The nature of the superconducting state in two-dimensional transition-metal dichalcogenides remains under active debate. A widely used description invokes so-called Ising superconductivity. In this work, we investigate theoretically this pairing state by employing single magnetic impurities as local probes of the superconducting condensate. We analyze the formation of Yu-Shiba-Rusinov bound states in the presence of Ising spin-orbit coupling and an in-plane magnetic field to study how their spectral properties encode the underlying pairing structure. We identify distinct features in the bound-state spectrum and tunneling response that differentiate this system from conventional superconductors. Our results demonstrate that magnetic impurities provide a sensitive probe of the structure of the superconducting state and yield experimentally accessible signatures of unconventional aspects of Ising superconductivity.

We demonstrate that the charge value of transport mechanisms heavily impacts the validity of thermodynamic uncertainty relations (TURs). Specifically, we show within the framework of full counting statistics, that the recently established quantum TUR can be violated by the presence of transport processes that carry more than one charge, like Andreev reflection processes in normal metal-superconductor junctions. We propose a modified quantum TUR, which incorporates the charge value and demonstrate that this charge-dependent quantum TUR can only be violated if the highest charge transport process exceeds this charge value. In particular, we establish that the breaking of the quantum TUR solely originates from the charge value of the highest charge transport process. Namely, our analytical considerations do not invoke the existence of superconductivity, and these considerations generally hold for non-interacting electronic transport which can be described by the scattering formalism.

We report measurements of the thermopower of atomic-size gold contacts realized by the mechanically controllable break junction (MCBJ) technique over a temperature range from 18 K to 295 K. A thermometer included in the lithographic structure close to the constriction provides a direct measurement of the temperature increase generated by heating one side of the contact with a focused laser beam. While the conductance histograms confirm the quantum nature of the transport, we observe a nonmonotonic temperature dependence of the ensemble-averaged thermopower with a minimum of -2 μVK^-1 at about 150 K. The values for the thermopower obtained at the lowest and the high temperature are compatible with values reported in the literature, but the nonmonotonic behavior in between disagrees with the expected linear dependence for quantum coherent conductors described by the Landauer formula. We develop a theoretical model based on an energy dependent transmission function that qualitatively reproduces the nonmonotonic behavior, but fails quantitatively. We therefore interpret our data as a result of phonon contributions to the thermopower beyond the Landauer model and with opposite sign than the classical phonon drag known from bulk systems. Our findings show that, firstly, the thermopower gives important insight into the transport properties of atomic-size structures and second that the linear approximation of the Landauer model has to be used with caution when studying more complex transport properties even for atomic contacts from free-electron metals.

Charge transport in superconducting junctions at finite voltages is governed by Andreev reflections, including multiple Andreev reflections, which are processes that enable multiple charge transfer, a hallmark that shot noise can directly quantify. Since the effective charge extracted from shot noise measurements varies with the transparency of the junction, systematic control of transparency is essential but experimentally challenging. Here, we present shot noise scanning tunneling microscopy measurements enabled by a newly developed amplifier, allowing access to different transparency regimes. We perform shot noise measurements on Pb(111) with tunable transparency at 2.2 K and observe that the shot noise evolves from a single electron tunneling regime to multiple charge transfer regime as transparency increases. Our results are quantitatively consistent with theoretical simulations of Andreev reflections and multiple Andreev reflections for a single-channel system. These results establish junction transparency as the key parameter governing the evolution of charge transport and demonstrate that noise-STM is a powerful platform for investigating microscopic charge transport mechanisms with controlled junction transparency at the atomic scale.

We demonstrate the first violation of the Leggett-Garg inequality and time-order noninvariance on public quantum computers using genuine noninvasive measurements. By gathering sufficiently large statistics, we have been able to violate Leggett-Garg inequality and time-order invariance. The detailed analysis of the data on 10 qubit sets from 5 devices available on IBM Quantum and one on IonQ reveals violations beyond 5 standard deviations in almost all cases. We implemented our protocols using fractional gates, newly available on the IBM Heron devices, allowing us to benchmark them in application to weak measurements. The noninvasiveness is supported by a qualitative and quantitative agreement with the model of weak disturbance. Moreover, our data expose statistically significant deviations from theoretical predictions that exceed declared device error rates, establishing weak measurement protocols as a sensitive benchmark for quantum hardware. These advances transform public quantum computers into practical testbeds for probing foundational questions of realism and temporal order with unprecedented accessibility and precision.

