Quantum illumination (QI) provides entanglement-enabled target detection enhancement, despite operating in an entanglement-breaking environment. Existing experimental studies of QI have utilized a Bayesian approach, assuming that the target is equally likely to be present or absent before detection, to demonstrate an advantage over classical target detection. However, such a premise breaks down in practical operational scenarios in which the prior probability is unknown, thereby hindering QI's applicability to real-world target detection scenarios. In this work, we experimentally implement the phase-conjugate receiver architecture previously envisioned for quantum illumination based target detection, and adopt the Neyman-Pearson criterion in lieu of the error probability for equally likely target absence or presence as our figure of merit for QI. We experimentally demonstrate an unconditional quantum advantage over the optimal classical-illumination protocol as benchmarked by the receiver operating characteristic, which examines detection probability versus false-alarm probability without resorting to known prior probabilities. Our work represents a critical advancement in adapting quantum-enhanced sensing to practical operational settings.

Classical vector waves can possess intricate spin angular momenta (SAM), which are perpendicular to the propagation direction, as revealed by the recent recognition of surprisingly transverse SAM in electromagnetic (EM) fields. In this paper, we employ the Hertz potential method to define structured vector fields and analytically decompose the SAM of the wave fields in two parts. Our approach of decomposition not only confirms that transverse SAM may originate from the first-order spatial inhomogeneity of the Poynting momentum, but also points out that for nonplanar vector waves with near fields, an extraordinary spin appears as the other distinct part of transverse spin. By four examples of vector beams, we further demonstrate that the proposed transverse spins prevail universally in both propagating and evanescent waves. This work renews our fundamental understanding of the decomposition of SAM for classical vector waves.

We investigate, both experimentally and theoretically, the eigenmodes of an electronic circuit in which gain and loss $RLC$ resonators are coupled through a capacitor. Due to the unavoidable magnetic loss in the inductors, we find that the eigenmode coalescence no longer emerges in contrast to the conventional non-Hermitian systems with spontaneous $\mathcal{PT}$-symmetry breaking. In particular, we find a transition from exponential decay to exponential growth in the amplitude of the periodic voltage oscillations of the resonators. The transition occurs near the exceptional points of the non-Hermitian circuit without considering the dissipations in inductors. We introduce a small resistor two orders of magnitude smaller than that of the $RLC$ resonators to mimic the energy dissipation in inductors and numerically solve the equivalent non-Hermitian Schrödinger equation. The numerical results can well reproduce experimental observations. Our above findings unambiguously indicate that the exponential growth behavior beyond the exceptional points is robust against some unavoidable dissipative perturbations.

We introduce a general framework for defining context-dependent time distributions in quantum systems using projective measurements. The time-of-flow (TF) distribution, derived from population transfer rates into a measurement subspace, yields a time-energy uncertainty relation of the form $\mathrm{\Delta }\mathcal{T}\mathrm{\Delta }H\ge \hslash /\left(6\sqrt{3}\right)\delta \theta$, where $\delta \theta$ quantifies the net population transfer measured by the projector. This bound applies to arbitrary projectors under unitary dynamics and reveals that time uncertainty is inherently measurement dependent. We demonstrate the framework with two applications: a general time-of-arrival–energy uncertainty relation and a driven three-level system under detuned coherent driving. The TF framework unifies timing observables across spin, atomic, and matter-wave systems and offers an experimentally accessible route to probing quantum timing in controlled measurements.

Adiabaticity is a cornerstone of many promising approaches to quantum control, computing, and simulation. In practice, however, there is always a trade-off. Although the deleterious effects of noise can be diminished by running a control schedule more quickly, this benefit comes at the expense of nonadiabaticity. To put these two unwanted effects on the same theoretical footing, we analyze the nonadiabatic error in qubit control as a form of entropy production, examining the mechanism by which fine-grained information is effectively lost despite the dynamics being fundamentally unitary. A crucial issue here is the question of how to define equilibrium under a time-dependent Hamiltonian. Using the Landau-Zener protocol as a test case, we show that entropy increases nearly monotonically when equilibrium is defined with respect to the effective Hamiltonian in the optimal superadiabatic frame. We then consider single-passage Landau-Zener-Stückelberg-Majorana interferometry, in which the initial state of the qubit is arbitrary. Violations of the second law of thermodynamics are possible but require exquisite control to achieve deliberately.

