Hybrid quantum dot-oscillator systems have become attractive platforms to inspect quantum coherence effects at the nanoscale. Here, we investigate a Cooper-pair splitter setup consisting of two quantum dots, each linearly coupled to a local resonator. The latter can be realized either by a microwave cavity or a nanomechanical resonator. Focusing on the subgap regime, we demonstrate that cross-Andreev reflection, through which Cooper pairs are split into both dots, can induce nonlocal correlations between the two resonators. Harnessing these correlations allows to establish and control a nonlocal photon transfer between them. The proposed scheme can act as a photonic valve with single-photon accuracy, with potential applications for quantum heat engines and refrigerators involving mesoscopic resonators.

One of the stunning consequences of quantum correlations in thermodynamics is the reversal of the arrow of time, recently shown experimentally in Micadei et al. [Nat. Commun. 10, 2456 (2019)], and manifesting itself by a reversal of the heat flow (from the cold system to the hot one). Here, we show that contrary to what could have been expected, heat flow reversal can happen without reversal of the arrow of time. Moreover, contrasting with previous studies, no initial correlations between system and bath are required. Instead, the heat flow reversal only relies on internal quantum coherences or correlations, which provides practical advantages over previous schemes: One does not need to have access to the bath in order to reverse the heat flow. The underlying mechanism is explained and shown to stem from the collective system-bath coupling and the impact of nonenergetic coherences (coherences between degenerate energy levels) on apparent temperatures. The phenomenon is first uncovered in a broad framework valid for diverse quantum systems containing energy degeneracy. By the end of the paper, aiming at experimental realisations, more quantitative results are provided for a pair of two-level systems. Finally, as a curiosity, we mention that our scheme can be adapted as a correlations-to-energy converter, which have the particularity to be able to operate at constant entropy, similarly to ideal work sources.

Compact microwave resonantors made of superconducting rings containing Josephson junctions (SQUIDs) are attractive candidates for building frequency-tunable metamaterials with low losses and pronounced nonlinear properties. We explore the nonlinearity of a SQUID metamaterial by performing a two-tone resonant spectroscopy. The small-amplitude response of the metamaterial under strong driving by a microwave pump tone is investigated experimentally and theoretically. The transmission coefficient $S_{21}$ of a weak probe signal is measured in the presence of the pump tone. When increasing the power of the pump, we observe pronounced oscillations of the SQUID's resonance frequency $f_{\mathrm{res}}$. The shape of these oscillations varies significantly with the frequency of the pump tone $f_{\mathrm{dr}}$. The response to the probe signal displays instabilities and sidebands. A state with strong second-harmonic generation is observed. We provide a theoretical analysis of these observations, which is in good agreement with the experimental results.

The anomalous ground-state phase shift in superconductor/antiferromagnet/superconductor Josephson junctions in the presence of the Rashba spin-orbit coupling is predicted and numerically investigated. It is found to be a consequence of the uncompensated magnetic moment at the superconductor/antiferromagnet interfaces. The anomalous phase shift exhibits a strong dependence on the value of the spin-orbit coupling and the sublattice magnetization with the simultaneous existence of stable and metastable branches. It depends strongly on the orientation of the Néel vector with respect to the superconductor/antiferromagnet interfaces via the dependence on the orientation of the interface uncompensated magnetic moment. This effect opens a way to control the Néel vector by supercurrent in Josephson systems.

The ability of cells to respond to external stimuli, by up and down regulation of genes, is a key strategy for survival in changing environmental conditions. Models based on equilibrium statistical mechanics have been able to successfully predict in vivo fold changes in gene expression by computing weights of configurational states of the promoter where genes can be expressed. These models are based on the same, perhaps unintuitive, assumption that transcription initiation—an inherently nonequilibrium process—can indeed be effectively described by the equilibrium binding of transcription factors (TFs) and polymerases to the promoter. The few earlier studies that independently test this assumption [P. H. von Hippel et al., Proc. Natl. Acad. Sci. USA 71, 4808 (1974)], were published before much of our modern day understanding of molecular biology was established, and their models fail to explain more recent experimental results. As such, it is not obvious that the original conclusions remain valid. Equilibrium models depend on fitted free energy differences between binding of TFs to specific operator sites versus nonspecific DNA. In this article we compare the fitted binding free energy of the well-studied LacI repressor to equilibrium binding constants measured in independent in vitro experiments. To make this comparison we take into account the distribution of binding energies of the transcription factor to the nonspecific DNA, and we adjust LacI binding constants to a common set of physiological conditions. We find that the fitted binding energies of the LacI repressor in vivo indeed agree with in vitro measured equilibrium binding constants, reestablishing the idea that equilibrium statistical mechanical models of transcriptional regulation should be viewed not merely as mathematical tools, but also as informative physical representations of underlying TF-DNA interactions.

