A qubit-oscillator junction connecting as a series two bosonic heat baths at different temperatures can display heat valve and diode effects. In particular, the rectification can change in magnitude and even in sign, implying an inversion of the preferential direction for the heat current with respect to the temperature bias. We perform a systematic study of these effects in a circuit QED model of qubit-oscillator system and find that the features of current and rectification crucially depend on the qubit-oscillator coupling. While at small coupling, transport occurs via a resonant mechanism between the subsystems, in the ultrastrong coupling regime the junction is a unique, highly hybridized system and current becomes largely insensitive to the detuning. Correspondingly, the rectification undergoes a change of sign. In the nonlinear transport regime, the coupling strength determines whether the current scales sub- or superlinearly with the temperature bias and whether the rectification, which increases in magnitude with the bias, is positive or negative. We also find that steady-state coherence largely suppresses the current and enhances rectification. An insight on these behaviors with respect to changes in the system parameters is provided by analytical approximate formulas.

Recent experiments show that the relative dielectric constant $\epsilon$ of water confined to a film of nanometric thickness reaches a strikingly low value of 2.1, barely above the bulk's 1.8 value for the purely electronic response. We argue that $\epsilon$ is not a well-defined measure for dielectric properties at subnanometer scales due to the ambiguous definition of confinement width. Instead, we propose the 2D polarizability ${\alpha }_{\perp }$ as the appropriate, well-defined response function whose magnitude can be directly obtained from both measurements and computations. Once the appropriate description is used, understanding the interplay between electronic and ionic contributions becomes critical, contrary to what is widely assumed. This highlights the importance of electronic degrees of freedom in interpreting the dielectric response of polar fluids under nanoconfinement conditions, as revealed by molecular dynamics simulations.

The interplay between spin and charge degrees of freedom in low-dimensional systems is a cornerstone of modern spintronics, where achieving all-electrical control of spin currents is a major goal. Spin-orbit interactions provide a promising mechanism for such control, yet understanding how spin and charge transport emerge from microscopic principles remains a fundamental challenge. Here, we develop a spin-dependent scattering matrix approach to describe spin and charge transport in a multiterminal system in the presence of Rashba spin-orbit interaction. Our framework generalizes the Büttiker formalism by offering expressions for spin and charge current densities as a function of the lead position, along with the corresponding linear response function. It simultaneously captures the effects of quantum confinement, the response to external magnetic fields, and the intrinsic properties of the electronic bands, offering a comprehensive description of the spin-charge interconversion mechanisms at play in a Hall bar, in agreement with experiments.

Deep neural-network quantum states have emerged as a leading method for studying the ground states of quantum magnets. Successful architectures exploit translational symmetry, but the root of their effectiveness and differences between architectures remain unclear. Here, we apply the ConvNext architecture, designed to incorporate elements of transformers into convolutional networks, to quantum many-body ground states. We find that it is remarkably similar to the factored vision transformer, which has been employed successfully for several frustrated spin systems, allowing us to relate this architecture to more conventional convolutional networks. Through a series of numerical experiments, we design the ConvNext to achieve greatest performance at lowest computational cost, and then apply this network to the Shastry-Sutherland and $J_1-J_2$ models, obtaining variational energies comparable to the state of the art, providing a blueprint for network design choices of translationally symmetric architectures to tackle challenging ground-state problems in frustrated magnetism.

A ring of capacitively coupled transmons threaded by a synthetic magnetic field is studied as a realization of a strongly interacting bosonic system. The synthetic flux is imparted through a specific Floquet modulation scheme based on a suitable periodic sequence of Lorentzian pulses that are known as “Levitons.” Such scheme has the advantage to preserve the translation invariance of the system and to work at the qubit sweet spots. We employ this system to demonstrate the concept of fractional values of flux quanta. Although such fractionalization phenomenon was originally predicted for bright solitons in cold atoms, it may be in fact challenging to access with that platform. Here, we show how fractional flux quanta can be read out in the absorption spectrum of a suitable “scattering experiment” in which the qubit ring is driven by microwaves.

Quantum non-Gaussian states of light have fundamental properties that are essential for a multitude of applications in quantum technology. However, many of these features are difficult to detect using standard criteria due to optical losses and detector inefficiency. As the statistics of light are unknown, the loss correction on the data is unreliable, despite the fact that the losses can be precisely measured. To address this issue, we employ a loss-mitigated verification technique utilizing quantum non-Gaussian witnesses, which incorporate the known optical losses and detector inefficiency into their derivation. This approach allows us to address the considerable challenge of experimentally demonstrating unheralded quantum non-Gaussian states of single photons and photon pairs.

