Accurate parameter estimation is a central task in quantum metrology and sensing, where quantum resources can provide precision beyond classical limits. In realistic settings, however, system-environment interactions lead to decoherence, reducing these strategies to their classical counterparts. Noise is typically classified as Markovian or non-Markovian, with the latter often preserving quantum coherence longer and thus supporting better metrological performance. Still, the absence of noise is generally considered ideal. In this work, we demonstrate that selectively engineered non-Markovian dynamics can outperform both the Markovian dynamics—including the corresponding quantum Cramér-Rao bounds—and the entirely noiseless case, corresponding to dispersion-free limit. We demonstrate these findings numerically for an all-optical setup, which is experimentally feasible and can be extended to other physical platforms. In general, our results open avenues for noise-assisted quantum metrology beyond conventional limits.

Entanglement is a central resource in quantum information science, yet its structure in high dimensions remains notoriously difficult to characterize. One of the few general results on high-dimensional entanglement is given by peel-off theorems, which relate the entanglement of a state to that of its lower-dimensional local projections. We build on this idea by introducing local extensions, the inverse process to peel-off projections, which provide a systematic way to construct higher-dimensional entangled states from lower-dimensional ones. This dual perspective leads to general bounds on how the Schmidt number can change under projections and extensions, and reveals mechanisms for generating bound entangled states of higher dimensionality. As a concrete application, we construct a positive-partial-transpose state of Schmidt number three in local dimensions $4\times 5$, the smallest system known to host such entanglement. We further extend this approach to identify an elegant family of generalized grid states with increasing Schmidt number, including explicit examples of a $7\times 7$ state with Schmidt number 4 and a $9\times 9$ state with Schmidt number 5, suggesting $(d+1)/2$ scaling in odd local dimensions $d\times d$. Taken together, our results provide a constructive toolkit for probing the scaling of bound entanglement in high dimensions.

We systematically investigate master equations involving double-bracket dissipators in the classical limit. Dissipators defined by double commutators arise naturally in dephasing dynamics, while those defined by double anticommutators are typically associated with noisy Hamiltonians. The classical limit of such master equations is derived by reformulating the dissipative open dynamics in phase space using the Wigner–Weyl transform and Moyal bracket formalism, and by identifying the leading-order terms in a systematic $\hslash$ expansion. We first analyze a double-commutator master equation associated with energy diffusion, and then turn to master equations containing a double anticommutator with the system Hamiltonian, recently derived in the context of noisy non-Hermitian systems. For both classes of double-bracket equations, we establish a gradient-flow representation of the dynamics within both the quantum and semiclassical formalisms. We further study the semiclassical evolution generated by double-bracket master equations for the harmonic (integrable) and driven anharmonic (chaotic) oscillators, considering both classical and quantum initial states. The dynamics are characterized through several observables, including mean position, momentum, and energy. The quantumness of the time-evolved state is assessed via the Wigner logarithmic negativity, and the interplay between double-bracket dissipation and chaotic dynamics is examined in detail. Finally, we extend our analysis to generalized master equations involving higher-order nested brackets, which arise naturally as a time-continuous formulation of spectral filtering techniques widely used in the numerical simulation of quantum systems.

In recent years, strong-field processes such as high-order harmonic generation (HHG) and above-threshold ionization driven by nonclassical states of light have become an increasingly popular field of study. The theoretical modeling of these processes often applies an approximate phase-space expansion of the nonclassical driving field in terms of coherent states, which has been shown to accurately predict the harmonic spectrum. However, its accuracy for the computation of quantum-optical observables like the degree of squeezing and photon statistics has not been thoroughly considered. In this work we introduce this approximative phase-space description and discuss its accuracy, and we find that it mischaracterizes the quantum-optical properties of the driving laser by making it an incoherent mixture of classical states. We further show that this error in the driving field description maps onto the light emitted from HHG, as neither sub-Poissonian photon statistics nor quadrature squeezing below vacuum fluctuations can be captured by the approximative phase-space description. Finally, to benchmark the approximative phase-space description, we consider the quantum HHG from a one-band model, which yields an exact analytical solution. Using the approximative phase-space representation with this specific model, we find a small quantitative error in the quadrature variance of the emitted field that scales with pulse duration and emitter density. Our results show that using this approximative phase-space description can mischaracterize quantum-optical observables. Attributing physical meaning to such results should therefore be accompanied by a quantitative analysis of the error.

