We show that projective measurements on quantum light can induce macroscopic cat states in many-electron systems driven by such light. Here we investigate the quantum dynamics of $N$ independent two-level electrons interacting with Schrödinger-cat or -kitten states of light. Without measurement, a macroscopic cat state of the electrons appears only in an ultrashort time window. In contrast, we demonstrate that photon-number parity or quadrature projective measurements can restore a macroscopic cat state in nonequilibrium electrons, even in the thermodynamic limit. These dynamics are captured by an external-field approximation, in which the electronic system evolves into a Rabi-oscillation cat state. Our results highlight the need for precise quantum measurement techniques for light to control macroscopic quantum states of matter driven by quantum light.

We study the emergence of quantum memory effects in a spin-boson system at finite temperature driven by an external time-periodic force. Quantifying memory effects by the trace-distance based measure for non-Markovianity and performing numerical simulations employing the hierarchical equations of motion approach, we find a pronounced peak structure when plotting the non-Markovianity measure as a function of the driving amplitude. This distinctive feature is interpreted using Floquet theory and the Floquet-Lindblad master equation, associating the peaks with the degeneracies of the quasienergy spectrum which lead to a strong enhancement of the relaxation times of the system. These results suggest strategies for the efficient control of non-Markovianity in open quantum systems by periodic driving.

An estimation method based on the split-step quantum walk (SSQW) demonstrated that by adjusting a single parameter in the initial state, it is feasible to attain the quantum Crameŕ-Rao bound in multiparameter estimation. This parameter introduces a broad category of local initial states that function as optimized probes. Dissimilar to the ordinary quantum walk (OQW), SSQW does not necessitate entangled or parameter-dependent initial states. The analytical findings derived in this investigation show that SSQW outperform OQW in achievable precision of multiparameter estimation in nearly all possible scenarios. Furthermore, in single-parameter estimation, the additional parameter can be utilized to adjust the dynamics of the walk in such a manner as to enhance the precision of the estimation through maximizing the elements of the quantum Fisher information matrix. The results of this study indicate that SSQW can remarkably enhance the estimation schemes through its rich topological properties.

We present a precise characterization of the onset of classicality that combines the formalism of quantum Darwinism with the tools from quantum metrology. We show that the quantum Fisher information provides a useful metric for assessing the rate at which classical objectivity emerges. Furthermore, our formalism allows us to explore how the choice of measurement impacts the precision with which an observer can determine the state of the system. For a paradigmatic example of the spin-star model, we demonstrate that optimal measurements lead to the emergence of classicality at an exponential rate. Although other measurements necessarily lead to slower emergence, we importantly show that suboptimal measurements can still saturate the Cramér-Rao bound. By recasting emergent classicality as an information acquisition protocol, our framework provides a precise operational description of quantum Darwinism.

Continuous-variable quantum key distribution, which can use coherent states, shows great compatibility with classical optical communications, enabling high secret key rates over a metropolitan area. However, conventional continuous-variable quantum key distribution implementations require phase compensation mechanisms that introduce operational complexity. Here, we propose a unidimensional discrete-modulated continuous-variable quantum key distribution protocol by using the variance extremum search method that eliminates the need for phase compensation. This protocol ensures security through semidefinite programming and can be expanded. Simulation results demonstrate that, under typical excess noise conditions, this protocol achieves transmission distances exceeding 200 km at the asymptotic limit and can achieve a distance of over 75 km under finite-size effects using only a few constellation points. This innovation streamlines the experimental setup and holds promise for enhancing the practicality and promoting the future deployment of continuous-variable quantum key distribution.

Using time-dependent density-functional theory, we investigate how a large permanent dipole moment modifies high-harmonic generation (HHG). Using LiH as an example, by systematically comparing spectra obtained with laser polarization parallel and perpendicular to the molecular axis, we identify distinctive features arising from pronounced Stark shifts induced by the dipole. In the parallel orientation, these shifts broaden resonance conditions and strongly enhance below-threshold harmonics, producing a plateaulike structure. A pronounced two-center interference minimum also appears near the maximum transient ionization potential. Furthermore, the variation of Stark shifts within each half optical cycle (HOC), together with their alternating signs between consecutive HOCs, introduces asymmetries in tunneling, propagation, and recombination. These asymmetries drive interference that manifests as spectral splits and blueshifts in the plateau and near-cutoff regions of the HHG spectrum.