We present a unified exact analytic solution of the radial Schrödinger equation with a class of four long-range potentials, three of which are two-scale. The physics of different potentials are encapsulated in the solution variable and parameters. Based on this solution and its asymptotic behaviors, we generalize Bo Gao’s formulation of quantum defect theory for single-scale potentials to two-scale regime. As an application of this two-scale quantum defect theory, we propose a concept of harmonic oscillator quantum defect of energy levels in a spherical harmonic trap, which characterizes the effect of the two-body interactions on their trap states. This concept is a generalization of the standard quantum defect for Coulomb potential, with a distinct feature that the equations followed by the two types of quantum defects induced by the potentials under study are formally the same yet with different parameters. Examples of these two quantum defects are illustrated for Rb and Cs atoms.

The rotation of a nanoparticle in a quantum system has many applications, from theory to experiments. This paper will treat nanoparticle rotational dynamics for spin-embedded nanorotors. We will model it as a rigid body that properly treats the rotation in the coframe of the nanorotor in the presence of external fields. Besides rotation, we will further investigate how to create large spatial superpositions in the inhomogeneous external magnetic field, such as in the case of the Stern-Gerlach apparatus. The spin-embedded nanorotors play a crucial role in creating matter-wave interferometers through their spin and external magnetic field interaction Hamiltonian. We aim to provide a holistic interpretation of the dynamics of three Euler angles, their quantum evolution, and the nanorotor's spatial motion in a Stern-Gerlach-type setup, where we will consider one-full-loop interferometry. We will then study how the quantum evolution of all the Euler angles leads to a spin coherence loss upon interference and what manifests the Einstein-de Haas effect in an external magnetic field. In particular, we show that by imparting rotation along the direction of the magnetic field, we can stabilize the nanorotor's libration mode. We will also extend our analysis to a case where the initial state of the libration mode is thermal and discuss the contrast loss due to interference of the nanorotor upon one-loop completion.

Quantum machine learning has gained attention for its potential to address computational challenges. However, whether those algorithms can effectively solve practical problems and outperform their classical counterparts, especially on current quantum hardware, remains a critical question. In this work, we propose a quantum machine learning method, called Parameter-Efficient Quantum Anomaly Detection (PEQAD), for practical image anomaly detection, which aims to achieve both parameter efficiency and superior accuracy compared to classical models. Emulation results indicate that PEQAD demonstrates favorable recognition capabilities compared to classical baselines, achieving an average accuracy of over 90% on benchmarks with significantly fewer trainable parameters. Theoretical analysis suggests that PEQAD has a comparable expressivity to classical counterparts while requiring only a fraction of the parameters. Furthermore, we demonstrate the first implementation of a quantum anomaly detection method for general image datasets on a superconducting quantum processor. Specifically, we achieve an accuracy of over 80% with only 16 parameters on the device, providing initial evidence of PEQAD’s practical viability and highlighting its significant reduction in parameter requirements.

When a droplet is deposited on a rough solid substrate, the classic Cassie-Wenzel theory (CWT) is often used to describe the wetting states. However, CWT fails to explain experimental observations of high static friction on hydrophobic surfaces with a critical roll-off angle close to ${90}^{\circ }$; the static friction is associated with the minimum energy required for the transition between the Cassie state and the Wenzel state. Here, we propose an alternative theory for calculating the static friction force of a droplet on a rough solid surface, based on a complete energy landscape method. Our results for static friction are orders of magnitude larger than those predicted by CWT and have been validated through quantitative comparisons with experiments.

The interplay between symmetry breaking, spin-orbit coupling, and magnetic frustration in noncentrosymmetric antiferromagnets has emerged as a fertile ground for discovering intertwined topological and quantum phenomena. Here, we report a cascade of magnetic phase transitions and a large topological Hall response in the newly discovered noncentrosymmetric antiferromagnetic (AFM) material ${\mathrm{Eu}}_2{\mathrm{Pd}}_3{\mathrm{Bi}}_4$ with a Neél temperature $T_N=7.6\mathrm{K}$. Under magnetic fields ($H$) parallel to the $c$ axis, two successive phase transitions at critical fields $H_1$ and $H_2$ are observed in magnetization at temperatures below $T_N$, which are attributed to the occurrences of sequential field-induced spin states. A pronounced topological Hall effect signal emerges exclusively within an intermediate field window $H_1<H<H_2$, signifying the formation of topological spin textures. The field-temperature phase diagram mapped via specific heat and magnetoelectric transport measurements incorporates five phase regimes: The paramagnetic state, a high-field polarized state, and three AFM states with distinct spin configurations. Our results identify ${\mathrm{Eu}}_2{\mathrm{Pd}}_3{\mathrm{Bi}}_4$ as a good candidate for scrutinizing sophisticated interplays between magnetic structure and nontrivial band topology.

We investigate the dependence of the maximum entropy method (MEM) reconstruction performance on the default model. The maximum entropy method is a reconstruction technique that utilizes prior information, referred to as the default model, to recover original signals from observed data, and it is widely used in underdetermined systems. The broad applications have been reported in fields ranging from the analysis of observational data in seismology and astronomy, to large-scale computations in quantum chemistry, and even in social sciences such as linguistics. However, a known drawback of MEM is that its results depend on the assumed default model. In this study, we employ the replica method to elucidate how discrepancies in the default model affect the reconstruction of signals with specific distributions. We report that for binary signals, even small discrepancies can induce phase transitions, leading to reconstruction failure. Additionally, by comparing MEM with reconstruction based on $L_1$-norm regularization, a method proposed in recent years, we demonstrate that MEM exhibits lower reconstruction accuracy under certain conditions.

