Recent works on the decoherent histories formalism suggest that slowly evolving macroscopic quantities (extensive sums of local observables) in quantum many-body systems can be consistently monitored: The existence of past measurements does not alter future outcome distribution. Here, for Gaussian weak measurements, we show that fluctuations of macroscopic quantities cannot be consistently monitored in general, in contrast to their intensive mean value. Exceptions include fluctuations at infinite temperature, at critical points, and in semiclassical systems. We analytically quantify nonconsistency in terms of susceptibility and obtain related results on entropy growth under noisy unitary.

Goos-Hänchen (GH) and Imbert-Fedorov (IF) shifts of paraxial wave packets in reflection from a planar interface, known collectively as optical shifts, appear inevitable within the framework of the wave theory. Yet, we demonstrate that such shifts can be completely eliminated at a given angle of incidence at a planar interface of homogeneous dielectric and absorbing media by classically entangling phase-space degrees of freedom of an incident wave packet. Thus, we can attain a geometric optics limit in the wave-packet reflection from a single planar interface. We identify a set of plasmonic materials (noble metals) and a parameter regime for which such cancellation is robust to modest errors in the incidence angle and source coherence, as well as polarization, providing practical guidelines for tailoring transverse beam displacements and angular deviations with standard metallic coatings.

We directly measure the branching ratio of the $5s4d{}^1{D}_2\to 5s5p{}^3{P}_2$ transition and the decay rate of the $5s5p{}^1{P}_1\to 5s4d{}^1{D}_2$ transition in neutral strontium. For more than four decades, these quantities lacked an experimental determination independent of theoretical input. We measure the branching ratio to be 0.177(4), significantly lower than the widely cited theoretical value of 0.322 [C. W. Bauschlicher, Jr. et al., J. Phys. B: At. Mol. Phys. 18, 1523 (1985)]. We also determine the decay rate to be $5.3\left(5\right)\times {10}^3{\mathrm{s}}^{-1}$, consistent with the value reported by Hunter et al. [Phys. Rev. Lett. 56, 823 (1986)] but substantially lower than the recent theoretical prediction of $9.25\left(40\right)\times {10}^3{\mathrm{s}}^{-1}$ [A. Cooper et al., Phys. Rev. X 8, 041055 (2018)]. These measurements provide an experimental benchmark for quantitative modeling of loss processes in laser cooling and single-atom fluorescence detection in optical tweezers with Sr.

We report a differential momentum measurement of nonsequential double ionization (NSDI) of Mg in strong laser fields, by combining laser ablation with the supersonic molecular beam technique. The experimental result shows that the momentum distribution of ${\mathrm{Mg}}^{2+}$ ions along the laser polarization exhibits a single-peaked structure. This finding, confirmed by our fully classical model, provides direct experimental evidence that the NSDI of Mg proceeds predominantly via the recollision excitation with a subsequent ionization pathway. Furthermore, we find a distinct discrepancy in the shape of the momentum distribution obtained from the experiment and the classical model. The discrepancy mainly arises from the inability of the model to account for the returning electron impact ionization and excitation cross sections. Our experimental approach can be readily extended to other atomic species that are solid at room temperature. It also has important implications for probing electron entanglement in NSDI of alkaline-earth metal atoms.

The Josephson diode, a nonreciprocal quantum element analogous to the familiar semiconductor p–n junction diode, has been realized in solid-state systems. However, it remains relatively unexplored in atomtronic circuits, which, in contrast to the solid state, allow real-time, in situ, and tunable control over key elements such as junction geometry, current-phase relationship, symmetry breaking, and interaction-induced nonlinearity. In this Letter, we propose and numerically demonstrate the realization of the Josephson diode effect in an atomtronic circuit consisting of a ring-shaped Bose-Einstein condensate and with optical barriers serving as Josephson junctions. Our implementation of this macroscopic nonreciprocal quantum phenomenon is based on realizing the required inversion symmetry breaking through asymmetric barrier placement and an asymmetric alternating current drive, enabling position- and drive-tunable diode effects with efficiencies up to 20% and 91%, respectively. While standard time-of-flight absorption imaging can readily observe these effects, we employ cavity optomechanics for cleaner, in situ, real-time, and nondestructive measurements of the relevant condensate dynamics. Our results establish a highly tunable platform for nonreciprocal Josephson transport, opening avenues for diode-based neutral-atom technologies in future quantum circuits.