We investigate the nonequilibrium expansion dynamics of dipolar bosons and fermions in finite one-dimensional lattice systems employing the multiconfigurational time-dependent Hartree method, which enables a numerically exact treatment of full many-body correlations, quantum fragmentation, and long-range interaction effects. By explicitly accounting for the interplay between quantum statistics, particle number, and long-range interactions, we uncover distinct dynamical regimes characterized by the emergence of coherent central high-density structures, near-ballistic expansion, and interaction-induced slowing down, as well as robust self-trapped states. Our analysis reveals that particle statistics fundamentally reshape the expansion dynamics: while bosonic systems exhibit correlation-induced fragmentation and interaction-driven slowing down, fermionic counterparts display markedly different transport behavior governed by Pauli blocking and exchange correlations. Furthermore, increasing particle number enhances correlation effects and amplifies the impact of long-range interactions, leading to nontrivial redistribution of density and coherence across the lattice. These findings demonstrate that beyond-mean-field correlations and the intrinsic nonlocality of dipolar interactions are essential for accurately describing expansion and transport in finite lattice geometries. Our results provide clear signatures of correlation-driven dynamical phenomena and establish finite dipolar systems as a versatile platform for probing nonequilibrium quantum many-body physics.

We investigate dressed bound states in an open cavity with a whispering-gallery-mode microring coupled to a two-level atom and a waveguide with a mirror at the right end. We demonstrate that the non-Hermiticity of an open cavity facilitates the formation of dressed bound states, which consist of vacancylike dressed bound states and Friedrich-Wintgen dressed bound states. By deriving analytical conditions for these dressed bound states, we show that when a two-level atom couples to the standing-wave mode that corresponds to a node of the photonic wave function the vacancylike dressed bound states occur, which are characterized by null spectral density at cavity resonance. Conversely, Friedrich-Wintgen dressed bound states can be realized by continuously adjusting system parameters and are indicated by the disappearance of the Rabi peak in the emission spectrum, which is a distinctive feature in the strong-coupling regime. Moreover, we extend our analysis to the non-Markovian regime and find that our results are consistent with those obtained under the Markovian approximation in the wideband limit. In the non-Markovian regime, we analyze dressed bound states for both zero and nonzero accumulated phase factors. For zero accumulated phase factors, the non-Markovian regime exhibits higher peak values and longer relaxation times for vacancylike dressed bound states compared to the Markovian regime, where the Friedrich-Wintgen dressed bound states are absent in the non-Markovian case. Finally, we establish the correspondence between the energy spectrum and bound-state conditions for nonzero accumulated phase factors and analyze the influence of various parameters on non-Markovian bound states. Our work exhibits bound-state manipulations through non-Markovian open quantum system, which holds great potential for building high-performance quantum devices for applications such as sensing, photon storage, and nonclassical light generation.

Macroscopic quantum superpositions are widely believed to be unobservable because large systems cannot be perfectly isolated from their environments. Here, we show that even under perfect isolation, intrinsic unitary dynamics in generic many-body systems, as analyzed through the eigenstate thermalization hypothesis and random matrix theory, can suppress the observable signatures of macroscopic coherence. Using the Greenberger-Horne-Zeilinger state as a representative example, we demonstrate that while fully correlated measurements can initially distinguish a macroscopic superposition from its corresponding classical mixture, generic many-body evolution renders them operationally indistinguishable for most times. By analyzing both distinguishability measures and established quantifiers of macroscopic quantumness, we find that equilibration not only hides coherence from accessible observables but also suppresses the corresponding signatures of macroscopic quantumness, in particular within the additive-local framework considered here. These results identify unitary thermalization, independent of environmental decoherence, as a fundamental mechanism that limits the observation of macroscopic quantum effects.

Environmental decoherence occurs when a quantum system interacts with its surroundings, progressively reducing quantum interference and coherence, complicating the preservation of critical quantum features over time, especially during experimental implementation. The quantum features of a state can be represented in phase space via the Wigner function, which manifests across multiple scales, with decoherence potentially influencing each scale differently, as examined in this work. We consider the compass state and its photon-added and photon-subtracted variants (optimized compass states) as our representative examples, each of which exhibits phase-space features with dimensions beyond the Planck scale, making them suitable for quantum sensing applications. We investigate the interaction of these states with a heat reservoir by employing a range of well-established theoretical tools. We observe that compass states with finer-scale phase-space features are more fragile to decoherence, with parameters favoring greater sub-Planckness in phase space concomitantly increasing the fragility of these compass states to decoherence. Our findings are then validated for generic quantum states interacting with the heat reservoir, for which we provide analytical and numerical investigations, exploring the relationship between quantum state robustness to decoherence and the sizes of their phase-space features; that is, phase-space features at smaller scales decay faster under decoherence, and vice versa.