We develop a formalism to study the resonant inelastic x-ray scattering (RIXS) response in metals based on the diagrammatic expansion for its cross section. The standard approach to the solution of the RIXS problem relies on two key approximations: short-range potentials and noninteracting conduction electrons. However, these approximations are inaccurate for charged particles in metals, where the long-range Coulomb interaction and dynamic screening effects are very important. In this work, we study how to extract important information about collective excitations in the Coulomb plasma, plasmons, and electron-hole pairs from RIXS data. We find that single- and multi-plasmon excitations can easily be distinguished by positions of the corresponding peaks, singularities, and their intensities. We also discuss the hybrid processes, where plasmon emission is accompanied by excitation of electron-hole pairs and study how they manifest themselves.

The efficient validation of quantum devices is critical for emerging technological applications. In a wide class of use cases the precise engineering of a Hamiltonian is required both for the implementation of gate-based quantum information processing as well as for reliable quantum memories. Inferring the experimentally realized Hamiltonian through a scalable number of measurements constitutes the challenging task of Hamiltonian learning. In particular, assessing the quality of the implementation of topological codes is essential for quantum error correction. Here, we introduce a neural-net-based approach to this challenge. We capitalize on a family of exactly solvable models to train our algorithm and generalize to a broad class of experimentally relevant sources of errors. We discuss how our algorithm scales with system size and analyze its resilience toward various noise sources.

We propose a method to probe and control the interactions between an ensemble of magnetic impurities in a superconductor via microwave radiation. Our method relies upon the presence of subgap Yu-Shiba-Rusinov (YSR) states associated with the impurities. Depending on the sign of the detuning, radiation generates either a ferro- or antiferromagnetic contribution to the exchange interaction. This contribution can bias the statistics of the random exchange constants stemming from the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. Moreover, by measuring the microwave response at the YSR resonance, one gains information about the magnetic order of the impurities. To this end, we estimate the absorption coefficient as well as the achievable strength of the microwave-induced YSR interactions using off-resonant radiation. The ability to utilize microwave fields to both probe and control impurity spins in a superconducting host may open new paths to studying metallic spin glasses.

The melting temperature of diamond has not yet been experimentally measured under static pressure and these data for cubic boron nitride (cBN) are scarce. This lack of information is understandable due to the difficulty of heating optically transparent diamond via absorption of intense laser radiation in a diamond anvil cell and obtaining a high temperature with traditional resistance heating methods in a large-volume, multiple-anvil apparatus. Here, we report the melting temperature of diamond ($5968\pm 457$ K) and cBN ($5689\pm 411$ K) at a pressure of 15 GPa based on a two-stage multiple-anvil apparatus coupled with a so-called flash-heating method and show that the previous theoretical predictions underestimate the melting points of these two superhard materials. Our results indicate that diamond has the highest melting point in nature, and strengthen our understanding of the melting behavior of those two superhard materials.

We consider a system of weakly coupled Rashba nanowires in the strong spin-orbit interaction (SOI) regime. The nanowires are arranged into two tunnel-coupled layers proximitized by a top and bottom superconductor such that the superconducting phase difference between them is $\pi$. We show that in such a system strong electron-electron interactions can stabilize a helical topological superconducting phase hosting Kramers partners of ${\mathbb{Z}}_{2m}$ parafermion edge modes, where $m$ is an odd integer determined by the position of the chemical potential. Furthermore, upon turning on a weak in-plane magnetic field, the system is driven into a second-order topological superconducting phase hosting zero-energy ${\mathbb{Z}}_{2m}$ parafermion bound states localized at two opposite corners of a rectangular sample. As a special case, zero-energy Majorana corner states emerge in the noninteracting limit $m=1$, where the chemical potential is tuned to the SOI energy of the single nanowires.

