 2 GHz, tunable by moderate out-of plane magnetic fields below 400 mT, and sub-mm footprint, two orders of magnitude smaller than conventional ferrite-based components. Using an electrode geometry predicted to minimize reflections, we achieve insertion losses as low as 2 to 4 dB. The self-matched design framework we utilize is broadly applicable, and can be implemented in a wide variety of non-reciprocal device architectures.

Controlling quantum phases of materials with vacuum field fluctuations in engineered cavities is a topical method for the optical manipulation of emergent phenomena. Here we demonstrate cavity-induced anisotropies in the electronic transport of a high-mobility two-dimensional electron system in a strong magnetic field. In particular, we show the suppression of longitudinal resistivity well below the resistivity at zero magnetic field. These cavity-induced effects occur at ultralow temperatures when the magnetic field lies between quantized Hall plateaus. We interpret our results as arising from the stabilization of thermally disordered quantum Hall stripes. Therefore, our work presents a demonstration of the cavity quantum electrodynamics control of a correlated electronic phase.

High-quality InAs quantum wells grown on InP are a promising platform for topological quantum information processing due to their large g-factor, strong Rashba spin-orbit interaction, and their compatibility with in-situ-deposited superconductors. In this work, we investigate InAs/InGaAs quantum wells grown on InP (001) wafers, focusing on how the layer structure and strain influence the electronic properties and surface morphology. By combining quantum transport measurements with atomic force microscopy, we show that the layer design predominantly affects the mobility anisotropy, which aligns well with the surface morphology. Surface characterization further reveals the mechanism of quantum well collapse when the layer thickness exceeds the strain limit. In addition, transport measurements demonstrate that quantum confinement has a clear impact on band nonparabolicity.

In quantum mechanics, empty space is not void but is characterized by vacuum-field fluctuations, which underlie phenomena such as the Lamb shift 1 , spontaneous emission, and the Casimir effect 2 . Due to their quantitatively small relative contributions in free-space atomic physics, they were traditionally overlooked in solid-state systems. Recently, however, the interplay between electronic correlations and quantum electrodynamical effects in low-dimensional systems has become a rapidly advancing area in condensed matter physics 3–5 , with substantial implications for quantum materials and device engineering. High-mobility two-dimensional electron gases in the quantum Hall regime 6 offer an ideal platform to investigate how vacuum electromagnetic fields affect strongly correlated electronic states. Here we demonstrate that adjusting the coupling strength between a two-dimensional electron gas and the vacuum fields of a hovering split-ring resonator leads to a significant reduction in exchange splitting at odd-integer filling factors, along with an enhancement of fractional quantum Hall gaps at filling factors 4/3, 5/3 and 7/5. Theoretical analysis indicates that these effects stem from an effective long-range attractive interaction mediated by virtual cavity photons in regions with strong vacuum electric field gradients. Our findings uncover a new mechanism by which cavity vacuum fields can reshape electronic correlations in quantum Hall systems, establishing a new approach for manipulating correlated quantum phases in low-dimensional materials and paving the way for engineering tailored many-body interactions in compact devices.

Controlling quantum phases of materials with vacuum field fluctuations in engineered cavities is a novel route towards the optical control of emergent phenomena. We demonstrate, using magnetotransport measurements of a high-mobility two-dimensional electron gas, striking cavity-induced anisotropies in the electronic transport, including the suppression of the longitudinal resistance well below the resistivity at zero magnetic field. Our cavity-induced effects occur at ultra-low temperatures (< 200 mK) when the magnetic field lies between quantized Hall plateaus. We interpret our results as arising from the stabilization of thermally-disordered quantum Hall stripes. Our work presents a clear demonstration of the cavity QED control of a correlated electronic phase.

Hybrid multiterminal Josephson junctions (JJs) are expected to harbor a novel class of Andreev bound states (ABSs), including topologically nontrivial states in four-terminal devices. In these systems, topological phases emerge when ABSs depend on at least three superconducting phase differences, resulting in a three-dimensional (3D) energy spectrum characterized by Weyl nodes at zero energy. Here, we realize a four-terminal JJ in a hybrid Al/InAs heterostructure, where ABSs form a synthetic 3D band structure. We probe the energy spectrum using tunneling spectroscopy and identify spectral features associated with the formation of a tri-Andreev molecule, a bound state whose energy depends on three superconducting phases and, therefore, is able to host topological ABSs. The experimental observations are well described by a numerical model. The calculations predict the appearance of four Weyl nodes at zero energy within a gap smaller than the experimental resolution. These topological states are theoretically predicted to remain stable within an extended region of the parameter space, well accessible by our device. These findings establish an experimental foundation to study high-dimensional synthetic band structures in multiterminal JJs, and to realize topological Andreev bands.