We study the possibility of detection of “spin-boson” entanglement by qubit-only measurements. Such entanglement is impossible to detect by previously proposed schemes that involve a fixed system-environment interaction because of inherent symmetries within the coupling and the initial state of the environment. We take advantage of the possibility of tuning the qubit-environment coupling, which is available in some qubit realizations. As an example, we study a superconducting transmon qubit interacting with a microwave cavity, which is one of such systems and is, furthermore, essential in the context of quantum information processing. We propose suitable Hamiltonian parameters for the preparation and measurement phases of the detection scheme that allow for an experimental test, and verify that the reported signal is still non-negligibly large at finite temperatures.

The quantum Rabi model (QRM) is a fundamental model for light-matter interactions; the finite-component quantum phase transition (QPT) in the QRM has established a paradigmatic application for critical quantum metrology (CQM). However, this paradigmatic application is restricted to a local regime of the QPT which has only a single critical point. In this work we propose a globalized CQM in the QRM by introducing an auxiliary nonlinear term which is realizable and can extend the critical point to a continuous critical regime. As a consequence, high measurement precision is globally available over the entire coupling regime from the original critical point of the QRM down to the weak-coupling limit, as demonstrated by the globally accessible diverging quantum Fisher information in the dynamics. We illustrate a measurement scheme using quadrature dynamics, with globally criticality-enhanced inverted variance as well as the scaling relation with respect to finite frequencies. In particular, we find that the globally high measurement precision still survives in the presence of decoherence. Our proposal paves the way to break the local limitation of the QPT of the QRM in CQM and enables a broader application, with implications for applicability in realistic situations.

We theoretically investigate the stochastic dynamics of two qubits subject to one- and two-site correlated continuous weak measurements. When measurements dominate over the local unitary evolution, the system's dynamics is constrained and part of the physical Hilbert space becomes inaccessible: a typical signature of the quantum Zeno (QZ) effect. In this work, we show how the competition between these two measurement processes gives rise to two distinct QZ regimes, which we dubbed standard and correlated, characterized by a different topology of the allowed region of the physical Hilbert space, being a simply and nonsimply connected domain, respectively. We develop a theory based on a stochastic Gutzwiller ansatz for the wave function that is able to capture the structure of the phase diagram. Finally we show how the two QZ regimes are intimately connected to the topology of the flow of the underlying non-Hermitian Hamiltonian governing the no-click evolution.

The quantum approximate optimization algorithm (QAOA) is a leading platform for combinatorial optimization in the noisy intermediate-scale quantum (NISQ) era. A central challenge for QAOA and related variational algorithms is how to obtain efficient, provably reliable gradient estimators. We develop a rigorous spectral-filtering framework for the per-layer two-point estimator, a hardware-efficient heuristic. The framework represents the single-parameter slice as a finite Fourier series and analytically quantifies systematic error via a computable bias kernel, yielding a uniform error bound that depends solely on the Hamiltonian, independent of the specific quantum state. Within this framework, we prove that a two-point rule cannot be exact beyond the two-eigenvalue case of the generator. Incorporating shot noise, we derive a closed-form mean-square error (MSE)–optimal step-size rule. Its constants are rendered computable via trace moments, providing practical envelope bounds that avoid spectral enumeration. The optimal step size scales as $M^{-1/6}$ and the minimal MSE as $M^{-2/3}$ in the total number of shots $M$, matching the classical optimum for noisy finite differences. The framework also unifies randomized shifts and multipoint schemes, elevating the two-point method from a heuristic to a gradient estimator with predictable performance, controllable error, and formal guarantees, thereby providing a rigorous scheme for optimizing QAOA and broader classes of variational quantum algorithms on NISQ hardware.

We present a theoretical investigation of coherent control over the orientation of an individual molecule strongly coupled with a cavity using chirped-pulse driving. Specifically, we explore the dynamics of carbonyl sulfide (OCS) molecules under the influence of two chirped pulses with different spectral phases. We compare two pulse configurations: one with equal chirp rates $({\beta }_{+}={\beta }_{-})$ and another with unequal chirp rates $({\beta }_{+}\ne {\beta }_{-})$. Numerical simulations reveal that chirped pulses enable precise control of the molecular orientation, achieving a maximum orientation degree of 0.5773. By analyzing the distribution of molecular polariton states, we show that chirped pulses can activate multiphoton processes, leading to deviations from the predictions of first-order Magnus expansion methods. Additionally, we demonstrate the robustness of the maximum orientation with respect to chirp amplitude and detuning, providing insights into the role of pulse parameters in optimizing control. This work introduces a strategy for controlling molecular orientation in cavity-based systems and offers valuable perspectives for future experimental applications.

