We investigate magnomechanically induced transparency in a cavity magnomechanical system with traveling-field-induced non-Hermitian coupling. The setup consists of a microwave cavity mode coupled to magnons in a single-crystal yttrium iron garnet sphere, where the magnon mode is further hybridized with a vibrational mechanical mode through magnetostrictive interaction. In the Hermitian regime, strong photon–magnon coupling generates a single transparency window in the cavity transmission, which splits into a double-window structure when the magnon is coherently dressed by the mechanical mode through magnomechanical coupling. This provides a flexible platform in which the transparency response can be engineered from single- to multi-window behavior through effective magnomechanical interaction. When the non-Hermitian coupling is introduced, the transmission and phase responses become strongly asymmetric due to the competition between coherent cavity–magnon hybridization and reservoir-mediated non-Hermitian interaction. In particular, near the balance condition between the coherent and non-Hermitian couplings, the ordinary symmetric MMIT amplitude is weakened and the transparency feature is converted into an asymmetric non-Hermitian interference profile. By tuning the cavity detuning, this asymmetric magnomechanical transparency evolves into Fano-type line shapes with strongly non-Lorentzian phase dispersion, and its evolution is further visualized in two-dimensional detuning maps. Finally, we analyze the associated group delay and show that both slow- and fast-light response can be controlled by varying the photon–magnon coupling, magnomechanical interaction, and non-Hermitian strength. These results establish non-Hermitian cavity magnomechanics as a useful platform for controlling transparency, Fano interference, and probe dispersion in hybrid magnonic systems.

In this work, we develop a kinetic theory of stimulated Brillouin scattering (SBS) based on a generalized Bhatnagar-Gross-Krook (BGK) model incorporating a velocity-dependent collision frequency. Starting from the Boltzmann equation, we derive an analytic expression for the SBS gain coefficient that explicitly captures the coupling between velocity-dependent relaxation and acoustic response. We show that this modification leads to a systematic reshaping of the gain spectrum, including broadening, peak enhancement, and pressure-dependent frequency shifts that are not captured by conventional BGK theory. Our results demonstrate improved agreement with experimentally measured gain coefficients, identifying velocity-resolved relaxation as a key physical mechanism underlying SBS in rarefied and intermediate collisional regimes. This framework provides a physically grounded extension of kinetic SBS theory and introduces a parameter for controlling and optimizing Brillouin gain in high-power laser systems.

We propose a theoretical scheme for controlling the Goos-Hänchen shift (GHS) of a reflected probe field in a polariton optomechanical system. The system comprises an optical mode, a molecular vibrational mode, and $N$ excitonic modes, where excitons couple to molecular vibrations via conditional displacement interactions and to photons through electric-dipole interactions. We show that the effective exciton-vibration coupling provides a powerful mechanism for coherent GHS control: in its absence, the system exhibits a pronounced GHS at resonance, while activating it strongly suppresses the shift. The effective cavity detuning and the cavity length serve as additional tunable parameters for GHS manipulation. Furthermore, increasing the collective exciton-optical coupling enhances the GHS. Our results establish a framework for probing the GHS in polariton optomechanical systems and offer different avenues for designing optical devices that exploit beam-displacement phenomena.

Inspired by the distinctive properties of universal photon blockade (UPB), we propose a two-photon Jaynes-Cummings model in an asymmetric cavity to achieve quantum nonreciprocity. Through theoretical analysis, we establish direction-dependent optimal conditions for perfect UPB in weak-driving limitation and subsequently validate these conditions through numerical solutions of the master equation. Our results demonstrate that in asymmetric cavities, the optimal atom-cavity coupling strength for UPB exhibits directionality, leading to a corresponding directional dependence in the equal-time second-order correlation function of the cavity field. This directional anisotropy enables remarkable photon statistics in the weak-coupling case: output photons display bunching in one propagation direction while manifesting antibunching in the opposite direction, thereby achieving giant nonreciprocal UPB. Our findings provide a framework for engineering robust quantum nonreciprocal effects, with promising applications in quantum information processing and optical communication systems.