Single-file transport, where particles cannot bypass each other, has been observed in various experimental setups. In such systems, the behavior of a tracer particle (TP) is subdiffusive, which originates from strong correlations between particles. These correlations are especially marked when the TP is driven and leads to inhomogeneous density profiles. Determining the impact of this inhomogeneity when several TPs are driven in the system is a key question, related to the general issue of bath-mediated interactions, which are known to induce collective motion and lead to the formation of clusters or lanes in a variety of systems. Quantifying this collective behavior, the emerging interactions, and their dependence on the amplitude of forces driving the TPs remains a challenging but largely unresolved issue. Here, considering dense single-file systems, we analytically determine the entire dynamics of the correlations and reveal out-of-equilibrium cooperativity and competition effects between driven TPs.

Dipolar quantum droplets are exotic quantum objects that are self-bound due to the subtle balance of attraction, repulsion, and quantum correlations. Here we present a systematic study of the critical atom number of these self-bound droplets, comparing the experimental results with extended mean-field Gross-Pitaevskii equation and quantum Monte Carlo simulations of the dilute system. The respective theoretical predictions differ, questioning the validity of the current theoretical state-of-the-art description of quantum droplets within the extended Gross-Pitaevskii equation framework and indicating that correlations in the system are significant. Furthermore, we show that our system can serve as a sensitive testing ground for many-body theories in the near future.

Detection and imaging of an electrically conductive object at a distance can be achieved by inducing eddy currents in it and measuring the associated magnetic field. We have detected low-conductivity objects with an optical magnetometer based on room-temperature cesium atomic vapor and a noise-canceling differential technique which increased the signal-to-noise ratio (SNR) by more than three orders of magnitude. We detected small containers with a few mL of salt water with conductivity ranging from 4–24 S/m with a good SNR. This demonstrates that our optical magnetometer should be capable of detecting objects with conductivity $<1\mathrm{S}/\mathrm{m}$ with a SNR $>1$ and opens up new avenues for using optical magnetometers to image low-conductivity biological tissue including the human heart which would enable noninvasive diagnostics of heart diseases.

How long does it take a quantum particle to return to its origin? As shown previously under repeated projective measurements aimed to detect the return, the closed cycle yields a geometrical phase which shows that the average first detected return time is quantized. For critical sampling times or when parameters of the Hamiltonian are tuned this winding number is modified. These discontinuous transitions exhibit gigantic fluctuations of the return time. While the general formalism of this problem was studied at length, the magnitude of the fluctuations, which is quantitatively essential, remains poorly characterized. Here, we derive explicit expressions for the variance of the return time, for quantum walks in finite Hilbert space. A classification scheme of the diverging variance is presented, for four different physical effects: the Zeno regime, when the overlap of an energy eigenstate and the detected state is small, and when two or three phases of the problem merge. These scenarios present distinct physical effects which can be analyzed with the fluctuations of return times investigated here, leading to a topology-dependent time-energy uncertainty principle.

We study the electron and spin transport in a van der Waals system formed by one layer with strong spin-orbit coupling and a second layer without spin-orbit coupling, in the regime when the interlayer tunneling is random. We find that in the layer without intrinsic spin-orbit coupling, spin-charge coupled transport can be induced by two distinct mechanisms. First, the gapless diffusion modes of the two isolated layers hybridize in the presence of tunneling, which constitutes a source of spin-charge coupled transport in the second layer. Second, the random tunneling introduces spin-orbit coupling in the effective disorder-averaged single-particle Hamiltonian of the second layer. This results in nontrivial spin transport and, for sufficiently strong tunneling, in spin-charge coupling. As an example, we consider a van der Waals system formed by a two-dimensional electron gas (2DEG)—such as graphene—and the surface of a topological insulator (TI) and show that the proximity of the TI induces a coupling of the spin and charge transport in the 2DEG. In addition, we show that such coupling can be tuned by varying the doping of the TI's surface. We then obtain, for a simple geometry, the current-induced nonequilibrium spin accumulation (Edelstein effect) caused in the 2DEG by the coupling of charge and spin transport.

