We demonstrate the emergence of novel topological phases in quantum Hall-superconductor hybrid structures driven by Landau-level mixing and spin-orbit coupling. For a narrow superconducting stripe atop a two-dimensional electron gas, hybridization of chiral Andreev edge states yields a rich phase diagram, including the unexpected realization of the long-sought p-wave superconducting state at even filling factors, thus allowing its detection at lower fields. These phases feature quantized nonlocal conductance from electron cotunneling at filling factor ν=1, coexisting with quantized crossed Andreev reflection at ν=2 leading to the appearance of a neutral mode. Numerical simulations and effective modeling reveal how spin-orbit coupling and geometry control these transitions, enabling realistic routes to engineer topology in proximized quantum Hall devices.

We report the first experimental observation of subgap transport in ferromagnetic insulator-superconductor-insulator-superconductor junctions realized in EuS/Al/AlOx/Al vertical stacks. Differential conductance measurements reveal multiple Andreev reflection peaks, with odd-order peaks split by the spin-splitting induced in the superconductor adjacent to EuS, while even-order peaks remain unaffected. Combining experiments with quasiclassical transport modeling, we extract the spin-splitting and an approximate distribution of transmission channels, finding that roughly a hundred highly transmitting (τ>0.3) channels will dominate transport. The observation of a Josephson current further confirms strong superconducting coupling through these channels. Our results demonstrate that a single spin-split superconductor is sufficient to observe the even-odd multiple Andreev reflection effect. Our work establishes EuS/Al junctions as a versatile platform to study subgap transport, Josephson coupling, and spin-polarized superconducting phenomena.

We study active steering protocols for weakly measured qubits in the presence of error channels due to amplitude and phase noise. If the error rate is sufficiently small, the protocol approaches and stabilizes a predesignated pure target state with high fidelity and high purity, and thus implements autonomous state stabilization. We present numerical simulation results for one and two qubits, taking Andreev qubit circuits as an example. As a function of the error rate, a sharp threshold separates an error-correcting weak-damping regime from a strong-damping regime where the target state cannot be reached anymore. At the threshold, the purity gap closes.

Superconducting interfaces have recently been demonstrated to contain a rich variety of effects that give rise to sizable thermoelectric responses and unexpected thermal properties, despite traditionally being considered poor thermoelectrics due to their intrinsic electron-hole symmetry. We review different mechanisms driving this response in hybrid normal-superconducting junctions, depending on the dimensionality of the mesoscopic interface. In addition to discussing heat to power conversion, cooling and heat transport, special emphasis is put on physical properties of hybrid devices that can be revealed by the thermoelectric effect.

The ultimate spatial limit to establish a Josephson coupling between two superconducting electrodes is an atomic-scale junction. The Josephson effect in such ultrasmall junctions has been used to unveil new switching dynamics, study coupling close to superconducting bound states or reveal non-reciprocal effects. However, the Josephson coupling is weak and the sensitivity to temperature reduces the Cooper pair current magnitude. Here we show that a feedback element induces a time-dependent bistable regime which consists of spontaneous periodic oscillations between two different Cooper pair tunneling states (corresponding to the DC and AC Josephson regimes respectively). The amplitude of the time-averaged current within the bistable regime is almost independent of temperature. By tracing the periodic oscillations in the new bistable regime as a function of the position in a Scanning Tunneling Microscope, we obtain atomic scale maps of the critical current in 2H-NbSe2 and find spatial modulations due to a pair density wave. Our results fundamentally improve our understanding of atomic size Josephson junctions including a feedback element in the circuit and provide a promising new route to study superconducting materials through atomic scale maps of the Josephson coupling.

Most superconductors exhibit spin-singlet pairing within a single band. In multiband systems with strong spin-orbit coupling, more exotic scenarios can emerge, including Cooper pairs between bands with distinct symmetries. Here, we present evidence of the formation of Cooper pairs between spin-nondegenerate helical surface bands in the superconductor β-Bi_{2}Pd. Scanning tunneling microscopy reveals anisotropic Yu-Shiba-Rusinov (YSR) resonances induced by vanadium impurities, with long-range amplitude modulations attributed to spin-conserving Bogoliubov quasiparticle interference (BQPI). Analysis of BQPI at the subgap YSR energy shows that only a selective subset of normal-state scattering processes contributes to Cooper pair formation, indicating interband mixing. We trace this selectivity to the hybridization of a helical surface band with others via the impurity.