We introduce a method for shaping a cold-atom cloud using a vortex laser beam with a polarization singularity at its center, which creates a point of vanishing intensity. Exploiting this feature we experimentally demonstrate two different schemes to create micron-scaled line- and sheetlike atomic density distributions. In the dynamic scheme, atoms in the bright beam regions are accelerated and therefore effectively removed from the cloud. In the dark-state scheme, these atoms are pumped into a state that does not interact with the shaping light. In both cases, an atomic distribution remains, either as a thin line or as a sheet when an additional polarizer is used. We find good agreement between the experimental results and our theoretical model, which predicts the method to be in principle not diffraction limited, paving the way for studies of phenomena arising in unconfined atomic ensembles on the micrometer scale.

We build the case that our published chiral soliton model [Phys. Rev. Lett. 123, 250402 (2019)] can lead to a genuine quantum time crystal for finite atom numbers. To do this we compare results from the chiral soliton model with exact numerical solutions for the ground state of the three-particle Schrödinger equation in the limit that the spatial profile of the soliton is largely independent of its center-of-mass motion. The connection between the two approaches is found via the Page-Wootters mechanism, and this is what allows for dynamical evolution even in the quantum ground state, as required for a genuine quantum time crystal.

In this work we introduce modulation asymmetry and nonlinearity asymmetry to investigate the quantum synchronization dynamics in a four-node optomechanical system with nonlinear Kerr media. Through quantum Langevin equations and a generalized synchronization measure $S_q^{\varphi }$, we demonstrate that modulation asymmetry induces robust antisynchronization with a stabilized phase difference near $\pi$. Conversely, nonlinearity asymmetry enables continuous phase modulation while preserving synchronization robustness. Notably, under conditions of high nonlinearity strength, synchronization demonstrates high sensitivity to frequency but minimal dependence on coupling configurations. In the specific case without nonlinearity, however, the synchronization behavior differs significantly between the two coupling topologies. These conclusions are expected to generalize to networks with more nodes, enabling the regulation of the synchronization phase.

We study the dynamics of an optomechanical system consisting of a single-mode optical field coupled to a mechanical oscillator, where the nonlinear interaction includes both linear and quadratic terms in the oscillator's position. We present an analytical solution to this quantum-mechanical Hamiltonian problem by employing the formalism of two-phonon coherent states. Quantum estimation theory is applied to the resulting state of the optical field, with a focus on evaluating the quantum Fisher information with respect to the strength of the quadratic coupling. Our estimation scheme employs balanced homodyne photodetection and demonstrates that the corresponding classical Fisher information can reach the quantum Fisher information limit, with the phase of the local coherent oscillator playing a crucial role.

Chiral metasurfaces supported by symmetry-protected bound states in the continuum (SP-BICs) possess exceptional capabilities for confining light fields and enhancing light-matter interactions. However, a fundamental trade-off persists in chiral metasurfaces: systematically achieving both ultrahigh-$Q$ resonances and maximum circular dichroism (CD). Here, we propose a robust strategy for chiral quasi-BIC metasurfaces, where perturbations are introduced to the symmetric eigenfield, enabling the systematic realization of ultrahigh $Q$-factors and near-unity CD. We demonstrate that the two holes within the nanodisks, acting as perturbations, whose position dominates the $Q$-factors, provide sufficient chiral degrees of freedom to tailor the C points along the $k_x$ and $k_y$ directions in momentum space. When the chiral perturbations are located at the eigenfield minima, this enables the achievement of $Q\approx 1.879\times {10}^5$ and CD = 0.99. This method enables the reconfiguration of C points from arbitrary initial positions and is broadly applicable to various eigenmodes, providing a systematic and robust framework for high-performance chiral optoelectronic and photonic devices.

Photon counting is a fundamental component in quantum optics and quantum information. However, implementing ideal photon-number-resolving (PNR) detectors remains experimentally challenging. Multiplexed PNR detection offers a scalable and practical alternative by distributing photons across multiple modes and detecting their presence using simple on-off detectors, thereby enabling approximate photon-number resolution. In this work, we establish a theoretical model for such detectors and prove that the estimation error in terms of photon number moments decreases inverse proportionally to the number of detectors. Thanks to the enhanced PNR capability, a multiplexed PNR detector provides an advantage in cat-state breeding protocols. Assuming a two-photon subtraction case, 7 dB of squeezing, and an array of 20 detectors of efficiency $95\%$, our calculation predicts fidelity $\sim 0.88$ with a success probability $\sim 3.8\%$, representing orders-of-magnitude improvement over previous works. Similar enhancement also extends to cat-state generation with the generalized photon number subtraction. With experimentally feasible parameters, our results suggest that megahertz-rate cat-state generation is achievable using an on-chip array of tens of on-off detectors.