We report on the emergence of the anomalous (paramagnetic) superfluid response in altermagnetic superconductors at arbitrary impurity concentrations. Due to anisotropic gapless superconductivity, altermagnetic superconductors with an out-of-plane Zeeman field display an anisotropic paramagnetic Meissner effect. The effect is strongest for parallel altermagnetic and Zeeman exchange field vectors and in the clean sample. The presence of nonmagnetic impurities leads to isotropisation and, consequently, weakens the effect; however, the paramagnetic response sustains intermediate amounts of impurities in the system. As demonstrated in recent experiments, microwave superfluid stiffness measurements can serve as a sensitive probe of gapless superconductivity.

Nanomechanical resonators are a powerful platform for studying nonlinear dynamics with high sensitivity and precision. We explore the nonlinear response of a high-Q nanomechanical string resonator in and beyond the Duffing regime and introduce a robust framework for accurately extracting its conservative nonlinearities. The method is based on the backbone curve obtained from ringdown measurements, making it inherently resilient to small frequency fluctuations while explicitly accounting for both symmetry-breaking and non-symmetry-breaking nonlinearities. To validate the approach, we perform complementary ringdown and frequency-response measurements on the nanostring resonator and benchmark the backbone-based extraction against established frequency-response techniques. The comparison confirms the accuracy of the proposed framework and demonstrates its advantages over conventional methods for nonlinear characterization.

Motivated by the orientation-dependent spin splitting and vanishing net magnetization of altermagnets, we study d -wave altermagnetic Josephson junctions (JJs) with a focus on the emergence and controllability of non-Hermitian degeneracies [or exceptional points (EPs)] where both eigenvalues and eigenvectors coalesce. Unlike conventional JJs, the altermagnetic order induces spin- and orientation-dependent Andreev bound states, offering additional control of the Josephson effect. We show that the position of EPs as a function of the superconducting phase difference ϕ can be tuned by the crystal orientation, strength of the altermagnetic order, Rashba spin-orbit coupling, and the degree of non-Hermiticity. Furthermore, the number of EPs is found to be directly tunable by altermagnetic properties. The total supercurrent associated with different transverse momentum k y shows a smooth behavior but sensitive to the crystal orientation and other properties of the altermagnetic JJ. These findings demonstrate a pathway to realizing distinctive non-Hermitian phenomena arising from the interplay among non-Hermiticity, Rashba interaction, superconductivity, and altermagnetism.

We demonstrate violation of objective realism in quantum world using unconstrained weak measurements. Instead of limited Leggett-Garg approach with artificial bounds on the observed values, we assume two identical and indepenent weak detectors and final conditioning. The experimental verification has been performed on public quantum computers, IBM and IonQ. Thanks to sufficiently large statistics, the violation is observed at the level of 10 standard deviations. The tests confirmed also high quality of parametric two-qubit gates offered by main quantum hardware providers.

Incorporating oxygen into metallic atomic-scale junctions modifies the interatomic bonding and may even promote the formation of monoatomic chains. In the specific case of copper oxide, first-principles studies have predicted the emergence of ferromagnetic ground states, attributing certain atomic configurations with spin filtering capabilities. By means of low-temperature transport measurements, we provide a series of experimental evidence indicating the presence of local magnetism in air oxidized mechanically controllable copper break junctions. Our findings include hysteretic magnetoresistance, zero-bias anomalies attributed to the Kondo effect and anomalous shot noise behavior. We provide an extension of the Landauer description of shot noise for quantum coherent transport to include energy-dependent transmission functions. The analysis of the anomalous shot noise in the presence of strong zero-bias anomalies arising from the Kondo effect allows to determine the spin polarization of the current which may reach even full polarization, confirming the spin-filtering capability of copper oxide atomic contacts.