We describe a method for simulating and characterizing realistic Gottesman-Kitaev-Preskill (GKP) cluster states, rooted in the representation of resource states in terms of sums of Gaussian distributions in phase space. We apply our method to study the generation of single-mode GKP states via cat state breeding, and the formation of multimode GKP cluster states via linear optical circuits and homodyne measurements. We characterize resource states by referring to expectation values of their stabilizers and witness operators constructed from them. Our method reproduces the results of standard Fock-basis simulations, while being more efficient and being applicable in a broader parameter space. We also comment on the validity of the heuristic Gaussian random noise model, through comparisons with our exact simulations: We find discrepancies in the stabilizer expectation values when homodyne measurement is involved in cluster state preparation, yet we find close agreement between the two approaches on average.

Research on quantum states often focuses on the correlation between nonlocal effects and local unitary invariants, among which local unitary equivalence plays a significant role in quantum state classification and resource theories. This paper focuses on the local unitary equivalence of multipartite quantum states in quantum information theory, aiming to determine a complete set of invariants that identify their local unitary orbits; these invariants are crucial for deriving polynomial invariants and describing the physical properties preserved under local unitary transformations. The study deeply explores the characterization of local unitary equivalence and the method of detecting entanglement using local unitary Bargmann invariants. Taking two-qubit systems as an example, it verifies the measurability of invariants that determine equivalence and establishes a connection between Makhlin fundamental invariants (a complete set of 18 local unitary invariants for two-qubit states) and local unitary Bargmann invariants. These Bargmann invariants, related to the traces of products of density operators and marginal states, can be measured through cycle tests (an extended form of swap tests).

This study investigates quantum Fisher information (QFI) with respect to a class of nonlocal operators $\tilde{O}$ in one-dimensional quantum chains. Based upon the triangular matrix-product-operator theory, we establish a universal scaling behavior of QFI with respect to $\tilde{O}$ for general infinite matrix-product states (iMPSs): $\mathcal{F}\sim {\varrho }_2{N}^2+{\varrho }_{1}N+{\varrho }_0$, where the coefficients ${\varrho }_i$ can be efficiently identified by iMPS algorithms. Applied to a spin-1 Bilinear-Biquadratic model and an XXZ model, the scaling coefficients $\left\{{\varrho }_i\right\}$ distinguish various phases through distinct scaling behaviors: For instance, the topologically ordered Haldane phase exhibits characteristic quadratic scaling (${\varrho }_2\ne 0$), while the trimer and dimer phases are featured with linear scaling (${\varrho }_1\ne 0$). Notably, since the QFI scaling behavior undergoes fundamental changes between distinct phases, these coefficients provide signals at first-order, second-order, and Berezinskii-Kosterlitz-Thouless type quantum phase transitions in these models. Furthermore, finite bond-dimension ($D$) scaling analysis reveals a dichotomy. For gapped phases, the coefficients converge rapidly as $D$ increases. Conversely, for critical systems, the coefficients exhibit $D$-scaling effects that depend on the specific model. By bypassing finite-size fittings with limited $N$, our approach accesses the thermodynamic-limit physics for QFI with respect to nonlocal operators, offering new avenues for characterizing exotic quantum phases in one-dimensional quantum systems.

In this paper, we investigate spin-polarized muonium-muonium and spin-polarized hydrogen-hydrogen scattering for total orbital angular momentum $L=0$ using the non-Born-Oppenheimer confined variational method combined with explicitly correlated Gaussian basis functions. Our approach yielded accurate $S$-wave phase shifts and elastic cross sections at scattering momenta below $0.6a_0^{-1}$, where $a_0$ is the Bohr radius. By utilizing the effective range formula, we determined the $S$-wave scattering lengths to be $3.70a_0$ and $1.35a_0$ for spin-polarized muonium-muonium and spin-polarized hydrogen-hydrogen scattering, respectively.

