Quantum measurements play a fundamental role in quantum mechanics. Especially, generalized quantum measurements provide a powerful and versatile tool to extract information from quantum systems. However, how to realize them on an arbitrary higher-dimensional quantum system remains a challenging task. Here we propose a simple recipe for the implementation of a general positive-operator valued measurement (POVM) on a higher-dimensional quantum system via a one-dimensional discrete-time quantum walk with a two-dimensional coin. Furthermore, using single photons and linear optics, we realize experimentally a symmetric, informationally complete (SIC) POVM on a three-dimensional system with high fidelity. As an application, we realize a qutrit state tomography with SIC-POVM and confirm that the infidelity scaling achieved by the qutrit SIC-POVM is as good as that based on mutually unbiased bases. Our study paves the way to explore physics and information in higher-dimensional quantum systems and finds applications in various quantum information processing tasks that rely on generalized quantum measurements.

The branching fraction of the rare decay ${\mathrm{\Lambda }}_b^0\to \mathrm{\Lambda }(1520){\mu }^{+}{\mu }^{-}$ is measured for the first time, in the squared dimuon mass intervals $q^2$, excluding the $J/\psi$ and $\psi (2S)$ regions. The data sample analyzed was collected by the LHCb experiment at center-of-mass energies of 7, 8, and 13 TeV, corresponding to a total integrated luminosity of $9{\mathrm{fb}}^{-1}$. The result in the highest $q^2$ interval, $q^2>15.0{\mathrm{GeV}}^2/c^4$, where theoretical predictions have the smallest model dependence, agrees with the predictions.

A narrow structure in the $p\overline{\mathrm{\Lambda }}$ system near the mass threshold, named as $X(2085)$, is observed in the process $e^{+}{e}^{-}\to p{K}^{-}\overline{\mathrm{\Lambda }}$ with a statistical significance greater than $20\sigma$. Its spin and parity are determined for the first time to be $J^P=1^{+}$ in an amplitude analysis, with a statistical significance greater than $5\sigma$ over other quantum numbers ($0^{-},1^{-}$ and $2^{+}$). The pole positions of $X(2085)$ are measured to be $M_{\mathrm{pole}}=(2084_{-2}^{+4}\pm 9)\mathrm{MeV}$ and ${\mathrm{\Gamma }}_{\mathrm{pole}}=(58_{-3}^{+4}\pm 25)\mathrm{MeV}$, where the first uncertainties are statistical and the second ones are systematic. The analysis is based on the study of the process $e^{+}{e}^{-}\to p{K}^{-}\overline{\mathrm{\Lambda }}$ and uses the data samples collected with the BESIII detector at the center-of-mass energies $\sqrt{s}=4.008$, 4.178, 4.226, 4.258, 4.416, and 4.682 GeV with a total integrated luminosity of $8.35{\mathrm{fb}}^{-1}$.

Nonlinear mechanical resonators display rich and complex dynamics and are important in many areas of fundamental and applied sciences. Here, we present a general strategy to generate mechanical nonlinearities for trapped particles by transverse driving in a funnel-shaped potential. Employing a trapped ion platform, we study the nonlinear oscillation, bifurcation, and hysteresis of a single calcium ion and demonstrate a 20-fold enhancement of the signal from a zeptonewton-magnitude harmonic force through the effect of vibrational resonance. Our results represent a first step in combining the rich nonlinear dynamics with the precision control over mechanical motions offered by atomic physics and open up possibilities for exploiting nonlinear mechanical phenomena in the quantum regime.

Anisotropic electron heating during electron-only magnetic reconnection with a large guide magnetic field is directly measured in a laboratory plasma through in situ measurements of electron velocity distribution functions. Electron heating preferentially parallel to the magnetic field is localized to one separatrix, and anisotropies of 1.5 are measured. The mechanism for electron energization is identified as the parallel reconnection electric field because of the anisotropic nature of the heating and spatial localization. These characteristics are reproduced in a 2D particle-in-cell simulation and are also consistent with numerous magnetosheath observations. A measured increase in the perpendicular temperature along both separatrices is not reproduced by our 2D simulations. This work has implications for energy partition studies in magnetosheath and laboratory reconnection.

