We develop a theory for the quantum-fluctuation spectrum of nonlinearly interacting whispering-gallery modes in a cylindrical microresonator with small radius variations. We show that the nonuniform axial spatial distribution of the modes can be used to optimize the coupler position, enabling both efficient pumping of the central mode and strong squeezing of correlated photon pairs in the sidebands.

Integrating quantum information protocols with metasurfaces opens new opportunities for nonlocal and high-capacity optical information processing. Here, we experimentally demonstrate remote multichannel vectorial meta-holography enabled by polarization entanglement between spatially separated photon pairs. By combining a compact, phase-robust Sagnac entanglement source with a full-Jones-matrix dielectric metasurface, we encode multiple elementary holographic contents and conditionally retrieve their individual and composite outputs through joint input-output polarization projections. The selected holographic content is identified through polarization-resolved coincidence measurements, whereas local single-channel detection alone does not provide access to the corresponding conditional channel information. Compared with single-channel detection, the coincidence reconstruction achieves a signal-to-noise ratio enhancement of 4.74 dB. This work establishes a compact quantum-correlated metasurface platform for multichannel holographic information processing, with potential applications in correlation-gated imaging, encrypted communication, and nonclassical optical information processing.

We investigate the dynamics of a quadratically coupled system under the influence of an external drive applied to the second oscillator, where the coupling facilitates a high-order synchronization with phase locking emerging in the form of $2:1$ between the oscillators. Our analysis reveals a synchronization blockade in the first oscillator, characterized by the complete suppression of conventional $1:1$ phase locking with the drive. Instead the first oscillator exhibits a $2:1$ phase-locking behavior arising from the anharmonicity induced by the quadratic coupling. In contrast, we observe that the directly driven second oscillator synchronizes with the drive, showing $1:1$ phase locking but notably at second-harmonic frequency (${\omega }_2=2{\omega }_d$). A classical mean-field analysis of the corresponding equations of motion reproduces this asymmetric phase-locking geometry, including the two symmetry-related 2:1 phase-locked states of the first oscillator and the 1:1 phase-locked state of the second oscillator. This demonstrates that the phase-locking structure itself can be understood from the nonlinear classical dynamics. The quantum analysis, however, reveals the microscopic origin of the synchronization blockade: the quadratic interaction imposes a two-phonon selection rule that suppresses the conventional single-phonon synchronization channel of the first oscillator. Furthermore, we show that the system exhibits mutual synchronization when both the oscillators satisfy the resonance condition, enabling coherent energy exchange facilitated by nonlinear quadratic coupling. The mutual synchronization shows synchronized regimes and also subtle suppression of synchronized regimes near resonance occurring due to spectral splitting of the energy states. Using perturbation analysis of the master equation within the low-excitation subspace, we analyze steady-state phase distribution and synchronization measures, supported by population statistics and spectral responses. We also propose possible experimental realizations in trapped ions and optomechanical setups. These findings highlight the crucial role of quadratic coupling in enabling nonclassical synchronization phenomena, offering deeper insights for quantum control strategies and the development of quantum information platforms.

Probability backflow is investigated for charged spin-$1/2$ particles in a uniform magnetic field within the lowest-Landau-level approximation. While single-mode Landau states exhibit only weak and nonrobust backflow, it is shown that two-mode interference gives rise to negative probability flux in both the Schrödinger and Pauli formulations. In the Pauli case, orbital and spin contributions to the probability current are explicitly separated, revealing an additional interference mechanism. The external magnetic field is found to act as a tunable control parameter, simultaneously controlling the spatial overlap of orbitals and the spin dynamics. In the two-mode regime, a resonance between orbital mismatch and spin precession leads to an enhancement of backflow. Finite spatial resolution is shown to not suppress the effect within realistic coarse graining. These results establish magnetic-field-controlled quantum backflow in Landau systems as an accessible interference phenomenon.

Conventional dynamical decoupling employs pulse sequences whose microwave implementation relies on the rotating-wave approximation (RWA), which constrains the driving strength so that it remains much smaller than the qubit energy-level splitting. This requirement fundamentally limits pulse speed and detection bandwidth, particularly in systems with small level splittings. Here, we develop a Floquet-engineered dynamical decoupling framework that operates beyond the RWA and can simultaneously achieve high-fidelity decoupling, faster spin manipulation, and enhanced robustness under strong driving. Based on these pulses, we construct Floquet dynamical-decoupling sequences that maintain decoupling performance while substantially extending the detectable ac magnetic-field frequency range. Numerical simulations using nitrogen-vacancy centers demonstrate reliable quantum sensing in regimes inaccessible to conventional RWA-based schemes, with the detection bandwidth scaling directly with the achievable Rabi frequency. Our results establish Floquet-based dynamical decoupling as a general strategy for broadband quantum sensing in strongly driven two-level systems.