The non-Hermitian model exhibits counterintuitive phenomena that are not observed in the Hermitian counterparts. To probe the competition between non-Hermitian and Hermitian interacting components of the Hamiltonian, we focus on a system containing non-Hermitian $XY$ spin chain and Hermitian Kaplan-Shekhtman-Entin-Aharony (KSEA) interactions along with the transverse magnetic field. We show that the non-Hermitian model can be an effective Hamiltonian of a Hermitian $XX$ spin-$\frac{1}{2}$ with KSEA interaction and a local magnetic field that interacts with local and nonlocal reservoirs. The analytical expression of the energy spectrum divides the system parameters into two regimes: in one region, the strength of Hermitian KSEA interactions dominates over the imaginary non-Hermiticity parameter, while in the other, the opposite is true. In the former situation, we demonstrate that the nearest-neighbor entanglement and its derivative can identify quantum critical lines with the variation of the magnetic field. In this domain, we determine a surface where the entanglement vanishes, similar to the factorization surface, known in the Hermitian case. On the other hand, when non-Hermiticity parameters dominate, we report the exceptional and critical points where the energy gap vanishes and illustrate that bipartite entanglement is capable of detecting these transitions as well. Going beyond this scenario, when the ground state evolves after a sudden quench with the transverse magnetic field, both the rate function and the fluctuation of bipartite entanglement quantified via its second moment can detect critical lines generated without quenching dynamics.

A recently proposed stochastic hidden-variable model for quantum mechanics has been claimed to involve “retrocausality” due to the appearance of equations of motion with future-time boundary conditions. We formulate an equivalent system of forward-time equations of motion that gives rise to the same trajectories as solutions, but involves only initial-time boundary conditions. The forward-time dynamics involves a guidance term for the dynamical variables, determined by the phase-space distribution corresponding to a quantum wave function. We show, however, that this particular guidance term can be recovered as the mean-field limit of averaged pairwise interactions among an ensemble of finitely many particles. The results establish a general tradeoff in stochastic models for quantum mechanics between reverse-time and guiding wave formulations.

Grothendieck constants $K_G\left(d\right)$ are intimately linked with Bell nonlocality: they bound the advantage of high-dimensional quantum strategies over local ones in a bipartite correlation scenario. However, apart from $d=2$, their values are unknown. Here, we exploit a recent Frank-Wolfe approach to provide good candidates for lower bounding some of these constants. The complete proof relies on solving difficult binary quadratic optimization problems. For $d\in \{3,4,5\}$, we construct specific rectangular instances that we can solve to certify better bounds than those previously known; by monotonicity, our lower bounds improve on the state of the art for $d⩽9$. For $d\in \{4,7,8\}$, we exploit elegant structures to build highly symmetric instances achieving even greater bounds; however, we can only solve them heuristically. We also recall the standard relation with violations of Bell inequalities and elaborate on it to interpret generalized Grothendieck constants $K_G(d\mapsto 2)$ as the advantage of complex $d$-dimensional quantum mechanics over real qubit quantum mechanics. Motivated by this connection, we also improve the bounds on $K_G(d\mapsto 2)$.

The localization transition can be exploited as a resource for achieving quantum-enhanced sensitivity in parameter estimation. We demonstrate that, by employing different classes of localization-inducing potentials, one can significantly enhance the precision of parameter estimation. Specifically, we focus on the precision measurement of the Stark strength parameter encoded in the low- and high-energy eigenstates of a one-dimensional fermionic lattice under the influence of Aubry-André-Harper localization-delocalization transition. For the ground state, we consider the single-particle system, in addition to the system at half filling. Our work reveals that Quantum Fisher Information (QFI) offers superior scaling with respect to the system size compared to the pure Stark case, leading to a better parameter estimation. However, experimental measurement of the QFI based on fidelity in a multibody system is a significant challenge. To address this, we suggest experimentally relevant operators that can be utilized to achieve precision surpassing the Heisenberg Limit (HL) or can even saturate the QFI scaling. These operators, relevant for practical experimental setups, provide a feasible pathway to harness the advantages offered by the localization-delocalization transition by exploiting two distinct localizing potentials for quantum-enhanced parameter estimation. In addition, to investigate the robustness of the proposed quantum sensors, we add noise in the form of thermal fluctuations, and demonstrate that the Fisher information of the thermal states can saturate the HL. In principle, this work demonstrates how introducing additional quantum-criticality-inducing control parameters could be utilized to enhance quantum sensitivity in the estimation of an unknown target parameter, and hence promises applications in wider contexts, e.g., in hybrid systems involving other classes of localization-inducing potential, such as Anderson localization, and even in systems involving other types of quantum criticalities, such as second-order and topological phase transitions.