Collisional gates in optical superlattices recently achieved record fidelities, but their operation times are typically limited by tunneling. Here we propose and analyze an alternative route to a fast $\sqrt{\mathrm{SWAP}}$ gate for two fermionic atoms in an optical superlattice based on optimized, time-dependent control of the short and long lattice depths. The gate is implemented by transiently releasing the atoms into a quasiharmonic confinement centered between the two sites. With an appropriately chosen contact interaction strength, a controlled collision accumulates the exchange phase required for $\sqrt{\mathrm{SWAP}}$ and generates entanglement. We employ a continuum, time-dependent Schrödinger-equation simulation that goes beyond a two-site Fermi-Hubbard description and benchmark it against experimentally implemented tunneling-based protocols, reproducing the observed single-particle tunneling and spin-exchange dynamics. For experimentally accessible lattice depths, we find that the proposed gate operates in $\sim 21\mu \mathrm{s}$, more than an order of magnitude faster than tunneling-based implementations, while achieving fidelities $\gtrsim 99\%$. We further analyze sensitivity to lattice-depth variations and show that a composite sequence improves robustness. Our results establish fast, collision-mediated entangling gates in superlattices as a promising building block for scalable neutral-atom quantum computation.

We investigate the dynamical formation of nonlinear patterns in one-dimensional ring condensates under bichromatic periodic modulation of the interaction strength. The stability phase diagram of the condensate's homogeneous density state is analytically derived by introducing a suitable biharmonic variant of the Mathieu equation and computing the associated Floquet spectrum. It reveals the complex interplay among the driving parameters, i.e., amplitude, frequencies, and the so-called frequencies' mixing angle, which dictate the instability onset and the selective enhancement of higher-order resonance tongues, thus offering precise control over the excited modes. These results are in agreement with time-dependent mean-field simulations evidencing the emergence of density wave modulations of specific momenta, while enabling a deeper understanding of the nonlinear stage of the relevant instability. Further insights on the ensuing unstable nonlinear dynamics are provided through a reduced five-mode model which captures the instability onset, the oscillatory behavior of the mode populations and the phase-space dynamics, in agreement with the mean-field predictions. Our study highlights the versatility of bichromatic driving to generate and control complex nonlinear patterns that are within reach in present day ultracold atom experiments.

A quantum interferometer can achieve phase sensitivity beyond the classical limit by using the nonclassical properties of the quantum state. The SU(1,1) interferometer, a commonly used quantum interferometer, can be realized by replacing the linear beam splitters in a Mach-Zehnder interferometer with nonlinear processes. The phase sensitivity of the SU(1,1) interferometer increases with an increase of internal photon number. Therefore, the bright-seeded SU(1,1) interferometer has an advantage of boosted sensitivity. However, the phase sensitivity of a bright-seeded SU(1,1) interferometer is usually susceptible to unavoidable optical losses in experiments, especially the internal losses, which limit its applications. Here, we propose a loss-resistant bright-seeded SU(1,1) quantum interferometer based on joint detection, in which intensity detection is implemented by subtracting part of the probe beam from the conjugate beam. We introduce a tunable parameter $t$ in this intensity detection and obtain the optimal $t$ as a function of internal losses. Such a scheme allows for better phase sensitivity in the presence of optical losses. Our results provide an alternate method for realizing a loss-resistant SU(1,1) interferometer and may promote the applications of the SU(1,1) interferometer in practical quantum measurements.

Exact quantum master equations serve as a fundamental tool for investigating quantum dissipative dynamics. However, deriving such equations—even when possible—is extremely tedious for multistate and multiatom systems. In this work, we focus exclusively on population dynamics of the extended Jaynes-Cummings model and develop a rigorous equation-of-motion approach for the double-excitation manifold. The resulting dynamical equations are simple and involve only two auxiliary functions. Numerical simulations confirm that the dynamics in the double-excitation manifold closely resembles that of the single-excitation manifold: both exhibit a quantum phase transition from a dissipative phase to a localized phase, with the double-excitation manifold showing a higher critical dissipation strength than its single-excitation counterpart.