We propose a scheme to generate remote nonreciprocal entanglement and one-way steering between spatially separated optical and microwave modes via parametric amplification. Both entanglement and one-way steering emerge when the system is driven from a specific direction, while being fully suppressed under reverse driving. This nonreciprocity originates from asymmetric detuning induced by directional optical squeezing. Remarkably, compared with the opposite driving direction, the bipartite entanglement is enhanced by approximately a factor of 1.5, while the one-way steering is amplified by about 3.8 times, due to a chiral interaction induced by directional optical squeezing. The enhancement and suppression of these quantum correlations can be actively controlled by tuning the squeezing phase. In addition, increasing the magnon-microwave coupling splits the initially single entanglement and steering peak into two peaks, thereby providing additional detuning windows for nonreciprocal operation. Our scheme provides an all-optical approach for the controllable switching of remote quantum correlations between optical and microwave fields, which may be useful for realizing remote hybrid chiral quantum networks.

Quantum instruments describe both the classical outcome and the updated quantum state in a measurement process. To do this in a nontrivial way, instruments must have the capability to interact coherently with the state that they measure. Here we develop a resource theory for instruments. We consider a relevant quantifier of the separation between interactive and noninteractive instruments and show that it admits three distinct operational interpretations in terms of quantum information tasks. These concern (i) the preservation of maximally entangled states after a local measurement, (ii) the average ability to preserve random states after measurement, and (iii) the ability to recover the classical information generated from measuring half of a maximally entangled state. We also introduce a natural set of allowed operations and show that the third task fully characterizes the resource content of instruments. Our general framework reproduces as special cases established resource theories for channels and measurements.

Magnetic field sensors based on nitrogen-vacancy (NV) centers have attracted considerable attention due to their high sensitivity and spatial resolution. However, conventional vector magnetometry typically relies on either the four axes of ensemble NV centers or three orthogonal reference magnetic fields. Here, we propose a microwave-free vector magnetometry scheme that employs a reference magnetic field applied to a single NV center. We investigate the influence of electric field coupling and ${}^{14}\mathrm{N}$ nuclear spin hyperfine splitting on the oscillation dynamics, employing a strong reference pulse magnetic field to mitigate nuclear spin depolarization near the ground-state level anticrossing region. Specifically, a straightforward Rabi technique is utilized to ascertain the magnitude and polar angle of the magnetic field based on the amplitude and frequency of oscillations. The magnitude, polar angle, and azimuthal angle of the target magnetic field are estimated by varying the strength of the reference magnetic field. This method can be extended to ensembles of NV centers. In comparison to conventional microwave driving techniques, this method obviates the need for microwave components and eliminates the requirement for devices that generate orthogonal three-dimensional magnetic fields, thereby streamlining the technical framework.

We examine the influence of a static dc electric field on electromagnetically induced transparency (EIT) resonances that involve highly excited Rydberg states. Our focus is on how these resonances are altered when the relative orientation between the laser polarization and the external electric-field vectors is varied. We experimentally demonstrate characteristic variations in the amplitude of the Stark-split EIT resonances, which can be explained by the selection rules in various geometries. We also present a simplified semianalytical model that closely resembles the experimental observations. We use these findings to obtain information about the spatially inhomogeneous electric field, produced by a biased wire, using EIT fluorescence measurements that agrees with the expected angular dependencies. These results suggest that simultaneous analysis of frequency shifts and amplitudes of Rydberg EIT resonances may enable vector electrometry of electrostatic fields, which is necessary for many quantum sensing applications.