Quantum superposition is often phrased as the ability to add state vectors. In practice, however, the physical quantity is a ray (a rank-one projector), so each input specifies only a projector and leaves a gauge freedom in the phases of its vector representatives. This becomes a real operational barrier when one asks for a device that, given two independently prepared unknown pure states, outputs a coherent state proportional to a prescribed linear combination. We identify the missing ingredient as not probabilistic but phase-like. One needs a physical scenario that fixes a single phase convention on the relevant set of rays, so that the overlaps become well-defined complex numbers. Thus, we formalize this through phase conventions and a single notion—dubbed “overlap determinability.” Our main result clarifies the no-go and constructive aspects of this statement: overlap determinability is a necessary operational phase resource for generic coherent superposition, while the converse is asserted only when the phase convention is physically supplied and implementable, as in the standard reference-state promise. This reformulation unifies modern no-universal-superposition results and reinterprets the exceptional yes-go protocols, which succeed precisely when side information supplies the required missing resource. We then show that granting universal access to such convention-fixed overlaps destabilizes the familiar foundational and computational constraints. It enables forbidden transformations akin to quantum cloning and yields superluminal signaling. It also permits reflections about unknown states, leading to exponentially fast amplitude amplification and a collapse of Grover's search bound to a logarithmic query complexity.

In this work we study a particular class of Bell inequalities involving only direct equality comparisons of outcomes. In the bipartite case, the scenario can be interpreted as a natural generalization of full-correlator inequalities (xor games) beyond binary outputs. We define the subpolytope of the local polytope corresponding to this scenario and solve it for several bipartite and multipartite scenarios by leveraging some structural properties. In doing so, we obtain thousands of new tight inequalities, many of which are also facets of the standard local polytope. We also define unanimous Bell inequalities, a particular case of the previous class applied to the multipartite setting in which only full-equality events (all outcomes equal) are considered. We show that such inequalities can always be written as deterministic nonlocal games, and we give a simple multipartite unanimous family and prove its local bound. We show that most of these inequalities admit quantum violations, and we also display aspects of their importance for nonlocality. For instance, we identify examples where such inequalities can act as dimension witnesses, outcome witnesses, and witnesses of genuine multipartite nonlocality, as well as being relevant to the Clauser-Horne-Shimony-Holt inequality. These results show that these simple and elegant inequalities by themselves provide a powerful tool for discovering new Bell inequalities and device-independent witnesses.

Two classically equivalent expressions of mutual information of probability distributions (classical bipartite states) diverge when extended to quantum systems, and this difference has been employed to define quantum discord, a quantifier of quantum correlations beyond entanglement. Similarly, equivalent expressions of classical Fisher information of parametrized probability distributions diverge when extended to quantum states, and this difference may be exploited to characterize certain complex natures of quantum states. Here, by complexity of quantum states, we mean some hybrid nature which intermingles the classical and quantum features. It is desirable to quantify complexity of quantum states from various perspectives. In this work, we pursue the idea of discord and introduce an information-theoretic quantifier of metrological complexity for quantum states (relative to the Hamiltonian that drives the evolution of quantum systems) via the notion of Fisher discord, which is defined by the difference between two important versions of quantum Fisher information: the quantum Fisher information defined via the symmetric logarithmic derivatives and the Wigner-Yanase skew information defined via the square roots of quantum states. We reveal basic properties of the quantifier of metrological complexity, and compare it with some other quantifiers of complexity. In particular, we show that stationary states (or equilibrium states of the quantum system) and all pure states exhibit zero metrological complexity in this setting. As illustrations, we evaluate the metrological complexity for various prototypical states in both discrete and continuous-variable quantum systems, and demonstrate its applications in detecting quantum phase transitions in the $XY$ spin model.

We report an experimental realization of backward retrieval in a spin-wave quantum memory based on a Stark-echo-modulated protocol in ${\mathrm{Eu}}^{3+}$:${\mathrm{Y}}_2{\mathrm{SiO}}_5$. By using Stark control, we preserve the full optical depth of the ensemble while suppressing coherent noise, enabling conditional storage fidelities above 97%. Our analysis shows that the present backward-retrieval efficiency is mainly limited by technical imperfections rather than by fundamental constraints. With realistic engineering improvements, backward retrieval in this protocol could move beyond the reabsorption-limited forward-emission regime. The protocol is also compatible with cavity-enhanced operation, offering an additional route toward higher efficiencies. These findings establish Stark-echo modulation as a practical and scalable route to high-efficiency, long-lived solid-state quantum memories.

