Accurate and robust quantum process tomography (QPT) is crucial for verifying quantum gates and diagnosing implementation faults in experiments aimed at building universal quantum computers. However, the reliability of QPT protocols is often compromised by faulty probes, particularly state preparation and measurement (SPAM) errors, which introduce fundamental inconsistencies in traditional QPT algorithms. We propose and investigate enhanced QPT for multiqubit systems by integrating the error matrix in a digital twin of the identity process matrix, enabling statistical refinement of SPAM error learning and improving QPT precision. Through numerical simulations, we demonstrate that our approach enables highly accurate and faithful process characterization. We further validate our method experimentally in superconducting quantum processors, achieving at least an order-of-magnitude fidelity improvement over standard QPT. Our results provide a practical and precise method for assessing quantum gate fidelity and enhancing QPT on a given hardware.

Circulators are nonreciprocal devices that enable directional signal routing. Nonreciprocity, which requires time-reversal symmetry breaking, can be produced in waveguides in which the propagation medium moves relative to the waveguide at a moderate fraction of the wave speed. Motivated by this effect, here we propose a design for nonreciprocal microwave transmission based on an extended, annular Josephson junction, in which the propagation medium consists of a train of moving fluxons. We show how to harness this to build a high-quality resonant microwave circulator, and we theoretically evaluate the anticipated performance of such a device.

We discuss a novel mechanism for generating dark matter from a fast-rolling scalar field, relevant for both inflation and rotating axion models, and apply it specifically to the (QCD) axion. Dark matter comes from scalar field fluctuations generated by the product of the curvature perturbation and the fast-rolling background field. These fluctuations can explain the totality of dark matter in a vast axion parameter space, particularly for the QCD axion, which will be targeted by upcoming experiments. We review the constraints on this mechanism and potential gravitational-wave signatures.

In this Letter, we present a new formulation of loss cone theory as a reaction-diffusion system, which accounts for loss cone events through a sink term and can be orbit averaged. It can recover the standard approach based on boundary conditions, and is derived from a simple physical model that overcomes many of the classical theoretical constraints. We test our formulation by computing the relaxed distribution of disruptive orbits in phase space, which has a simple analytic form and agrees with the pericenter of tidal disruption events at disruption predicted by nonaveraged models. This formulation of the problem is particularly suitable for including more physics in tidal disruptions and the analogous problem of gravitational captures—e.g., strong scatterings, gravitational waves emission, physical stellar collisions, and repeating partial disruptions—that can all act on timescale shorter than two-body relaxation and might cause the tension between the observed vs theoretically predicted population of tidal disruptions.

During the late stages of a binary neutron star inspiral, dynamical tides induced in each star by its companion become significant and should be included in complete gravitational-wave (GW) modeling. We investigate the coupling between the tidal field and quasinormal modes in hybrid stars and show that the discontinuity mode ($g$ mode) intrinsically associated with first-order phase transitions and buoyancy contributes non-negligibly compared with the fundamental $f$ mode. We find that the $g$-mode overlap integral can reach up to $\sim 10\%$ of the $f$-mode value for hybrid star masses in the range $1.4-2.0M_{\odot }$, with the largest values generally associated with larger density jumps. This leads to a GW phase shift due to the $g$ mode of $\mathrm{\Delta }{\varphi }_g\lesssim 0.1-1\mathrm{rad}$ (i.e., up to $\sim 5\%-10\%$ of $\mathrm{\Delta }{\varphi }_f$), with the largest shifts occurring for masses near the phase transition. At higher masses, the shifts remain smaller and nearly constant, with $\mathrm{\Delta }{\varphi }_g\lesssim 0.1\mathrm{rad}$ (roughly $\sim 1\%$ of $\mathrm{\Delta }{\varphi }_f$). These GW shifts may be relevant even at the design sensitivity of current second-generation GW detectors in the most optimistic cases. Moreover, if a $g$ mode is present and lies near the $f$-mode frequency, neglecting it in the GW modeling can lead to systematic biases in neutron star parameter estimation, resulting in radius errors of up to $1\%-2\%$. These results show the importance of dynamical tides to probe neutron stars’ equation of state, and to test the existence of dense-matter phase transitions.

This Letter investigates the formation of quantum droplets in curved spacetime, highlighting the significant influence of curvature on the formation and properties of these objects. While our computations encompass various dimensions, we primarily focus on two dimensions. Our findings reveal a novel class of curvature-driven quantum effects leading to the formation of quasistable liquid droplets, suggesting a feasible pathway for experimental observation, particularly in microgravity environments.

Interference between scattering channels is observed in resonant inelastic x-ray scattering (RIXS) at the Ne $K$ threshold. Final states with $|2p^{-1}np⟩$ as the main configuration are populated via $|1s^{-1}{n}^{\prime }p⟩$ resonances, where large-amplitude one-electron ($n^{\prime }=n$) channels interfere with small-amplitude two-electron ($n^{\prime }\ne n$) channels. The interference is manifested in an asymmetric line profile where the two-electron resonance occurs in the tail region of the one-electron resonance. In RIXS, this slowly varying tail plays the role of the continuum for the Fano effect. The asymmetric profile is well modeled by means of a single asymmetry parameter, determined by the amplitudes of the scattering channels and the energy separation between the resonances.