We analyze the elastic scattering of vortex electrons by chiral molecules. In this process, the incident vortex electron acts as a chiral probe, analogous to the incident circularly polarized photon in the photoeffect. This suggests that chiral asymmetry in the scattering observables similar to photoelectron circular dichroism (PECD) is expected. Indeed, we show that the angular distribution of electrons scattered by a single target molecule averaged over molecular orientations depends on the handedness of the incident electron and the molecule and remains unchanged if both handednesses are switched simultaneously, in analogy with PECD. We also show that although this chiral asymmetry disappears for a homogeneous gas target, it survives for scattering by any finite cloud of randomly oriented molecules. Thus the asymmetry is observable, at least in principle, under realistic experimental conditions. The predictions of the theory are illustrated by calculations for a chiral glyceraldehyde molecule described within the molecular zero-range potential model.

The spin and orbital are two basic degrees of freedom that play significant roles in exploring exotic quantum phases in optical lattices with synthetic spin-orbit coupling (SOC) and high-orbital bands, respectively. Here, we combine these two crucial ingredients by proposing an alternative orbital optical Raman lattice scheme to explore exotic high-orbital Bose condensates with Raman-induced SOC in a square lattice. We find that both the SOC and $p$-orbital interactions influence the condensed state of bosons. Their interplay results in two novel high-orbital many-body quantum phases: a uniform angular momentum superfluid phase, which exhibits a global topological chiral orbital current characterized by a uniform Chern number, and a two-dimensional topological spin-orbital supersolid phase, which is characterized by the spin and orbital angular momentum density-wave patterns and by the chiral and antichiral topological edge excitations with a staggered Chern-number distribution as predicted here for supersolid phases. Our scheme may open a new avenue for exploring exotic SOC and high-orbital physics in optical lattices and is expected to advance the experimental realization of novel supersolids in higher dimensions.

We analyze the zero-energy collision of three identical bosons in the same internal state with total orbital angular momentum $L=2$, assuming short-range interactions. By solving the Schrödinger equation asymptotically, we derive two expansions of the wave function when three bosons are far apart or a pair of bosons and the third boson are far apart. The scattering hypervolume $D$ is defined for this collision. Unlike the scattering hypervolume defined by one of us in 2008 [S. Tan, Phys. Rev. A 78, 013636 (2008)], whose dimension is length to the fourth power, the dimension of $D$ studied in the present paper is length to the eighth power. We then derive the expression of $D$ when the interaction potentials are weak, using Born's expansion. We also calculate the energy shift of such three bosons with three different momenta $\hslash {\mathbf{k}}_1, \hslash {\mathbf{k}}_2$, and $\hslash {\mathbf{k}}_3$ in a large periodic box. The obtained energy shift depends on $D^{\left(0\right)}/{\mathrm{\Omega }}^2$ and $D/{\mathrm{\Omega }}^2$, where $D^{\left(0\right)}$ is the three-body scattering hypervolume defined in the previously mentioned work by Tan for the three-body $L=0$ collision and $\mathrm{\Omega }$ is the volume of the periodic box. We also calculate the contribution of $D$ to the three-body $T$-matrix element for low-energy collisions. We then calculate the shift of the energy and the three-body recombination rate due to $D^{\left(0\right)}$ and $D$ in the dilute homogeneous Bose gas. The contribution to the three-body recombination rate constant from $D$ is proportional to $T^2$ if the temperature $T$ is much larger than the quantum degeneracy temperature but still much lower than the temperature scale at which the thermal de Broglie wavelength becomes comparable to the physical range of interaction.

We present a fiber-integrated approach for generating optical-bottle beams based on a microcavity-cascade optical tweezers (MCOT) probe. A 650 nm laser excites the ${\mathrm{LP}}_{21}$ mode in a single-mode fiber through controlled core offset, and the resulting quadrupolar field undergoes multipath interference inside a thermally tapered microcavity. Internal reflections introduce phase shifts that form closed three-dimensional dark regions surrounded by high-intensity fringes, enabling stable optical-bottle beams. Finite-element simulations and experiments confirm the repeatable trapping and release of low-refractive-index particles in an aqueous environment, demonstrating robust three-dimensional confinement. The fabrication process of the composite fiber probe is straightforward and compatible with fiber-optic integration, providing a compact and tunable platform for particle manipulation. Owing to its simplicity, stability, and scalability, the proposed method offers potential for applications in optical trapping, microfluidics, and bioengineering.