In conventional superconductors, Cooper pairs form in an even-parity spin-singlet state. Noncentrosymmetric superconductors, which lack inversion symmetry, exhibit antisymmetric spin-orbit coupling (ASOC) that can combine even-parity spin-singlet and odd-parity spin-triplet pairs into a mixed-parity order parameter. Spin-triplet components are highly beneficial for superspintronic devices. Whether ASOC alone - without strong electronic correlations - is sufficient to generate a measurable triplet component remains a central open question. Here, we resolve this question in Nb_18Re_82 (Nb-Re), a weakly-correlated noncentrosymmetric metal whose superconducting pairing symmetry has been actively debated. Using low-temperature scanning tunneling spectroscopy on single crystals with four distinct crystallographic orientations, find a pronounced orientation-dependent anisotropy in the local density of states. Supported by a symmetry-constrained model, we show that the complete set of tunneling spectra requires a mixed-parity order parameter with the triplet amplitude reaching up to half of the singlet component. These results reconcile the conflicting reports in the literature on Nb-Re and demonstrate that ASOC is sufficient to foster a sizable spin-triplet component even without strong electronic correlations, suggesting that mixed-parity superconducting states may be more widespread than previously assumed. Since Nb-Re can be readily fabricated in thin-film form, these findings position it as an accessible platform for superspintronic devices and establish orientation-resolved tunneling spectroscopy as a general protocol for the detection of mixed-parity order parameters.

Abstract Noise measurements provide a valuable tool for revealing spin polarization effects in the electronic transport through quantum coherent conductors. We present an extension of the Landauer description of shot noise to include energy dependent transmission functions and apply it to explore local magnetic correlations in air oxidized copper contacts, for which first-principle studies have predicted the emergence of ferromagnetic ground states, attributing certain atomic configurations with spin filtering capabilities. By means of low-temperature transport measurements, we provide comprehensive experimental evidence, including hysteretic magnetoresistance and zero-bias anomalies (ZBAs) attributed to the Kondo effect, for the presence of local magnetism. The analysis of the anomalous shot noise in the presence of ZBAs allows us to determine the spin polarization of the current which may reach even full polarization, confirming the spin filtering capability of copper oxide atomic contacts.

Activation processes govern noise-induced switching between long-lived states. In an equilibrium double well, the thermally activated switching rate exhibits a prefactor with a nonmonotonic dependence on environmental coupling, a foundational crossover known as Kramers turnover. Here, we demonstrate a Kramers turnover analogue in a Kerr parametric oscillator, a driven-dissipative nonlinear system featuring two stable phase states. First, we analytically establish turnover physics in this out-of-equilibrium setting. There, the strong physical correlation between the activation barrier and intrinsic damping fundamentally obscures the underlying turnover physics. To overcome this limitation, we rescale the rotating-frame dynamics and introduce a tunable effective friction controlled entirely by the parametric drive. This rescaling comes at the cost of a concurrent rescaling of the effective temperature. Exploiting this simultaneous scaling, we leverage the effective temperature to extract the turnover directly from temperature-dependent observations. Subsequently, measuring noise-induced phase slips in a micro-electromechanical device, we observe a distinct crossover in the prefactor's temperature dependence. Our results unambiguously isolate the out-of-equilibrium turnover regime and highlight that the competition between dissipation and fluctuations profoundly shapes activation dynamics also beyond equilibrium.

We show that an emergent geometric symmetry generates non-trivial topology in quantum-dot-based multi-terminal Josephson junctions. It confines same-spin Andreev bound state crossings to an analytic one-dimensional manifold of the synthetic Brillouin zone, where interdot coupling selects Weyl nodes of charge ±1 in the singlet sector and doubly degenerate cones of charge ±2 in the doublet sector, at gate-tunable locations. The mechanism yields a spectroscopic detection protocol and a design principle for fabricating devices with non-trivial topological signatures.

Multiterminal superconducting junctions have revitalized the investigation of the Josephson effect. One of the most interesting aspects of these hybrid systems is the occurrence of multi-Cooper pair tunneling processes that have no analog in two-terminal devices. Such correlated tunneling events are also intimately connected to the Andreev bound states (ABSs) supported by these structures. Josephson junctions with four superconducting terminals have attracted special attention because they are predicted to support ABSs with nontrivial topological properties. Here, we present a theoretical study of sextets, which are correlated tunneling processes involving three Cooper pairs and four different superconducting terminals. We investigate how sextets can be identified from the analysis of the current-phase relation, show how sextets are connected to the hybridization of ABSs, and discuss their existence in recent experiments on four-terminal devices realized in hybrid Al/InAs heterostructures.