Spin polarization measurements were performed in three 2D electron gases in GaAs with densities ne=9.1, 7.2 and 6.5 x1010 cm-2, in the quantum Hall regime. Full spin polarization at nu = 1 surrounded by rapid depolarization due to Skyrmion formation was observed in all devices, consistent with past measurements. Depolarization of the nu=4/3, 8/5 states and repolarization of the nu=5/3 state was also measured, in remarkable agreement with a non-interacting, disorder-free Composite Fermion model. Optical power and temperature dependent measurements of the nu = 1 state suggest a regime of non-linear optics.

We present a novel semiconductor-superconductor hybrid material based on a molecular beam epitaxially grown InAsSb surface quantum well with an in-situ deposited Nb top layer. Relative to conventional Al-InAs based systems, the InAsSb surface quantum well offers a lower effective mass and stronger spin-orbit interaction, while the Nb layer has a higher critical temperature and a larger critical magnetic field. The in-situ deposition of the Nb results in a high-quality interface that enables strong coupling to the InAsSb quantum well. Transport measurements on Josephson junctions reveal an induced superconducting gap of 1.3 meV. Furthermore, a planar asymmetric SQUID is realized, exhibiting gate-tunable superimposed oscillations originating from both the individual Josephson junction and the full SQUID loop. The large induced superconducting gap combined with strong spin-orbit interaction position this material as an attractive platform for experiments exploring gate-tunable superconductivity and topological superconducting devices.

AbstractSemiconductor‐superconductor hybrid materials are used as a platform to realize Andreev bound states, which hold great promise for quantum applications. These states require transparent interfaces between the semiconductor and superconductor, which are typically realized by in‐situ deposition of an Al superconducting layer. Here a hybrid material is presented, based on an InAs 2D electron gas (2DEG) combined with in‐situ deposited Nb and NbTi superconductors, which offer a larger operating range in temperature and magnetic field due to their larger superconducting gap. The inherent difficulty associated with the formation of an amorphous interface between III‐V semiconductors and Nb‐based superconductors is addressed by introducing a 7 nm Al interlayer. The Al interlayer provides an epitaxial connection between an in‐situ magnetron sputtered Nb or NbTi thin film and a shallow InAs 2DEG. This metal‐to‐metal epitaxy is achieved by optimization of the material stack and results in an induced superconducting gap of approximately 1 meV, determined from transport measurements of superconductor‐semiconductor Josephson junctions. This induced gap is approximately five times larger than the values reported for Al‐based hybrid materials and indicates the formation of highly‐transparent interfaces that are required in high‐quality hybrid material platforms.

We study the current scaling of the oscillatory magnetoresistance peak height in microwave induced magnetoresistance oscillations, over the temperature range [Formula: see text], by examining the change in response observed under a supplementary [Formula: see text] current bias, when the [Formula: see text] also produces an overall giant negative magnetoresistance in the high mobility GaAs/AlGaAs system. A "universality" is observed in the decay of the magnetoresistance peak height over the examined temperatures, sizes, and applied [Formula: see text]. The results suggest that the photoexcited resistance maxima are sensitive not to the DC current, but rather to the current density [Formula: see text].

The performance of superconducting microwave circuits is strongly influenced by the material properties of the superconducting film and substrate. While progress has been made in understanding the importance of surface preparation and the effect of surface oxides, the complex effect of superconductor film structure on microwave losses is not yet fully understood. In this study, we investigate the microwave properties of niobium resonators with different crystalline properties and related surface topographies. We analyze a series of magnetron sputtered films in which the Nb crystal orientation and surface topography are changed by varying the substrate temperatures between room temperature and 975 K. The lowest-loss resonators that we measure have quality factors of over 10 ^6 at single-photon powers, among the best ever recorded using the Nb on sapphire platform. We observe the highest quality factors in films grown at an intermediate temperature regime of the growth series (550 K) where the films display both preferential ordering of the crystal domains and low surface roughness. Furthermore, we analyze the temperature-dependent behavior of our resonators to learn about how the quasiparticle density in the Nb film is affected by the niobium crystal structure and the presence of grain boundaries. Our results stress the connection between the crystal structure of superconducting films and the loss mechanisms suffered by the resonators and indicate that even a moderate change in temperature during thin film deposition can significantly affect the resulting quality factors.