Bilayers consisting of two-dimensional (2D) electron and hole gases separated by a 10 nm thick AlGaAs barrier are formed by charge accumulation in epitaxially grown GaAs. Both vertical and lateral electric transport are measured in the millikelvin temperature range. The conductivity between the layers shows a sharp tunnel resonance at a density of $1.1\times {10}^{10}{\mathrm{cm}}^{-2}$, which is consistent with a Josephson-like enhanced tunnel conductance. The tunnel resonance disappears with increasing densities and the two 2D charge gases start to show 2D-Fermi-gas behavior. Interlayer interactions persist causing a positive drag voltage that is very large at small densities. The transition from the Josephson-like tunnel resonance to the Fermi-gas behavior is interpreted as a phase transition from an exciton gas in the Bose-Einstein-condensate state to a degenerate electron-hole Fermi gas.

With the rapid development of ultrafast experimental techniques for the research of carrier dynamics in solid-state systems, a microscopic understanding of the related phenomena, particularly a first-principle calculation, is highly desirable. Nonadiabatic molecular dynamics (NAMD) offers a real-time direct simulation of the carrier transfer or carrier thermalization. However, when applied to a periodic supercell, there is no cross-$k$-point transitions during the NAMD simulation. This often leads to a significant underestimation of the transition rate with the single-$k$-point band structure in a supercell. In this work, based on the surface hopping scheme used for NAMD, we propose a practical method to enable the cross-$k$ transitions for a periodic system. We demonstrate our formalism by showing that the hot electron thermalization process by the multi-$k$-point NAMD in a small silicon supercell is equivalent to such simulation in a large supercell with a single $\mathrm{\Gamma }$ point. The simulated hot carrier thermalization process of the bulk silicon is compared with the recent ultrafast experiments, which shows excellent agreements. We have also demonstrated our method for the hot carrier coolings in the amorphous silicons and the ${\mathrm{GaAlAs}}_2$ solid solutions with the various cation distributions.

Quantum impurity models (QIMs) are ubiquitous throughout physics. As simplified toy models they provide crucial insights for understanding more complicated strongly correlated systems, while in their own right are accurate descriptions of many experimental platforms. In equilibrium, their physics is well understood and have proven a testing ground for many powerful theoretical tools, both numerical and analytical, in use today. Their nonequilibrium physics is much less studied and understood. However, the recent advancements in nonequilibrium integrable quantum systems through the development of generalized hydrodynamics (GHD) coupled with the fact that many archetypal QIMs are in fact integrable presents an enticing opportunity to enhance our understanding of these systems. We take a step towards this by expanding the framework of GHD to incorporate integrable interacting QIMs. We present a set of Bethe-Boltzmann type equations which incorporate the effects of impurity scattering and discuss the new aspects which include entropy production. These impurity GHD equations are then used to study a bipartioning quench wherein a relevant backscattering impurity is included at the location of the bipartition. The density and current profiles are studied as a function of the impurity strength and expressions for the entanglement entropy and full counting statistics are derived.

In the systems showing critical behavior, various response functions have a singularity at the critical point. Therefore, as the driving field is tuned toward its critical value, the response functions change drastically, typically diverging with universal critical exponents. In this Letter, we quantify the inequality of response functions with measures traditionally used in economics, namely by constructing a Lorenz curve and calculating the corresponding Gini index. The scaling of such a response function, when written in terms of the Gini index, shows singularity at a point that is at least as universal as the corresponding critical exponent. The critical scaling, therefore, becomes a single parameter fit, which is a considerable simplification from the usual form where the critical point and critical exponents are independent. We also show that another measure of inequality, the Kolkata index, crosses the Gini index at a point just prior to the critical point. Therefore, monitoring these two inequality indices for a system where the critical point is not known can produce a precursory signal for the imminent criticality. This could be useful in many systems, including that in condensed matter, bio- and geophysics to atmospheric physics. The generality and numerical validity of the calculations are shown with the Monte Carlo simulations of the two dimensional Ising model, site percolation on square lattice, and the fiber bundle model of fracture.

Spin-wave amplification techniques are key to the realization of magnon-based computing concepts. We introduce a novel mechanism to amplify spin waves in magnonic nanostructures. Using the technique of rapid cooling, we create a nonequilibrium state in excess of high-energy magnons and demonstrate the stimulated amplification of an externally seeded, propagating spin wave. Using an extended kinetic model, we qualitatively show that the amplification is mediated by an effective energy flux of high energy magnons into the low energy propagating mode, driven by a nonequilibrium magnon distribution.