We theoretically study spin pumping into a spin-nematic state in a junction system composed of a ferromagnetic insulator and a spin-nematic insulator, described by using the spin-1 bilinear-biquadratic model. We analyze an increase of the Gilbert damping in ferromagnetic resonance (FMR) due to an interfacial exchange coupling within a mean-field theory based on the Schwinger boson method. We find that the two Schwinger bosons contribute in distinct ways to spin pumping. We report a detailed dependence of the spin pumping on a resonant frequency, a magnetic field, and an interface type.

Since the discovery of the Andreev reflection process at normal-metal/superconductor junctions and the corresponding Andreev bound states in superconductor/normal-metal/superconductor junctions, various multiterminal Josephson junctions have been studied to explore many exotic phases of quantum matter, where the formation of Andreev bound states in the normal region account for dissipationless supercurrent and play a central role in determining exotic properties. Recently, an intriguing aspect of the multiterminal Josephson junctions has been proposed to study the topological properties, wherein the Andreev bound states acquire topological characteristics upon tuning the phase differences of superconducting terminals. In this work, we investigate topologically nontrivial phases in four-terminal Josephson junctions based on square and graphene lattices. Additionally, we apply a gating potential that smoothly drives the Andreev bound states from a topologically nontrivial state to a topologically trivial state. Furthermore, we observe that the gating potential in our setup produces similar physics of the topological Andreev bound states of the double (single) quantum-dot multiterminal Josephson junctions when the gating potential is small (large) compared to the superconducting energy gap.

Sustaining the growth of the data volume generated by artificial intelligence and the internet of things demands to develop schemes for data storage and processing operating at terahertz frequencies, unrestrained by thermal throttling. The optical drive of coherent magnetic collective excitations, namely magnons, represents a promising route. The ability to arbitrarily and nonthermally increase the magnon frequencies with laser pulses could enable this progress. However, this effect has not been reported to date. To achieve it, here, we explore the optical resonant excitation of high-momentum magnons, which experimentally are observed to couple to low-momentum magnons, modifying the frequencies and amplitudes thereof. This evidence, not caused by laser heating, is explained with a resonant light-scattering mechanism coupling high- and low-momentum eigenmodes across momentum space. Our results disclose routes to inducing instabilities and phase transitions via mode softening and potentially even light-driven Bose-Einstein condensation of magnons and superconductivity mediated by high-momentum spin-fluctuations.

The transition-metal dichalcogenides featuring Ising spin-orbit coupling in so-called Ising superconductors offer a unique system to study the interplay of singlet and triplet superconductivity. The presence of high critical fields, spectral properties such as the mirage gap, and field-tunable charge and spin currents in Ising superconductor Josephson junctions are some of the important features. In this Letter, we study an Ising superconductor Josephson junction with a transparent interface and show that Andreev bound states are spin split due to a relative misorientation of in-plane fields in the superconducting contacts. Correspondingly, supercurrent-phase relations display a strongly nonsinusoidal behavior. Introducing additional spin-polarized channels with low transmission results in a nonreciprocal current-phase relation with a diode effect that can be tuned by the in-plane exchange fields. The diode efficiency reaches high values of the order of 40% and is not sensitive to disorder in the junction. Such structures can be realized in van der Waals heterostructures of two-dimensional superconductors and magnets. Published by the American Physical Society 2025

Advances in circuit quantum electrodynamics have enabled the generation of arbitrary nonclassical microwave states, enabling progress in physics. Here, we present a theoretical study of the electrical current in a Josephson tunnel junction interacting with a nonclassical electromagnetic environment. This allows us to generalize classical transport phenomena like photon-assisted tunneling and Shapiro steps to the quantum regime. We predict that the analysis of the supercurrent in such a setup enables the complete reconstruction of quantum states of the electromagnetic environment, something that is not possible with normal tunnel junctions.