Doubly differential ionization cross sections (DDCSs) of ethanol were measured for incident electron energies ranging from 60 eV to 1 keV, over an angular range of ${30}^{\circ }$–${135}^{\circ }$. The energies of the secondary electrons spanned from 4 eV up to approximately half of the primary electron energy. The experimental results were compared with calculations based on the distorted-wave Born approximation, including postcollision interaction between the two outgoing electrons. Overall, the experimental and theoretical data show reasonably good agreement within the experimental uncertainties. Singly differential ionization cross sections (SDCSs) were derived by integrating the DDCSs over the emission angles. These SDCSs are well reproduced by the generalized binary-encounter Bethe (GBEB) model. Total ionization cross sections (TICSs), obtained by integrating the SDCSs over the secondary electron energy, also show good agreement with the predictions of the GBEB model.

Radiative nonlinear four-wave mixing can monitor the evolution of electronic wave packets, providing access to lifetimes and quantifying the light-induced couplings between excited states. We report the observation of quantum beats in an autoionizing electronic wave packet in krypton, probed using this technique. Analysis of the signal reveals that these beats originate from the contribution of previously unassigned, doubly excited states interacting with singly excited ones. We introduce a minimal theoretical model, based on multichannel quantum-defect theory, which quantitatively reproduces both the wave-packet dynamics and the static spectrum. This work combines a versatile, noncommensurate XUV-IR-based experimental scheme with a tractable model, establishing a powerful approach for the metrology and control of complex, correlated electronic states.

Two-component Bose-Einstein condensates in the miscible phase can support polarization solitary waves, known as magnetic solitons. By calculating the interaction potential between two magnetic solitons, we elucidate the mechanisms and conditions for the formation of bound states—or molecules—and support these predictions with dynamical simulations. We analytically determine the dissociation energy of bound states consisting of two oppositely polarized solitons and find good agreement with full numerical simulations. Collisions between bound states—either with other bound states or with individual solitons—produce intriguing dynamics. Notably, collisions between a pair of bound states exhibit a dipole-like behavior. We anticipate that such bound states, along with their rich collision dynamics, are within reach of current experimental capabilities.

Using density functional theory, we have theoretically studied the formation and the stability of vortices in quantum liquid droplets composed of a mixture of hyperfine states of potassium. Following the experimental setup that produced quantum droplets for the first time, we work with squeezed drops that are compressed in one direction. By squeezing the drops even more, toward a quasi-two-dimensional geometry, we study the minimum atom number able to show a stable vortex and obtain that this number is significantly smaller than previous predictions for spherical droplets. The reduction of the critical atom number for forming a stable vortex could make their experimental observation in these droplets, which is still lacking, more feasible. Contrary to results obtained in heteronuclear mixtures, where the energetically preferred vortices are partially filled with the species not participating in the rotation, our results show a relevant stability island of fully empty vortices. Increasing the number of particles in the drop and the speed of rotation, we estimate the transition line between empty and filled vortices.

Dicke superradiance describes an ensemble of $N$ permutationally invariant two-level systems collectively emitting radiation with a peak radiated intensity scaling as $N^2$. Individual Dicke states are typically entangled. However, the density matrix during superradiant decay is a mixture of such states, raising the subtle question of whether the total state is entangled or separable. We resolve this by showing analytically that for any $N$, starting from the fully excited state, the collective decay preserves separability for all times. This answers a longstanding question on the role of entanglement in Dicke superradiance and underscores that, despite collective dissipation, separable states remain separable under these dynamics.

We study the optical response of a binary system of identical atoms in which one of them is excited by an incoherent pump. Applying the diagrammatic formalism developed by Donaire [Phys. Rev. A 104, 043704 (2021)], we show how scattering, absorption, stimulated emission, spontaneous emission, and resonant energy transfer can be tailored by varying (1) the interatomic distance, which governs the interference effects of the emitted radiation, and (2) the pump rate, which determines the population of the atomic levels. It is found that, for sufficiently strong pumping, the collective component of the extinction cross section becomes negligible, regardless of the interatomic distance, as optical gains compensate for losses, and the total extinction cross section is reduced to less than half of its value in the absence of pumping. In contrast, at weak pumping and short interatomic distances, interference effects lead to a significant suppression of the extinction cross section relative to that of two noninteracting atoms.