We use charge sensing to detect entropy changes in a double quantum dot defined by electrostatic gating of a GaAs/AlGaAs heterostructure. This system can be tuned to be two separate systems, like two independent, artificial atoms, or a single coherent system, like a molecule. We study entropy changes in both regimes due to changes in the occupation of the system. First we recover the single-dot result for each dot, that the occupation of the dot by a single electron corresponds to an increase in the entropy of k_Blog 2. Next we examine two different charge transitions in the "molecular" regime, and how it reveals itself in terms of the measured entropy. We also uncover a realization of Pauli blockade that clutters the entropy signal. By applying a rate equation model, we demonstrate the effect's nonequilibrium origins and exclude it from the analysis of the system's entropy. Understanding these experiments in this simplest coupled system enables the study of the entropy in other, more complicated coupled quantum systems, such as ones with topological or highly entangled ground states.

The capacitance and differential conductance of MBE-grown AlGaAs/GaAs p-i-n diodes are investigated. In these devices, the p-doped layer, an adjacent intrinsic spacer, and a central barrier are composed of AlGaAs. A voltage bias applied between the doped layers enables the accumulation of two-dimensional electrons and holes at the barrier, forming a closely spaced electron-hole bilayer (EHB) system. Surprisingly, the details of the doping scheme have a profound effect on the characteristics of the EHB. In particular, with the dopants placed in a layer with a larger band gap, large conductance and capacitance oscillations are observed, making the observation of excitonic effects virtually impossible. In this article, we report new experiments and analyses that clarify not only the origins but also the quantitative details of the oscillations.

In light of recent developments demonstrating the impact of cavity vacuum fields inducing the breakdown of topological protection in the integer quantum Hall effect, a compelling question arises: what effects might cavity vacuum fields have on fundamental constants in solid-state systems? In this work we present an experiment that assesses the possibility of the von Klitzing constant itself being modified. By employing a Wheatstone bridge, we precisely measure the difference between the quantized Hall resistance of a cavity-embedded Hall bar and the resistance standard, achieving an accuracy down to 1 part in 105 for the lowest Landau level. While our results do not suggest any deviation that could imply a modified Hall resistance, our work represents pioneering efforts in exploring the fundamental implications of vacuum fields in solid-state systems.

We have characterized the electronic properties of a high-mobility two-dimensional electron system in modulation doped InAsSb quantum wells and compare them to InSb quantum wells grown in a similar fashion. Using temperature-dependent Shubnikov-de Haas experiments as well as FIR transmission we find an effective mass of $m^{\ast} \approx$ 0.022$m_{e}$, which is lower than in the investigated InSb quantum well, but due to a rather strong confinement still higher than in the corresponding bulk compound. The effective $g$-factor was determined to be $g^{\ast} \approx$ 21.9. These results are also corroborated by $k \cdot p$ band structure calculations. When spin polarizing the electrons in a tilted magnetic field, the $g$-factor is significantly enhanced by electron-electron interactions, reaching a value as large as $g^{\ast}$ = 60 at a spin polarization P = 0.75. Finally, we show that due to the low effective mass the quantum Hall effect in our particular sample can be observed up to a temperature of 60 K and we propose scenarios how to increase this temperature even further.

Magnetometers based on the extraordinary magnetoresistance (EMR) effect are promising for applications which demand high sensitivity combined with room temperature operation but their application for magnetic field sensing requires further optimization. A key challenge is to obtain Ohmic metal/semiconductor contacts with low contact resistances in EMR devices comprising semiconductors with low carrier densities and high electron mobilities, yet, this topic remains scarcely investigated experimentally. By annealing high-mobility InSb in argon with systematically increasing temperatures, we experimentally demonstrate how the contact resistance to InSb films can be improved by two orders of magnitude by annealing to the micro-Ohm cm2 range without degrading the high mobility. We further show that lowering the contact resistance monotonously increases the room temperature magnetoresistance at 2 T from 700 to 65,000 high-mobility InSb thin films and suggest that it can best be explained by multiple band conduction.

We study the effect of the pseudospin ferromagnetism with the aid of an electrically detected electron spin resonance in a wide AlAs quantum well containing a high quality two-dimensional electron system. Here, pseudospin emerges as a two-component degree of freedom, that labels degenerate energy minima in momentum space populated by electrons. The built-in mechanical strain in the sample studied imposes a finite "Zeeman" splitting between the pseudospin "up" and "down" states. Because of the anisotropy of the electron spin splitting we were able to independently measure the electron spin resonances originating from the two in-plane valleys. By analyzing the relative resonance amplitudes, we were able to investigate the ferromagnetic phase transitions taking place at integer filling factors of the quantum Hall effect when the magnetic field is tilted. The pseudospin nature of these transitions is demonstrated.