Quantum imaginarity is a distinct resource in quantum information theory, yet its nonlocal properties have not been fully explored. Here, we report an experimental study of the nonlocal advantage of quantum imaginarity (NAQI), in which local measurements on one subsystem can steer the average imaginarity of the conditional states of the other subsystem beyond the corresponding classical bound. The $l_1$-norm of imaginarity inequality is adopted as an experimentally accessible witness. A hybrid optimization algorithm that combines a genetic algorithm with sequential quadratic programming is developed to avoid local optima and accelerate the search for optimal measurement settings. Using polarization-encoded photonic qubits, we prepare two classes of two-qubit Bell-diagonal states and experimentally characterize the imaginarity of the conditional states following local measurements. Clear violations of the $l_1$-norm of imaginarity inequality are observed, providing an experimental certification of NAQI. We further investigate the relationship among NAQI, the nonlocal advantage of quantum coherence (NAQC), and Bell nonlocality (BN) based on their respective inequality criteria. For the two-qubit Werner states considered in this work, the regions of states that violate the respective criteria satisfy ${\mathcal{D}}_{\mathrm{NAQC}}\subset {\mathcal{D}}_{\mathrm{NAQI}}\subset {\mathcal{D}}_{\mathrm{BN}}$. Our work provides an experimental realization of NAQI and a comparison of different forms of quantum nonclassicality in a photonic platform.

In this work we propose a quantum-walk model, named controlled intermittent quantum walks (CIQWs), where one performs a continuous quantum walk for a period of time under a certain control signal and then adjusts the control signal and performs a continuous quantum walk for another period of time under the new control signal, repeating the operation in this way. We apply the model to tackle the fundamental problem of finding an unknown marked vertex on a graph, known as spatial search, obtaining a universal quantum search algorithm. For any graph with any number of marked vertices, our CIQW-based algorithm achieves a quadratic speedup in query complexity over classical random-walk-based algorithms. Compared to state-of-the-art results in quantum spatial search, our algorithm achieves significantly lower query complexity in certain cases. Furthermore, by using the simpler Laplacian matrix as the Hamiltonian for our continuous-time quantum walk, we facilitate a more concrete and concise search algorithm. These features distinguish our work and add a fresh perspective to the growing body of quantum spatial search algorithms.

Discriminating between noisy quantum processes is a central primitive for quantum communication, metrology, and computing. While discrimination limits for finite-dimensional channels are well understood, the continuous-variable setting—particularly under experimentally relevant energy constraints—remains significantly less developed. In this work, we establish an energy-constrained chain rule for the Belavkin-Staszewski channel divergence, which yields a fundamental upper bound on the error exponents achievable by fully adaptive, energy-constrained quantum channel discrimination protocols. We then derive efficiently computable bounds on asymmetric error exponents for energy-constrained discrimination of bosonic dephasing and loss-dephasing channels. Specifically, we show that three operationally relevant quantities—the measured relative entropy, the Umegaki relative entropy, and the geometric Rényi divergence—admit semidefinite program (SDP) formulations when the input energy is bounded and the Hilbert space is suitably truncated. Applying these tools, we demonstrate that optimal probes for these channels under energy constraints are Fock diagonal, and we also enable numerically precise evaluation of bounds on achievable error exponents across discrimination strategies ranging from separable to fully adaptive. The resulting SDPs provide practical benchmarks for quantum-limited sensing in low-energy bosonic platforms.

Modern precision measurements, such as interferometry for detecting gravitational waves, rely on the estimation of optical phases encoded in light fields. Here we propose to exploit the collectively enhanced output field of a driven-dissipative many-body quantum system as a light source in order to improve the precision of estimating optical phases. Pronounced temporal correlations of such output fields benefit the sensitivity of measurement protocols, which we show theoretically by employing a boundary time crystal as a light source. The fundamental bound on the precision of such estimation shows scaling with the number of constituents $N$ of the many-body system as $N^4$ while scaling linearly with the measurement time $T$. We discuss this scaling from the perspective of both the resources employed to build the light source and the resources produced by the light source. We show that a measurement scheme, in which the phase-shifted light field is guided into an auxiliary replica system, which serves as a detector that is sensitive to nontrivial temporal correlations of light, can saturate the fundamental bound on precision at an optimal operating point.

We present an in-depth study of the Dicke quantum battery introduced in Phys. Rev. Lett. 134, 130401 (2025), in particular, showing explicitly that the $N^2$ power charging advantage found in the extensive regime agrees with the quantum speed limits for open systems dynamics. We also study the coherent energy extraction by an external driving field, thus extending previous results of Monsel et al. [Phys. Rev. Lett. 124, 130601 (2020)] to an $N$-atom “charger.”