Quantum digital signatures (QDS) provide information-theoretic security for data integrity, authenticity, and nonrepudiation by exploiting the laws of quantum mechanics. While existing QDS protocols have achieved significant efficiency improvements via one-time universal hashing, they rely on trusted quantum devices, requiring precise characterization and stable calibration of state preparation and measurement processes, which in practice cannot be fully guaranteed. Here, we propose a device-independent QDS protocol based on the violation of Bell inequalities and the entropy accumulation theorem, thereby ensuring information-theoretic security against general coherent attacks without assumptions on the internal functioning of the devices. We analyze the protocol performance under realistic noise and loss models and show that it is feasible with current experimental technology over multikilometer distances. Furthermore, by leveraging state-of-the-art single-photon sources, the achievable distance can be extended to several tens of kilometers. Our results establish device-independent QDS as a practical and scalable cryptographic primitive for secure applications in quantum networks with untrusted devices.

We investigate the effect of nonresonant two-photon excitation on the design of Rydberg two-qubit phase gates for nonindependently addressed qubits. We show that Stark shifts due to nonresonant intermediate states introduce additional phases, constraining the set of entangling gates that can be realized via two-photon transitions in the strong-Rydberg-blockade limit. For the two controlled- z (cz) gates distinguished by the sign of the trace of their matrix representations, we demonstrate high-fidelity implementations using a three-pulse sequence, without additional single-qubit operations to correct the two-qubit phase, by controlling the absolute phases and field amplitudes of the pulses applied to each qubit. Based on these insights, we introduce robust control schemes tailored to each cz gate, yielding high-fidelity protocols with pulse sequences containing either an even or an odd number of pulses.

We study asymmetric reflection and nonreciprocal transmission in a non-Hermitian optomechanical system, which contains a ring optomechanical resonator, a tapered-fiber waveguide, and a nanotip. Benefiting from the combination of coherent and dissipative coupling, phase-controlled nonreciprocal optical coupling can be generated, leading to asymmetric reflection. Meanwhile, the equivalent synthetic magnetism induced by a directionally fixed strong pump can break Lorentz reciprocity, thereby enabling nonreciprocal transmission. When dissipative and coherent couplings are balanced and the phase is matched, exceptional points (EPs) emerge where a contrast ratio of up to $100\%$ can be realized. Remarkably, these direction-dependent behaviors can be dynamically modulated by tuning the optical coupling and can even be completely reversed by controlling the phase to transform odd EPs into even EPs. Furthermore, the system allows the signal from a specific input port to be transmitted to the two output ports, thereby realizing effective quantum routing. Our study not only reveals the critical role of EPs in the direction-dependent behavior but also provides an efficient strategy for developing high-performance integrated nonreciprocal devices.

Spatial walk-off limits second-harmonic generation in birefringent crystals by separating the fundamental and second-harmonic beams, particularly for tightly focused fields. We show that wavelength-scale periodic modulation of the refractive index can suppress this limitation by reshaping the angular dispersion near an existing birefringent phase-matching point. The modulation couples two second-harmonic propagation directions so that their transverse energy flows cancel, eliminating the leading walk-off contribution while phase matching is recovered on the modified dispersion branch. We derive simple design conditions for this regime and confirm the mechanism with spatially resolved simulations that include pump depletion and grating-induced modification of both frequencies. For representative type-I second-harmonic generation in beta barium borate, the modulated structure can maintain spatial overlap and enhance the interaction at a millimeter scale. The approach is therefore most relevant to compact, walk-off-limited frequency converters and tightly focused beams.

Topological invariants play a central role in structured light, providing robust, discrete descriptors of complex optical fields. In an effort to systematically expand such invariants, we detail and generalize two complementary mechanisms for extracting lower-dimensional components of a higher-dimensional field: one associated with phase vortices, in which a gauge structure is relevant, and one based on polarization, for which it is not. Together, these mechanisms form part of a broader framework for defining topological invariants in high-dimensional fields, enabling high-density and robust information encoding through topological field engineering.