Most superconducting mechanisms pair electrons within the same band, forming spin singlets. However, the discovery of multi-band superconductivity has opened new scenarios for pairing, particularly in systems with strong spin-orbit coupling. Here, we reveal inter-band pairing in the superconductor by mapping the amplitude of sub-gap Yu-Shiba-Rusinov (YSR) states around Vanadium adatoms deposited on its surface. The surface of is characterized by spin-helical-like bands near the Fermi level. Scanning tunneling spectroscopy reveals anisotropic YSR amplitude oscillations around the impurity, driven by spin-conserving Bogoliubov quasiparticle interference (BQPI). Analysis of the BQPI patterns at the YSR energy exposes inter-band pairing in this material. Interestingly, only a small subset of all possible inter-band scattering processes observed in the normal state contribute to the BQPI patterns. Combining experimental data and theory, we demonstrate that the observed band selectivity results from the hybridization of the band coupled with the impurity with other bands. Our findings reveal unconventional pairing mechanisms in and highlight the crucial role of spin-orbit interactions in their formation.

Strain provides a powerful knob to tailor the electronic properties of semiconductors. Simple yet accurate approximations that capture strain effects in demanding simulations of mesoscopic nanostructures are therefore highly desirable. However, for III-V compounds, key materials for quantum applications, such approaches remain comparatively underdeveloped. In this work, we derive a compact, effective Hamiltonian that describes the conduction band of zincblende III-V semiconductors incorporating strain effects. Starting from the eight-band k·p model with Bir-Pikus corrections, we perform a folding-down procedure to obtain analytical expressions for conduction-band strain-renormalized parameters, including the effective mass, chemical potential, spin-orbit coupling, and g-factor. The model reproduces full multiband results under small to moderate strain, while retaining a form suitable for device-scale calculations. We benchmark the model for bulk deformations and apply it to representative nanostructures, such as core.shell nanowires and planar heterostructures. Our results provide a practical and versatile tool for incorporating strain into the design of III-V semiconductor devices, enabling reliable predictions of their properties with direct implications for spintronic, straintronic, optoelectronic, and topological quantum technologies.

Connecting two half-filled quantum dots to two superconducting leads induces a competition of bonds, with the dots forming either an interdot exchange bond or two individual Yu-Shiba-Rusinov (YSR) screening bonds with the leads. Defining a qubit using these singlet parity bonding states provides dot charge noise protection, attributed to the chargeless nature of the screening quasiparticles, and magnetic noise protection, as the bonds guard against magnetic polarization. In this paper, we propose embedding a Double Quantum Dot (DQD) Josephson junction in parallel with a transmon to enable circuit Quantum Electrodynamics (cQED) measurements and operation of a YSR bond qubit. We demonstrate that, under realistic parameters, two-tone spectroscopy of the DQD can be performed, revealing a significant parameter regime suitable for qubit operation. Additionally, coherent manipulations of the bond states can be achieved through dot gates, and single-shot readout is enabled by measurements of a capacitively coupled resonator. Finally, we analyze noise sources and estimate gate noise on couplings as the primary source of qubit decoherence. Since this qubit is protected against nuclear Overhauser fields and does not rely on spin-orbit interactions for operation, a broader range of material platforms becomes available compared to current Andreev spin qubits.

Detection and control of Andreev bound states (ABSs) localized at semiconductor-superconductor interfaces are essential for their use in quantum applications. Here we investigate the impact of ABSs on the supercurrent through a Josephson junction containing a quantum dot (QD). Additional normal-metal tunneling probes on both sides of the junction unveil the ABSs residing at the semiconductor-superconductor interfaces. Such knowledge provides an ingredient missing in previous studies, improving the connection between theory and experimental data. By varying the ABS energies using electrostatic gates, we show control of the switching current, with the ability to alter it by more than an order of magnitude. Finally, the large degree of ABS tunability allows us to realize a three-site Andreev molecule in which the central QD is screened by both ABSs. This system is studied simultaneously using both supercurrent and spectroscopy.