Recent advances in strong x-ray laser techniques enable the study of nonlinear multiphoton ionization in extreme high-frequency fields. Although the stabilization regime in such fields is theoretically established, its modified properties in the nondipole regime for long laser pulses remains unknown. Here we numerically investigate the strong-field ionization of hydrogen or a hydrogenlike ion in a long XUV laser pulse in the nondipole regime. Our study of the time-dependent quantum dynamics reveals a quasiperiodic modulation of the ionization yield as a function of pulse duration. We demonstrate that the Coulomb-field-induced slow oscillation of the ionized electron wave packet during the interaction is responsible for the observed modulation of the ionization yield. Furthermore, we scrutinize the unusual photon momentum sharing between the photoelectron and the ion in this extreme regime. These effects are observable in upcoming x-ray free-electron laser facilities.

Measurement plays a crucial role in a quantum system beyond just learning about the system state: it changes the postmeasurement state and hence influences the subsequent time evolution; further, measurement can even create entanglement in the postmeasurement conditional state. In this work, we study how a careful choice of parameters for a typical measurement process on cold atom systems—phase-contrast imaging—strongly impacts both what the experimentalist observes and the measurement backaction on the system, including the creation and diffusion of quasiparticles emerging from the quantum many-body dynamics. We focus on a Bose-Einstein-condensate array in the low-temperature, low-momentum limit. Our theoretical investigation identifies regimes in which the imaging light probes either bare atomic or Bogoliubov quasiparticle dynamics. Furthermore, we show that, together with a weak global drive, appropriate tuning of the measurement bandwidth enables direct probing of Bogoliubov quasiparticles while controlling the measurement-induced creation and diffusion of quasiparticles across momentum modes. These results clarify how the measurement protocol determines both the observed excitations and the corresponding measurement backaction in weakly interacting Bose-Einstein condensates.

We report on an imaging scheme for quantum gases that enables simultaneous detection of two spin states with single-atom resolution. It utilizes the polarization of the emitted photons during fluorescence by choosing appropriate internal states of lithium-6 atoms in a magnetic field. This scheme can readily be implemented to obtain in situ spin correlations in a wide variety of experimental settings.

The dyadic Green’s function is an important tool of computational photonics, giving deeper insights into light-matter interaction. We present an operator approach to the derivation of the dyadic Green’s function of a generic anisotropic planarly layered medium for both electric and magnetic fields. The resulting Green’s function is expressed through the evolution operators (a kind of transfer matrices) of the comprising layers and the surface impedance tensors, the singular term being naturally separated from other terms. The operator approach to the Green’s function simplifies both the conceptual understanding of the problem and the subsequent practical applications, some of which are demonstrated here. The proposed approach can be easily generalized to the case of spherical and cylindrical layers, as well as bi-anisotropic layered media. The obtained results can be applied in nanophotonics engineering problems.

Resonances associated with complex-frequency poles are ubiquitous across physics and can arise in any open system, ranging from subwavelength particles and cavities to biological structures. When two such resonances coalesce, they form exceptional points (EPs), non-Hermitian singularities known to produce unusual spectral and dynamical behavior. However, the dynamics of the response of such resonances and exceptional points to a complex-frequency drive has remained largely unexplored. Here, we experimentally observe the temporal response of complex-frequency resonances and theoretically study it for exceptional points, demonstrating that the passive system's initial response for a decaying drive grows linearly, with enhanced growth at EPs. Closed-form theory for general resonators, extended to higher-order modes, predicts efficient power transfer with $t$ and $t^2$ scaling for complex single poles and EPs, respectively, at all times. We analyze these effects in subwavelength optical scatterers and experimentally in an electrical circuit analogue, with excellent agreement, and explore configurations that capture EP-enhanced growth.

We investigate the bipartite and tripartite entanglement and the mechanical ground-state cooling in a hybrid microwave-optomechanical circuit. This system, which consists of a common microwave $LC$ resonator, two single-Cooper-pair transistors (SCPTs), and two micromechanical resonators, when driven in a specific regime, behaves equivalently as an optomechanical system with a single cavity field interacting with two mechanical resonators. The cavity couples to the mechanical modes via radiation-pressure, cross-Kerr (CK) type nonlinearities, and an interaction between the two mechanical resonators. First, we demonstrate that adjusting the gate-charge deviation and the detuning frequency enables manipulation of multipartite entanglement within this system. Then, our results indicate that distinct types of CK couplings enhance either the cooling efficiency or the entanglement generation. More specifically, the three-mode CK coupling contributes to multipartite entanglement, whereas the conventional CK coupling plays a key role in simultaneous ground-state cooling of the mechanical modes. This observed competition between cooling efficacy and entanglement formation yields insights for the optimal design of hybrid quantum systems built for multitask operations.

Rydberg atoms possess strong electric-dipole transitions and tunable energy levels, making them promising candidates for microwave to optical conversion on integrated superconducting atom chips. Achieving strong coupling of the atoms to, e.g., the microwave field of an on-chip resonator requires placing the atoms within tens of micrometers from the chip surface. However, inhomogeneous stray electric fields originating from the surface can induce position-dependent Stark forces, resulting in atomic motion and leading to time-dependent shifts of the Rydberg energy levels. We experimentally investigate these effects using time-of-flight and spectroscopic techniques, observing substantial level shifts and signal loss attributable to field-induced atomic motion. A theoretical model incorporating an exponentially decaying surface field with a superimposed bias accurately reproduces the observed dynamics. To mitigate the level shift, we introduce a Stark echo sequence that dynamically reverses the force. This approach suppresses the atomic motion and maintains the atomic resonance. The method relies solely on global field control and is compatible with atom-resonator coupling architectures, providing a robust strategy for preserving coherence of Rydberg atoms in inhomogeneous electric fields near surfaces.