We investigate supercurrent interference patterns measured as a function of magnetic field in ballistic graphene Josephson junctions. At high doping, the expected ${\mathrm{\Phi }}_0$-periodic “Fraunhofer” pattern is observed, indicating a uniform current distribution. Close to the Dirac point, we find anomalous interferences that are close to a $2{\mathrm{\Phi }}_0$ periodicity, similar to that predicted for topological Andreev bound states carrying a charge of $e$ instead of $2e$. This feature persists with increasing temperature, ruling out a nonsinusoidal current-phase relationship. It also persists in junctions in which sharp vacuum edges are eliminated. Our results indicate that the observed behavior may originate from an intrinsic property of ballistic graphene Josephson junctions, though the exact mechanism remains unclear.

We present a thorough study of the thermodynamics of a one-dimensional repulsive Bose gas, focusing in particular on corrections beyond the Luttinger-liquid description. We compute the chemical potential, pressure, and contact as a function of temperature and gas parameter with an exact thermal Bethe ansatz. In addition, we provide interpretations of the main features in the analytically tractable regimes, based on a variety of approaches (Bogoliubov, hard core, Sommerfeld, and virial). The beyond-Luttinger-liquid thermodynamic effects are found to be nonmonotonic as a function of gas parameter. Such behavior is explained in terms of nonlinear dispersion and “negative excluded volume” effects, for weak and strong repulsion, respectively, responsible for the opposite sign corrections in the thermal next-to-leading term of the thermodynamic quantities at low temperatures. Our predictions can be applied to other systems including super Tonks-Girardeau gases, dipolar and Rydberg atoms, helium, quantum liquid droplets in bosonic mixtures, and impurities in a quantum bath.

In the known topological semimetals, conventional charge carriers exist in addition to relativistic quasiparticles, and thus a disentangling of their conduction properties remains challenging. Here, we address an unsaturated extreme magnetoresistance (XMR) with a marked deviation from the semiclassical $B^2$ behavior that is commonly credited to the presence of topologically protected electronic states. For the topologically trivial semimetal LuAs, we observe a nonsaturating XMR with a nonquadratic magnetic-field dependence gained up to nearly 60 T. Remarkably, this diamagnetic material exhibits a very large magnetostriction that provides solid evidence for a field-dependent variation of electron and hole concentrations. We show that an underlying strain-induced change in the charge-carrier densities can give rise to an unsaturated XMR even in a moderately imbalanced semimetal. Our finding is of importance as well for topological semimetals in which the number of conventional charge carriers can be continuously altered with increasing field, and hence some of their high-field properties may not necessarily reflect the presence of massless quasiparticles.

The proper description of step structures in the exchange correlation potential, of charge localization, and a reasonable prediction of band gaps have been long-standing, serious challenges for semilocal density functionals. In practice, obtaining all of these properties from the functional derivative of an energy functional was possible only at the price of incorporating exact exchange. We here show that they can be achieved at significantly lower, semilocal computational expense by using kinetic-energy density-dependent functionals. The key to obtaining these features is a functional construction strategy that focuses on the derivative discontinuity and the density response.

Hierarchical aggregation is generally viewed as a kinetic phenomenon governed by kinetic growth laws, such as in the Smoluchowski equation, and modeled using diffusion or reaction limited kinetic growth models. Some aggregates, especially those controlled by surface grafting or surfactants, display reversible stability. For these equilibrated aggregates a simple thermodynamic model is proposed to describe the size distribution and the enthalpy and entropy of aggregation. The model uses the average degree of aggregation, $z_{i\left(i-1\right)}$, as the central quantifying parameter. Here $i$ is an index reflecting the hierarchical level of structure in an aggregate, for instance, composed of crystals ($i=0$), clustered primary particles ($i=1$), aggregates ($i=2$), and agglomerates of aggregates ($i=3$). A change in Gibbs free energy for aggregation is given by $\mathrm{\Delta }{G}_{i\left(i-1\right)}=-RTln\left(1/z_{i\left(i-1\right)}\right)$ for each level ($i>0$). This expression is advantageous since the degree of aggregation is directly determined in small-angle neutron and x-ray scattering, by transmission electron microscopy, simulation, or through spectroscopy. The atomistic hierarchical model enables an understanding of the mechanism of equilibrium aggregation since it provides expressions for entropy and enthalpy of aggregation at each structural/thermodynamic level. The model can be extended to describe pseudoequilibrium for industrially relevant materials such as condensation polymers. Applications in organic pigments and wormlike micelles are also briefly demonstrated.