We introduce a new type of supercurrent valve based on full-shell nanowires. These hybrid wires consist of a semiconductor core fully wrapped in a thin superconductor shell and subjected to an axial magnetic field. Due to the tubular shape of the shell, the superconductor phase acquires an integer number n of 2 pi twists or fluxoids that increases in steps with applied flux. By connecting two such hybrid wires, forming a Josephson junction (JJ), a flux-modulated supercurrent develops. If the two superconducting sections of the JJ have different radii R1 and R2, they can develop equal or different fluxoid numbers n1, n2 depending on the field. If n1 not equal n2, the supercurrent is blocked, while it remains finite for n1 = n2. This gives rise to a fluxoid valve effect controlled by the applied magnetic field or a gate voltage at the junction. We define a fluxoid-valve quality factor that is perfect for cylindrically symmetric systems and decreases as this symmetry is reduced. We further discuss the role of Majorana zero modes at the junction when the full shell nanowires are in the topological superconducting regime.

We probed a gate-tunable InAs nanowire Josephson weak link by coupling it to a microwave resonator. Tracking the resonator frequency shift when the weak link is close to pinch-off, we observe that the ground state of the latter alternates between a singlet and a doublet when varying either the gate voltage or the superconducting phase difference across it. The corresponding microwave absorption spectra display lines that approach zero energy close to the singlet-doublet boundaries, suggesting parity flipping transitions, which are in principle forbidden in microwave spectroscopy and expected to arise only in tunnel spectroscopy. We tentatively interpret them by means of an ancillary state isolated in the junction acting as a reservoir for individual electrons.

We explore superconducting quantum circuits where several leads are simultaneously connected beyond the tunneling regime, such that the fermionic structure of Andreev bound states in the resulting multiterminal Josephson junction influences the states of the full circuit. Using a simple model of single channel contacts and a single level in the middle region, we discuss different circuit configurations where the leads are islands with finite capacitance and/or form loops with finite inductance. We find situations of practical interest where the circuits can be used to define noise protected qubits, which map to the bifluxon and $0{-}\pi$ qubits in the tunneling regime. We also point out the subtleties of the gauge choice for a proper description of these quantum circuits dynamics.

Andreev bound states are fermionic states localized in weak links between superconductors which can be occupied with spinful quasiparticles. Microwave experiments using superconducting circuits with InAs/Al nanowire Josephson junctions have recently enabled probing and coherent manipulation of Andreev states but have remained limited to zero or small fields. Here we use a flux-tunable superconducting circuit in external magnetic fields up to 1T to perform spectroscopy of spin-polarized Andreev states up to ~250 mT, beyond which the spectrum becomes gapless. We identify singlet and triplet states of two quasiparticles occupying different Andreev states through their dispersion in magnetic field. These states are split by exchange interaction and couple via spin-orbit coupling, analogously to two-electron states in quantum dots. We also show that the magnetic field allows to drive a direct spin-flip transition of a single quasiparticle trapped in the junction. Finally, we measure a gate- and field-dependent anomalous phase shift of the Andreev spectrum, of magnitude up to approximately $0.7π$. Our observations demonstrate new ways to manipulate Andreev states in a magnetic field and reveal spin-polarized triplet states that carry supercurrent.

Hybrid superconductor-semiconductor devices offer highly tunable platforms, potentially suitable for quantum technology applications, that have been intensively studied in the past decade. Here we establish that measurements of the superconductor-to-normal transition originating from Joule heating provide a powerful spectroscopical tool to characterize such hybrid devices. Concretely, we apply this technique to junctions in full-shell Al-InAs nanowires in the Little-Parks regime and obtain detailed information of each lead independently and in a single measurement, including differences in the superconducting coherence lengths of the leads, inhomogeneous covering of the epitaxial shell, and the inverse superconducting proximity effect; all-in-all constituting a unique fingerprint of each device with applications in the interpretation of low-bias data, the optimization of device geometries, and the uncovering of disorder in these systems. Besides the practical uses, our work also underscores the importance of heating in hybrid devices, an effect that is often overlooked.