Understanding whether a polaritonic phenomenon is fundamentally quantum or classical is essential for building accurate theoretical models and guiding experimental design. Here, we address this question in the context of polaritonic spectra and report an intriguing new feature: the twin polariton, an additional splitting beyond the primary resonant polariton splitting originating from vacuum field fluctuations. We show that the twin polariton persists in the many-molecule limit under permutationally symmetrical initial-state constraint and that it follows the same linear dependence on coupling strength as the primary polariton splitting. This establishes a novel mechanism by which a quantum feature (the twin polariton) can be tuned through a classical one (the primary polariton), offering new opportunities to probe and control the fundamental nature of polaritonic systems.

Diamond’s unique physical properties make it a versatile material for a wide range of nonlinear and quantum photonic technologies. However, unlocking diamond’s full potential as a nonlinear photonic material with nonzero second-order susceptibility, ${\chi }^{(2)}\ne 0$, requires symmetry breaking. In this Letter, we use a nanoscale cavity to demonstrate second-harmonic generation (SHG) in diamond, and demonstrate, for the first time, that the magnitude of diamond’s effective ${\chi }^{(2)}$ strongly depends on the electronic configuration of defects in the diamond crystal, such as nitrogen-vacancy centers. The modification of ${\chi }^{(2)}$ arises from photoionization from the negative to neutral charge state, and is manifested by quenching of SHG upon green illumination. Toggling the green illumination allows for optical switching of the device’s ${\chi }^{(2)}$. Optical control of ${\chi }^{(2)}$ by defect engineering opens the door for second-order nonlinear processes in diamond.

We experimentally investigate the impact of a flat, horizontal disk onto a boiling liquid, i.e., a liquid in thermal equilibrium with its vapor phase. We observe exceptionally high impact pressures deviating strongly from the inertial scaling found for impact in a noncondensable environment, coinciding with the rapid collapse of the vapor pocket entrapped below the disk. We explain our findings, which are relevant for the safe transportation of cryogenic fuels, as a result of vapor condensation, leading to accelerated vapor pocket contraction at high impact velocity and low vapor density.

The recently observed nonreciprocal current-induced zero-resistance state (CIZRS) in twisted trilayer $\mathrm{graphene}/{\text{WSe}}_2$ heterostructure has posed a significant theoretical challenge. In the experiment, the system shows a zero-resistance state only when a sufficiently large current is applied in a particular direction, while stays in an incipient superconducting state with small resistance when the current is small or flows in the opposite direction. In this Letter, we provide a theory of CIZRS. We show that the threefold degenerate Fulde-Ferrell (FF) states are stabilized by the valley polarization and trigonal warping effects of trilayer $\mathrm{graphene}/{\text{WSe}}_2$ heterostructures. Moreover, a current flowing in a particular direction breaks the threefold degeneracy and favors a particular FF pairing domain. We therefore propose that the incipient superconducting state is naturally understood as a multidomain state where the interdomain supercurrent is difficult to flow due to the tiny Josephson coupling caused by the mismatch of Cooper-pair momenta between different FF domains. Nevertheless, a sufficiently large current in a particular direction can selectively populate a certain FF state and create monodomain pathways with zero resistance. Crucially, due to the threefold symmetry of the system, a current flowing in the opposite direction can fail to generate the zero-resistance pathways, thus giving rise to the observed nonreciprocity. Finally, we suggest that the long-sought-after triangular finite-momentum state can also be realized in valley-polarized superconductors.

$\beta \text{−}{\mathrm{PdBi}}_2$ is one of the candidates for topological superconductors with a superconducting (SC) transition temperature ($T_{\mathrm{c}}$) of 5.3 K, in which parity mixing of a spin singlet and spin triplet has been anticipated, being crucial for the further understanding of the relationship with inversion symmetry and parity mixing in the superconductivity. In this Letter, we measured the SC gap in a high-quality single crystal of $\beta \text{−}{\mathrm{PdBi}}_2$ by using high-resolution laser angle-resolved photoemission spectroscopy below $T_{\mathrm{c}}$. We found the isotropic SC gaps in momentum space for multiple bands, and observed that the difference between the SC gap of the topological surface bands and the bulk bands is about 0.1 meV, consistent with other experimental results. These direct and quantitative experimental results support the possibility of $\beta \text{−}{\mathrm{PdBi}}_2$ as a topological superconductor, characterized by unique crystal and electronic band structures.