We study the occurrence of long-lived metastable states in the $4f^{13}5d6s$ electron configuration of ${\mathrm{Yb}}^{+}$. By optical pumping of a single trapped ion on the ${}^2{F}_{7/2}^{\text{o}}\to {(7/2,0)}_{7/2}$ transition at 377.5 nm, we prepare a wide range of metastable electronic states. We use a cotrapped control ion to sympathetically cool the spectroscopy ion, allowing us to accurately time its subsequent decay. We record a strong decay signal corresponding to a lifetime of 0.92(8) s and a weaker decay signal with a lifetime of 9.8($+2.9,-2.0$) s and find evidence of a much longer lifetime, $>30$ s. We identify the metastable states with these lifetimes qualitatively and corroborate our results with atomic structure calculations that support the observed lifetimes and decay paths. These long-lived states provide opportunities in qubit- and qudit-state detection and optical clocks.

We show that superpositions of oppositely squeezed states—non-Gaussian Schrödinger-cat-like states—exhibit enhanced nonclassical features and provide an entanglement advantage in the small-squeezing regime. These states possess photon-number structures distinct from conventional coherent-state cat states, and we analyze their Wigner functions and the entanglement generated when they are injected into a $50:50$ beam splitter. As a practical application, we demonstrate that they enable a high-quality heralded single-photon source whose second-order intensity correlation function is smaller than that obtained from a pure two-mode squeezed vacuum state. We further propose a linear-optical heralding scheme that approximates these superpositions without requiring strong Kerr nonlinearities. Our results indicate that the superposition of oppositely squeezed states is a promising non-Gaussian resource for quantum information processing, particularly for single-photon generation.

A central problem in quantum information is determining quantum-classical boundaries. In the quasiprobability framework, a state is called classical if it is represented by a quasiprobability distribution that is positive, and thus a probability distribution. In recent years, the Kirkwood-Dirac (KD) distributions have gained much interest due to their numerous applications in modern quantum-information research. A particular advantage of the KD distributions is that they can be defined with respect to arbitrary observables. Here, we show that if two $d$-dimensional observables are picked at random, the set of classical (positive) states of the resulting KD distribution is a minimal polytope of dimension $2(d-1)$ with $2d$ explicitly known vertices. This implies minimality of the sets of KD-real observables, of KD-positive measurement elements, and of KD-positivity-preserving unitaries. We show how these results have implications on robust observations of nonclassical phenomena, on classical simulations of quantum circuits, and on foundations of quantum theory.

A key result in entanglement theory is that the addition of a catalyst dramatically enlarges the set of possible state transformations via local operations and classical communication (LOCC). However, it remains unclear what is the interplay between classical communication and quantum catalysis. Here, our aim is to disentangle the effect of the catalyst from that of classical communication. To do so, we explore a class of state transformations termed catalytic local operations (CLO) and compare it to LOCC and to stochastic LOCC augmented by bounded quantum communication. We show that these classes are incomparable and capture different facets of quantum state transformations.

We propose a method to compute expectation values of observables in the stationary state of a (Markovian) bosonic open quantum system. Using a hierarchy of semidefinite relaxations, we obtain finer and finer upper and lower bounds to any expectation value of interest. The bounds are rigorous, robust to stationary state degeneracies, and numerically improve as the occupation number increases on the examples we considered. This makes it adapted to the simulation of stationary states of bosonic qubits and, in particular, dissipatively stabilized cat qubits.

Decoy-state QKD with phase-randomized weak coherent pulses is typically analyzed assuming independent, precisely prepared intensities. Real sources, however, can exhibit correlated intensity drift across rounds, potentially leaking intensity information and breaking the standard decoy-state reduction to linear programs. Cauchy-Schwarz (CS) constraints can restore security by coupling $n$-photon yields across intensities, but they introduce nonlinear square-root constraints that are commonly handled via outer linearization around channel-model-based reference points. We propose a reproducible alternative: first solve the full CS-constrained parameter-estimation problems using the interior-point nonlinear solver IPOPT, then use the resulting candidate solution as the linearization point for the outer optimization that certifies a valid lower bound on the asymptotic key rate. Simulations for both coarse-grained model-independent correlations and fine-grained truncated-Gaussian models show consistently tighter key-rate bounds than canonical reference points, and in some cases allow certifying optimality when both optimization stages coincide.