We introduce the correlation-efficient time-evolution (CETE) algorithm for simulating quantum many-body dynamics. CETE recasts each step of time evolution as a time-independent correlation problem: the ansatz begins from a mean-field single Slater determinant and is then correlated to capture the true time-evolved state. We derive this exact ansatz from a contraction of the time-dependent Schrödinger equation onto the space of two electrons. Unlike conventional evolution by sequential short-time propagators, which must both correlate and decorrelate the state as the degree of correlation fluctuates in time, CETE correlates only once. This substantially reduces circuit depth, extending accessible simulation times on near-term quantum devices. We demonstrate the approach by simulating the electronic time evolution of the hydrogen molecule and the helium hydride ion, highlighting the potential for the CETE algorithm to simulate strongly correlated systems on near-term devices.

Quantum sensing with qubits has advanced fundamental physics searches, but higher dimensional systems offer largely unexplored potential. We show that in ultralight dark matter searches spin-1 NV-center qutrits exploiting common mode noise suppression achieve an order of magnitude improvement in axion-electron coupling sensitivity over qubit-based schemes. While enhanced quantum Fisher information in multilevel systems is known abstractly, we demonstrate its explicit realization in a realistic open quantum system via an optimal protocol saturating the Cramer-Rao bound, yielding a universal fourfold enhancement. This principle applies broadly to multilevel quantum platforms, including superconducting, neutral atom, and trapped-ion systems, establishing higher dimensional sensing as a powerful probe of new physics.

Nuclear optical clocks, a novel type of optical clock, align well with future development trends of optical clocks. As the core material for developing solid-state nuclear optical clocks, it is important to study Th-doped crystals with comprehensive advantages for the advancement of this field. The nonlinear crystal $\mathrm{Sr}{\mathrm{B}}_4{\mathrm{O}}_7$ (SBO) can directly frequency double a 296.8 nm laser to generate 148.4 nm excitation light, which matches the nuclear transition band of ${}^{229}\mathrm{Th}$. Meanwhile, SBO can be used as a host crystal for Th doping. This study reports the successful growth of Th-doped SBO, achieving the doping of thorium in a nonlinear oxide host. A 1-mm-thick Th:SBO crystal ($1.04\times {10}^{18}\mathrm{c}{\mathrm{m}}^{-3}$) retained high VUV transmittance at 148.4 nm (52.08%), only marginally lower than SBO (57.29%). Th:SBO crystal is expected to be a promising candidate for solid-state optical nuclear clocks.

The open quantum Rabi model describes a two-level system coupled to a harmonic oscillator. A Gaussian phase transition for the nonequilibrium steady states has been predicted when the bosonic mode is soft and subject to damping. We show that oscillator dephasing is a relevant perturbation, which leads to a non-Gaussian phase transition and an intriguing cascade of instabilities for $k\mathrm{th}$-order bosonic operators, as well as a jump in the steady-state qubit polarization. For the soft-mode limit, the equations of motion form a closed hierarchy and spectral properties can be efficiently studied. To this purpose, we establish a fruitful connection to non-Hermitian Hamiltonians. The results for the phase diagram, stability boundaries, and relevant observables are based on mean-field analysis, exact diagonalization, perturbation theory, and Keldysh field theory.