The nature of time as emergent for a system by separating it from its environment has been put forward by Page and Wootters [Phys. Rev. D 27, 2885 (1983)] in a quantum mechanical setting neglecting interaction between system and environment. Here, we add strong support to the relational concept of time by deriving the time-dependent Schrödinger equation for a system from an energy eigenstate of the global Hamiltonian consisting of system, environment, and their interaction. Our results are consistent with concepts for the emergence of time where interaction has been taken into account at the expense of a semiclassical treatment of the environment. Including the coupling between system and environment without approximation adds a missing link to the relational time approach opening it to dynamical phenomena of interacting systems and entangled quantum states.

Quantum neural networks (QNNs) have become an important tool for understanding the physical world, but their advantages and limitations are not fully understood. Some QNNs with specific encoding methods can be efficiently simulated by classical surrogates, while others with quantum memory may perform better than classical classifiers. Here we systematically investigate the problem-dependent power of quantum neural classifiers (QCs) on multiclass classification tasks. Through the analysis of expected risk, a measure that weighs the training loss and the generalization error of a classifier jointly, we identify two key findings: first, the training loss dominates the power rather than the generalization ability; second, QCs undergo a U-shaped risk curve, in contrast to the double-descent risk curve of deep neural classifiers. We also reveal the intrinsic connection between optimal QCs and the Helstrom bound and the equiangular tight frame. Using these findings, we propose a method that exploits loss dynamics of QCs to estimate the optimal hyperparameter settings yielding the minimal risk. Numerical results demonstrate the effectiveness of our approach to explain the superiority of QCs over multilayer Perceptron on parity datasets and their limitations over convolutional neural networks on image datasets. Our work sheds light on the problem-dependent power of QNNs and offers a practical tool for evaluating their potential merit.

We give a tight characterization of the relation between loss tolerance and error rate of the most popular protocol for quantum position verification, which is based on BB84 states. Combining it with classical information, we show, using semidefinite programming, for the first time a fault-tolerant protocol that is secure against attackers who preshare a linear amount of entanglement (in the classical information), arbitrarily slow quantum information and that tolerates a certain amount of photon loss. We also extend this analysis to the case of more than two bases, showing even stronger loss tolerance for that case. Finally, we show that our techniques can be applied to improve the analysis of one-sided device-independent quantum key distribution protocols.

Using $7.33{\mathrm{fb}}^{-1}$ of $e^{+}{e}^{-}$ collision data taken with the BESIII detector at the BEPCII collider, we report the first experimental study of the purely leptonic decay $D_s^{*+}\to e^{+}{\nu }_e$. Our data contain a signal of this decay with a statistical significance of $2.9\sigma$. The branching fraction of $D_s^{*+}\to e^{+}{\nu }_e$ is measured to be $({2.1}_{{-0.9}_{\mathrm{stat}}}^{+1.2}\pm {0.2}_{\mathrm{syst}})\times {10}^{-5}$, corresponding to an upper limit of $4.0\times {10}^{-5}$ at the 90% confidence level. Taking the total width of the $D_s^{*+}$ [$(0.070\pm 0.028)\mathrm{keV}$] predicted with the radiative $D_s^{*+}$ decay from the lattice QCD calculation as input, the decay constant of the $D_s^{*+}$ is determined to be $f_{D_s^{*+}}=({214}_{{-46}_{\mathrm{stat}}}^{+61}\pm 44_{\mathrm{syst}})\mathrm{MeV}$, corresponding to an upper limit of 354 MeV at the 90% confidence level.

We observe synchronization in a thermal ($35–60°\mathrm{C}$) atomic (Rb) ensemble driven to a highly excited Rydberg state (principle quantum number $n$ ranging from 43 to 79). Synchronization in this system is unexpected due to the atomic motion; however, we show theoretically that sufficiently strong interactions via a global Rydberg density mean field cause frequency and phase entrainment. The emergent oscillations in the vapor’s bulk quantities are detected in the transmission of the probe laser for a two-photon excitation scheme.

We consider theoretically a driven-dissipative quantum many-body system consisting of an atomic ensemble in a single-mode optical cavity as described by the open Tavis-Cummings model. In this hybrid light-matter system, the interplay between coherent and dissipative processes leads to superradiant pulses with a buildup of strong correlations, even for systems comprising hundreds to thousands of particles. A central feature of the mean-field dynamics is a self-reversal of two spin degrees of freedom due to an underlying time-reversal symmetry, which is broken by quantum fluctuations. We demonstrate a quench protocol that can maintain highly non-Gaussian states over long timescales. This general mechanism offers interesting possibilities for the generation and control of complex fluctuation patterns, as suggested for the improvement of quantum sensing protocols for dissipative spin amplification.