We develop a finite-temperature Gaussian-state formulation of quantum energy teleportation in the one-dimensional harmonic chain. For Gibbs states, the optimized measurement-feedback protocol reduces to thermal two-point functions. In the single-site protocol, the extracted energy is governed by a single correlator, which makes the thermodynamic and near-critical limits analytically tractable. At fixed finite temperature, the extracted energy is exponentially screened with distance and is controlled by a thermal correlation length for both critical and noncritical $\alpha$. In the zero-temperature critical limit taken after the thermodynamic limit, we derive the exact asymptotic law $E_{\mathrm{ext}}\sim d^{-4}$. Numerical results confirm both regimes and resolve the crossover responsible for the apparent drift of the effective decay exponent at intermediate distances. We also analyze squeezed Gaussian measurements and show that they modify the extraction prefactor: $p$ squeezing enhances the extracted energy, whereas $q$ squeezing suppresses it without changing the large-distance scaling.

Continuous-time quantum walks on a lattice spread ballistically and converge to a limiting distribution for the rescaled position. On the hard-wall half line the boundary reflects the walker but does not change the bulk dispersion, so the ballistic front remains set by the maximal group velocity. We ask how close one can get to this front when the initial state is constrained to occupy only the first $M$ sites. We show that optimizing the moment-generating function of the limiting-velocity distribution over this finite-support class reduces to the principal eigenvalue of an explicit $M\times M$ Hermitian matrix, which yields the optimal state by direct diagonalization. For large $M$, a near-front scaling limit produces a continuum description that controls the entire peak region. In particular, the maximal mean drift approaches the front with an inverse-square deficit in $M$ and a constant prefactor ${\pi }^2/4$. Numerical results verify both the finite-$M$ spectral formulation and the predicted scaling.

Quantum imaginarity is an important resource in quantum information theory, associated with the operational role of complex numbers in quantum mechanics. Here we investigate how imaginarity behaves under coherent superposition of orthogonal pure states. For orthonormal components, we express the geometric imaginarity in terms of a conjugation-overlap matrix. We characterize these matrices as complex symmetric contractions and determine the minimum dimension required for their realization. In the two-component setting, we derive the exact feasible region of the off-diagonal conjugation overlap and a sharp bound on its modulus. Moreover, the conjugation-overlap matrix of an orthonormal qubit pair is unitary, yielding the full range attainable by varying the relative phase. For multicomponent superpositions, we identify the exact condition for attaining maximal imaginarity in superpositions of real components and show that pairwise compatibility is insufficient for global realizability, although pairwise constraints still yield lower bounds. When only the diagonal conjugation overlaps are specified, the attainable imaginarity values form a closed interval whose end points can be determined by convex optimization. Our results show that the imaginarity of a superposition depends not only on the imaginarities of its components but also on their relative phases and the global compatibility of their conjugation overlaps.

Quantum information processing, communication, and sensing networks are being developed with various qubit platforms that use different encoding schemes. Connecting quantum network nodes to distribute entanglement requires matching photon qubit basis encoding. In this work we implement an interconversion protocol which converts photon qubit encoding from the polarization basis to the time-bin basis, transmits the photons through a transport fiber with large fluctuations in polarization, and converts back to polarization encoding for ease of measurement. This interconversion scheme faithfully transmits a polarization Bell state across the transport fiber by converting sources of infidelity to changes in transmission rate. These results illustrate a practical approach for interfacing distinct qubit platforms to enable modular and flexible operation in heterogeneous quantum networks.

We present fully correlated calculations of angularly resolved one- and two-photon photoionization delays in ${\mathrm{H}}_2\mathrm{O}$ for different ionization channels and light-polarization directions. By solving the time-dependent Schrödinger equation in full dimensionality, we obtain molecular-frame photoelectron angular distributions, Wigner delays, and two-photon photoionization delays for the three lowest cationic states of water. The comparison between Wigner and two-photon delays reveals that the interaction with the infrared (IR) field strongly reshapes the angular dependence of the delays, leading to pronounced anisotropies and directional asymmetries that are absent in the one-photon delays. Within a given sideband, the resulting angular variations can reach up to $\approx 80$ as for a fixed vibrational state, while differences between vibrational levels of the residual cation at a fixed emission angle are typically limited to $\approx 15$ as. The continuum-continuum contribution induced by the IR field is found to strongly depend on the photoelectron emission direction and molecular orientation, indicating that isotropic approximations commonly employed for atomic targets are not generally adequate for molecular systems. These results highlight the importance of orientation-resolved measurements for accessing the full complexity of molecular photoionization dynamics.