We reveal that the entropic uncertainty relation with a quantum memory is able to intrinsically connect local generic contextuality addressed in the pioneering work by Spekkens [Phys. Rev. A 71, 052108 (2005)] and nonlocal quantum resources such as entanglement and Bell nonlocality. Based on the constructed optimal set for any given single-qubit state, we prove rigorously a faithful criterion to witness the generic contextuality in the scenario of local quantum state preparation. Furthermore, within the framework of quantum resource distribution, it is proved that there exist quantitative trade-off relations between local preparation contextuality and bipartite entanglement or Bell nonlocality in a shared quantum system, which are captured by two inequalities where the local and nonlocal quantum resources can coexist. The faithful criterion and quantitative inequalities are all experimentally testable, which are verified through two independent, well-designed demonstrations on the Quafu quantum cloud platform.

We present a scalable chiral waveguide architecture for the on-demand nonlinear conversion of a single pump photon into a Stokes and anti-Stokes pair. Our model is implemented using a four-level atom coupled to three waveguide modes and controlled by an external light beam in the double-$\mathrm{\Lambda }$ configuration. With a suitable choice of atom-level symmetry, pump-photon coherence time, and control-beam intensity, this model allows to achieve near perfect or slightly discounted conversion efficiencies, when external decay channels are neglected or included, as well as ideal pair-state purities and strong cross-correlations. This suggests a promising route toward quasi-deterministic biphoton generation, while other approaches struggle to simultaneously optimize these figures of merit.

Typicality arguments replace the postulated mixed‐state ensembles of statistical mechanics with pure states sampled uniformly at random, explaining why most microstates of large systems exhibit thermal behavior. This paradigm has been revived in quantum contexts, where entanglement is deemed essential, but no clear quantitative link between entanglement structure and typicality has been established. Here, we study pure quantum states with controlled multipartite entanglement and show that when entanglement grows with system size $N$, fluctuations in macroscopic observables decay exponentially with $N$, whereas if entanglement remains finite, one recovers only the classical $1/\sqrt{N}$ suppression. Our work thus provides a quantitative connection between entanglement structure and the emergence of typicality, demonstrating that entanglement is crucial for thermalization in small quantum systems but unnecessary to justify equilibrium in macroscopic ensembles. This unifies the classical and quantum foundations of statistical mechanics by clarifying precisely when and why entanglement becomes relevant.

Recent advances in quantum simulators permit unitary evolution interspersed with locally resolved midcircuit measurements. This paves the way for the observation of large-scale space-time structures in quantum trajectories and opens a window for the in situ analysis of complex dynamical processes. We demonstrate this idea using a paradigmatic dissipative spin model, which can be implemented, e.g., on Rydberg quantum simulators. Here, already the trajectories of individual experimental runs reveal surprisingly complex statistical phenomena. In particular, we exploit free-energy functionals for trajectory ensembles to identify dynamical features reminiscent of hydrophobic behavior observed near the liquid-vapor transition in the presence of solutes in water. We show that these phenomena are observable in experiments and discuss the impact of common imperfections, such as readout errors and disordered interactions.

Computing the exact dynamics of many-body quantum systems becomes intractable as system size grows. Here, we present a symmetry-based method that provides an exponential reduction in the complexity of a broad class of such problems—qubits coupled to one-dimensional electromagnetic baths. We identify conditions under which partial permutational symmetry emerges and exploit it to group qubits into collective multilevel degrees of freedom, which we term “superspins.” These superspins obey a generalized angular momentum algebra, reducing the relevant Hilbert space dimension from exponential to polynomial. Using this framework, we efficiently compute many-body superradiant dynamics in large arrays of qubits coupled to waveguides and ring resonators, showing that—unlike in conventional Dicke superradiance—the total spin length is not conserved. At long times, dark states become populated. We identify configurations where these states exhibit metrologically useful entanglement. Our approach enables exact treatment of complex dissipative dynamics beyond the fully symmetric limit and provides a rigorous benchmark for approximate numerical methods.