Topological phases in frustrated quantum magnetic systems have captivated researchers for decades, with the chiral spin liquid (CSL) standing out as one of the most compelling examples. Featuring long-range entanglement, topological order, and exotic fractional excitations, the CSL has inspired extensive exploration for practical realizations. In this Letter, we demonstrate that CSLs can emerge in a kagome lattice driven by vacuum quantum fluctuations over the noninteracting vacuum within a single-mode gyrotropic cavity. The gyrotropic cavity imprints quantum fluctuations with time-reversal symmetry breaking and stabilizes a robust CSL phase without external laser excitation. Moreover, we identify experimentally accessible observables—such as average photon number and transport properties—that reveal connections between photon dynamics and the emergent chiral order. Our findings establish a novel pathway for creating, controlling, and probing topological and symmetry-breaking quantum phases in strongly correlated systems.

Spontaneous symmetry breaking (SSB) occurs when a many-body system governed by a symmetric Hamiltonian, and prepared in a symmetry-broken state by the application of a field coupling to its order parameter $O$, retains a finite $O$ value even after the field is switched off. SSB is generally thought to occur only in the thermodynamic limit $N\to \infty$ (for $N$ degrees of freedom). In this limit, the time to restore the symmetry once the field is turned off, either via thermal or quantum fluctuations, is expected to diverge. Here, we show that SSB can also be observed in finite-size quantum spin systems, provided that the spin is half-integer and three conditions are met: (1) the ground state of the system has long-range correlations, (2) the Hamiltonian conserves the (spin) parity of the order parameter, and (3) $N$ is odd. Using a combination of analytical arguments and numerical results (based on time-dependent variational Monte Carlo and rotor + spin-wave theory), we show that SSB on finite-size systems can be achieved via a quasiadiabatic preparation of the ground state—which, in U(1)-symmetric systems, is shown to require a symmetry-breaking field vanishing over timescales $\tau \sim O(N)$. In these systems, the symmetry-broken state exhibits spin squeezing with Heisenberg scaling.

Conference key agreement aims to establish shared, private randomness among many separated parties in a network. Device-independent conference key agreement (DICKA) is a variant in which the source and the measurement devices used by each party need not be trusted. So far, DICKA protocols largely fall into two categories: those that rely on violating a joint Bell inequality using genuinely multipartite entangled states and those that concatenate many bipartite protocols. The question of whether a hybrid protocol exists, where a multipartite Bell inequality can be violated using only bipartite entanglement, was asked by Grasselli et al. [Quantum 7, 980 (2023)]. We answer this question affirmatively, by constructing an asymptotically secure DICKA protocol achieving the same rate as the concatenation of bipartite device-independent quantum key distribution, yet relying on a single joint Bell violation. Our results prompt further discussion on the benefits of multipartite entanglement for DICKA over its bipartite alternative, and we give an overview of different arguments for near-term devices.

We have implemented a theoretical approach to provide time- and vibrationally resolved photoelectron spectra and ionization time delays of polyatomic molecules as those expected from current high energy resolution reconstruction of attosecond beatings by interference of two-photon transitions setups. Its application to the ${\mathrm{H}}_2\mathrm{O}$ molecule reveals that two-photon ionization delays extracted from the calculated photoelectron spectra do not vary monotonically with photon energy and can increase or decrease along the vibrational progressions. The difference between one- and two-photon ionization delays is substantially larger when the molecular cation is left in the symmetric stretching mode (up to 35 as, i.e., comparable to the actual ionization delays) rather than in the bending mode, thus showing a strong vibrational selectivity in the laser-photoelectron interaction. Hence, the variation of calculated two-photon delays with the final vibrational state does not simply reflect the energy dependence of the Coulomb phases, as usually assumed in a structureless electronic continuum, but also the changes in the molecular geometry associated with the different vibrational modes. As vibrational excitation accompanying the emission of a photoelectron is the rule rather than the exception in polyatomic molecules, the present Letter shows the importance of considering nuclear motion for a correct interpretation of ionization time delays.

We propose a spin-orbit strategy for generating dual-wavelength quantum skyrmions realized either as entangled photon pairs at dual wavelengths or as heralded single-photon states at a given wavelength—regimes neither previously conceptualized nor demonstrated. By coupling a two-photon entangled state to an electrically tunable liquid crystal topological defect, we engineer both nonlocal and local skyrmionic topologies in a reconfigurable platform. We highlight with examples how this approach may open new directions for engineered topological quantum states that exploit the topological richness of liquid crystals.

We demonstrate the existence of transient two-dimensional surfaces where a random-walking particle escapes to infinity in contrast to localization in standard flat two-dimensional space. We first prove that any rotationally symmetric two-dimensional membrane embedded in flat three-dimensional space cannot be transient. Then we formulate a criterion for the transience of a general asymmetric two-dimensional membrane. We use it to explicitly construct a class of transient two-dimensional manifolds with a nontrivial metric and height function but “zero average curvature,” which we dub “tablecloth manifolds.” The absence of the logarithmic infrared divergence of the Laplace-Beltrami operator in turn implies the absence of weak localization, nonexistence of bound states in shallow potentials, and breakdown of the Mermin-Wagner theorem and Kosterlitz-Thouless transition on the tablecloth manifolds, which may be realizable in both quantum simulators and corrugated two-dimensional materials.