We present a general framework for engineering two-dimensional subwavelength topological optical lattices using spatially dependent atomic dark states in a $\mathrm{\Lambda }$-type configuration of the atom-light coupling. By properly designing the spatial profiles of the laser fields inducing coupling between the atomic internal states, we show how to generate subwavelength Kronig-Penney-like geometric scalar potential accompanied by narrow and strong patches of the synthetic magnetic field localized in the same areas as the scalar potential. These sharply peaked magnetic fluxes are compensated by a smooth background magnetic field of opposite sign, resulting in zero net flux per unit cell while still enabling topologically nontrivial band structures. Specifically, for sufficiently narrow peaks, their influence is minimum, and the behavior of the system in a remaining smooth background magnetic field resembles the Landau problem, allowing for the formation of nearly flat energy bands with unit Chern numbers. Numerical analysis confirms the existence of ideal Chern bands and the robustness of the topological phases against nonadiabatic effects and losses. This makes the scheme well suited for simulating quantum Hall systems and fractional Chern insulators in ultracold atomic gases, offering a platform for exploring strongly correlated topological phases with high tunability.

Through molecular dynamics simulations, we investigate the phase separation and aggregation dynamics of active dumbbell particles in two dimensions subjected to shear. We find that the growth of the phase-separated region is arrested when shear is applied, with the average cluster size plateauing toward a value $R_s$ that remains constant over time. While activity enhances the resilience of clusters against shear-induced breakup, $R_s$ decreases with growing shear rate $\dot{\gamma }$, with an intermediate regime where $R_s\propto {\dot{\gamma }}^{-1}$. We find that clusters in the stationary state are progressively less polarized and increasingly elongated with increasing shear. At the same time, we find a breaking in chiral symmetry of both rotation direction and internal organization of clusters: Typically, dumbbells point toward the cluster center with a small nonzero angle, such that the active torque opposes the shear torque, with cluster's angular velocity well captured by a simplified analytical model. We argue that this conformation makes clusters more stable against shear.

Behavioral adoptions of individuals are influenced by their peers in different ways. While in some cases an individual may change behavior after a single incoming influence, in other cases multiple cumulated attempts of social influence are necessary for the same outcome. These two mechanisms, known as simple and complex contagion, often occur together in social contagion phenomena, yet their distinguishability based on the observable contagion dynamics is challenging. In this paper we define a social contagion model evolving on temporal networks where individuals can switch between contagion mechanisms. We explore three spreading scenarios: predominated by simple or complex contagion, or where the dominant mechanism changes during the unfolding process. We propose analytical and numerical methods relying on global spreading observables to identify which of these three scenarios characterizes a social spreading outbreak. This work offers insights into social contagion dynamics on temporal networks, without assuming prior knowledge about the contagion mechanism driving the adoptions of individuals.

We study the impact of noise on the dynamics of entanglement in the transverse-field Ising chain, with the field quenched linearly across one or both of the quantum critical points of the model. Taking concurrence as a measure of entanglement, we find that a quench generates entanglement between nearest- and next-nearest-neighbor spins, with noise reducing the amount of entanglement. Focusing on the next-nearest-neighbor concurrence, known to exhibit Kibble-Zurek scaling with the square root of the quench rate in the noiseless case, we find a different result when noise is present: The concurrence now scales logarithmically with the quench rate, with a noise-dependent amplitude. This is also different from the “anti-Kibble-Zurek” scaling of defect density with quench rate when noise is present, suggesting that noisy entanglement generation is largely independent of the rate of defect formation. Intriguingly, the critical timescale beyond which no entanglement is produced by a noisy quench scales as a power law with the strength of noise, with the same exponent as that which governs the optimal quench time for which defect formation is at a minimum in a standard quantum annealing scheme.

Many-body localization is a dynamical phenomenon characteristic of strongly interacting and disordered many-body quantum systems that fail to achieve thermal equilibrium. From a quantum information perspective, one of the most relevant fingerprints of this phenomenon is the logarithmic growth of the entanglement entropy over time. We perform intensive numerical simulations, applied to a paradigmatic model system, showing that the local ergotropy, the maximum extractable work via local unitary operations on a small subsystem in the presence of Hamiltonian coupling, dynamically witnesses the change from ergodic to localized phases. Within the many-body localized phase, both the local ergotropy and its quantum fluctuations slowly vary over time with a characteristic logarithmic law analogous to the behavior of entanglement entropy. This showcases how directly leveraging local control, instead of local observables or entropies analyzed in previous works, provides a thermodynamic marker of localization phenomena based on the locally extractable work.