We consider a spin system coupled to a noisy environment via an interaction Hamiltonian $H$ which is a polynomial of degree $n$ in the spin operators $S_i$ ($n$-magnetic Hamiltonian), and show that the use of the quantum super-Zeno effect, in conjunction with a spin-anticoherent subspace, is highly effective in protecting the spin from the noise. When $n=1$, as in the case of unwanted rotations induced by stray magnetic fields, our scheme provides a quantum gyroscope, a concept that generalizes naturally to higher $n$ values (a quantum $n$-gyroscope). The gist of the proposed protocol is to use the quantum super-Zeno effect to confine a state to the subspace, and then exploit anticoherence to freeze its evolution within it.

We investigate the dynamics of correlation propagation in the one-dimensional Fermi-Hubbard model with $\mathrm{SU}\left(N\right)$ symmetry when the repulsive-interaction strength is quenched from a large value, at which the ground state is a Mott-insulator with $1/N$ filling, to an intermediate value. From approximate analytical insights based on a simple model that captures the essential physics of the doublon excitations, we show that entanglement in the initial state leads to collective enhancement of the propagation velocity $v_{\text{SU}\left(N\right)}$ when $N>2$, becoming equal to the velocity of the Bose-Hubbard model in the large-$N$ limit. These results are supported by numerical calculations of the density-density correlation in the quench dynamics for $N=2,3,4,$ and 6.

The study of the boundary between classically simulable and computationally complex quantum dynamics is fundamental to understanding which physical resources may enable enhanced information-processing capabilities. We investigate this within the framework of quantum reservoir computing by introducing a tunable $N$-qubit random circuit model, where a fraction $p$ of Clifford gates are probabilistically substituted with nonstabilizing conditional-$\widehat{T}$ gates. We establish a direct correspondence between the reservoir's performance on temporal processing tasks and its entanglement-spectrum statistics and long-range nonstabilizer resource content. To assess scalability, we study the scaling of the antiflatness of states in the large-$N$ limit at a fixed circuit depth ratio $d/N\sim \mathcal{O}\left(1\right)$. This is taken as a witness to the concentration of measures, a known impediment to learning in thermalizing systems. We demonstrate that the learnability and scalability of the reservoir can be continuously controlled by the parameter $p$, allowing us to navigate from classically tractable to maximally expressive quantum dynamics. These architecture-agnostic results provide a general strategy for designing tunable and expressive quantum reservoirs, highlighting how certain nonclassical properties control average-case intrinsic learnability and functionality.

We study the structure of multipartite entanglement in monitored quantum circuits exhibiting measurement-induced phase transitions (MIPTs). Using a one-dimensional Clifford circuit subject to local measurements with a probability $p$, we show numerically that the entanglement depth, corresponding to the size of the largest cluster of entangled qubits, scales as a power law with system size on both sides of the transition. The power law exponent is 1 in the entangling phase and continuously decreases to 0 as $p\to 1$ in the disentangling phase. In addition, we find that the spatial support of the largest cluster exhibits an approximate fractal geometry with a tunable fractal dimension controlled by the measurement rate. We argue that this structure arises from a competition between unitary-driven coagulation of entangled clusters and measurement-induced fragmentation, giving rise to a fractal steady state reminiscent of classical coagulation-fragmentation models. Away from the MIPT critical point, the fractal dimension matches the entanglement depth power law exponent. These results show that multipartite entanglement structure provides a fresh perspective on the emergent quantum correlations in monitored quantum circuits and noisy quantum dynamics.

Understanding the mechanism governing the quantum-to-classical transition in finite-dimensional systems from the viewpoint of phase-space quasiprobability remains challenging. In this Letter, we establish an operational connection between discrete rounds of generalized measurements and continuous-time decoherence, with an explicit correspondence between the number of measurement rounds and the evolution time. Operationally, we show that a single round of such a generalized measurement eliminates quasiprobability negativity in finite-dimensional systems. From the decoherence perspective, this loss of negativity occurs abruptly at a critical time. In particular, this critical time can be shorter than the conventional decoherence time, indicating that the latter does not always faithfully track the disappearance of nonclassicality. Our results provide insight into the quantum-to-classical transition in finite-dimensional systems from the viewpoint of phase-space quasiprobability and suggest feasible experimental quantum-circuit tests.