The realization of high-fidelity quantum control is crucial for quantum information processing, particularly in noisy environments where control strategies must simultaneously achieve precise manipulation and effective noise suppression. Conventional optimal control designs typically require numerical calculations of the system dynamics. Recent studies have demonstrated that long short-term memory (LSTM) can accurately predict the time evolution of open quantum systems. Based on LSTM-predicted dynamics, we propose an optimal control framework for rapid and efficient optimal control design in open quantum systems. As illustrative examples, we apply the proposed framework to design optimal control for adiabatic speedup in a two-level system and for quantum state transfer in a spin chain, both under non-Markovian environments. For adiabatic speedup, our optimization procedure involves two steps: driving trajectory optimization and zero-area pulse optimization. Fidelity improvements for both steps have been obtained, demonstrating the effectiveness of the scheme. Furthermore, this effectiveness is validated for quantum state transfer in a spin chain, which is a high-dimensional control problem. Our optimal control design scheme utilizes predicted dynamics to generate optimized controls, offering broad application potential in quantum computing, communication, and sensing.

Response uncertainty relations in stochastic thermodynamics extend precision bounds to the sensitivity of observables under external perturbations. Here we derive a quantum response kinetic uncertainty relation for continuously monitored Markovian open quantum systems in the steady state of the Lindblad master equation. The response precision of a measured trajectory observable is bounded by two contributions: the conventional quantum dynamical activity and a perturbation-induced intersubspace transition term. The latter is absent in the classical limit and captures a genuinely quantum part of the response cost. We identify simple conditions under which either contribution vanishes, and we further clarify the structure of the intersubspace term through a symmetry-resolved decomposition and exact sector-selection rules. The bound and its structure are illustrated in a driven two-level atom.

We develop a deterministic and fully explicit framework for preparing an arbitrary three-qubit pure state from its computational-basis amplitudes. Our approach exploits bipartite Schmidt structure: we classify three-qubit states into five types with respect to a $1|2$ bipartition and provide concrete, concurrence-based criteria that identify the type and extract the required Schmidt data directly from the target amplitudes. For each type, we derive an explicit circuit template built from elementary single-qubit rotations and cnot gates, together with an end-to-end parameter map that specifies all gate angles and phases without procedural ambiguity. The resulting constructions are arranged to use only cnot gates between adjacent qubits, making them directly deployable on restricted-connectivity hardware. As an application, we group widely used three-qubit states into four representative classes and obtain class-adapted circuits whose parameters are read off from the amplitude and phase data; in several regimes, these specialized circuits reduce entangling-gate count and overall depth relative to universal templates.

Variational quantum algorithms have emerged as a cornerstone of contemporary quantum algorithms research. While they have demonstrated considerable promise in solving problems of practical interest, efficiently determining the minimal quantum resources necessary to obtain such a solution remains an open question. In this work, inspired by concepts from classical machine learning, we investigate the impact of overparametrization on the performance of variational algorithms. Our study focuses on the quantum approximate optimization algorithm (QAOA), a prominent variational quantum algorithm designed to solve combinatorial optimization problems. We investigate if circuit overparametrization is necessary and sufficient to solve such problems in QAOA, considering two representative problems: MAX-CUT and MAX-2-SAT. For MAX-CUT on 2-regular graphs we observe that overparametrization is both sufficient and necessary. To establish this, we analytically show that the optimal circuit depth for such problems scale as $n/2$ for even $n$. For a more general case of random graphs, the overparametrization is observed to be sufficient, yet necessary only for a statistically dominant fraction of instances. In sharp contrast, for MAX-2-SAT, underparametrized circuits suffice to solve most instances. This result highlights the potential of QAOA in the underparametrized regime, supporting its utility for current noisy devices.