Pseudoentropy is a complex-valued generalization of entanglement entropy defined from a reduced transition matrix. We study the pseudoentropy associated with a real-time transition matrix between an initial pure state and its unitary time evolution. For a subsystem $A$, we show that the short-time behavior of real-time pseudoentropy is governed by the correlation between the physical Hamiltonian $H$ and the modular Hamiltonian $K_A=-log{\rho }_A$ of the initial reduced state, $S_A(t,0)=S_A\left(0\right)-it\langle K_A(H-\langle H\rangle )\rangle +O\left(t^2\right)$. For Hermitian dynamics, the initial imaginary response is controlled by the symmetrized covariance of $H$ and $K_A$ with an overall minus sign, while the initial real response is governed by their commutator. Thus the imaginary part of real-time pseudoentropy is not merely a branch artifact: it is a time-oriented modular response generated by the correlation between microscopic time evolution and subsystem coarse graining. We clarify the relation of this result to the known first law of pseudoentropy, derive an all-order expression in a Schmidt-diagonal model, recover thermal pseudoentropy as a special case, illustrate the covariance or commutator decomposition in a two-qubit model, and confirm the covariance response in transverse-field Ising-chain quenches, including a finite-size study of a modular susceptibility near the Ising critical region. We discuss how this amplitude-level oriented response can be related to ordinary entropy production, and also give a concrete $\mathcal{PT}$-symmetric toy-model illustration of the non-Hermitian extension.

Classical simulation of open quantum system dynamics remains challenging due to the exponential growth of the Hilbert space, the need to accurately capture dissipation and decoherence, and the added complexity of memory effects in the non-Markovian regime. We develop an efficient algorithm for simulating both Markovian and non-Markovian dynamics in large one-dimensional quantum systems. Extending the tensor jump method, which combines time-dependent variational principle-based tensor-network evolution with a Suzuki-Trotter decomposition of stochastic trajectories, our approach incorporates time-dependent decay rates—treating positive rates as time-inhomogeneous Markovian processes and negative rates via the influence martingale formalism to unravel time-local non-Markovian dynamics. We further introduce the concept of the "influence radius" to achieve a resource-efficient framework enabling scalable simulations of open-system dynamics in the non-Markovian regime, as demonstrated for a one-dimensional transverse-field Ising chain comprising up to 100 spin qubits.

We investigate the fluorescence and relaxation dynamics of Cs atoms embedded in a cryogenic argon matrix using absorption, fluorescence, and polarization spectroscopy, complemented by diatomic-in-molecule simulations. The data reveal complex emission spectra, large Stokes shifts, and slow light-induced evolution, indicating strong host-guest interactions and substantial lattice reorganization. The absorption spectrum contains two prominent tripletlike structures superimposed on a broad background that may arise from a distribution of low-symmetry, defect-related, or grain-boundary environments. Excitation-resolved fluorescence reveals related behavior within these two spectral groups, compatible with two dominant families of trapping environments, although contributions from additional sites and multiple relaxation pathways cannot be excluded. Polarization measurements indicate distinct relaxation behavior for one excitation band, but neither the experimental data nor the present calculations permit a unique structural assignment.

Optical pumping is fundamental to high-precision measurement using thermal alkali-metal atoms in vapor cells. In applications such as atomic magnetometry, buffer gases (e.g., ${\mathrm{N}}_2$ or $\mathrm{He}$) at specific pressures are introduced to quench fluorescence and mitigate wall relaxation. In the high-pressure limit (e.g., the ${\mathrm{N}}_2$ pressure $p_{{\mathrm{N}}_2}>1\mathrm{atm}$), where collisional broadening exceeds hyperfine splittings of the atoms, optical pumping theory provides a clear description of the angular momentum exchange between photons and atomic spins. However, in many magnetic sensing scenarios, the high-pressure approximation becomes inadequate as its pressure conditions are not strictly satisfied. Consequently, an explicit description of optical pumping under realistic pressures is critical for selecting operating points and enhancing system performance. To address this, we develop a unified theoretical framework of optical pumping in the quasi-high-pressure regime, where collisional broadening is comparable to the ground-state hyperfine splitting. We demonstrate that light absorption, spin polarization, and magnetic-resonance linewidth in this regime differ significantly from those predicted by the high-pressure limit and offer favorable operating conditions. Our study extends conventional modeling and offers critical guidance for atomic magnetometry operating under realistic buffer gas pressures.

We design laser pulses to transform the oriented model formic acid (HCOOH) from its achiral ground state ($A^{\prime }$) to chiral configurations in the first excited state ($A^{\prime \prime }$). Optimal control of the production of the target enantiomer followed by enantiomer flips with highest enantiomeric excess is achieved by means of a linearly polarized laser pulse with optimal duration, field strength, frequency, and direction of polarization. The approach is motivated by effects of directionality of x-ray or laser pulses on different aspects of chirality of HCOOH discovered by the team of Dörner using cold target recoil ion momentum spectroscopy.