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.

Thermal noise is the predominant instability in the provision of ultrastable laser frequency, referencing to an optical cavity. Reducing the thermal-noise limit of a cavity means either making it larger to spread thermal fluctuations, reducing the sensitivity of the cavity to temperature, or lowering the temperature. We report on a compact photonic resonator made of solid fused silica that we cool in a cryogenic environment. We explore a null in the resonator's frequency sensitivity due to the balance of thermal expansion and thermo-optic coefficients at a temperature of 9.5 K, enabling laser stabilization with a long-term frequency drift of 4 mHz/s on the 195 THz carrier. The robustness of fused silica to cryogenics, the capability for photonic design to mitigate thermal noise and drift, and operation at a modest 9.5 K temperature offer unique options for ultrastable laser systems.

We investigate anomalously slow coarsening in a dilute two-dimensional superfluid closed with respect to particle and energy exchange with the environment. The dynamics is demonstrated to be closely connected both to a nonthermal fixed point (NTFP) in a far-from-equilibrium quantum system and to Kraichnan-Kolmogorov turbulence. During a universal dynamical regime associated with an anomalous NTFP, vortex dynamics are understood to be governed by three-vortex collisions that trigger vortex-antivortex annihilation events, leading to a subdiffusive decay of the vortex density and thus growth of the characteristic interdefect length scale, ${\ell }_{\text{v}}\sim t^{\beta }$ with $\beta \approx 1/5$. It is found that, during the same time when this power law in time is seen, the moments of the superfluid velocity circulation $\mathrm{\Gamma }$ around an area of spatial extent $r$ exhibit power-law scaling ${\mathrm{\Gamma }}^2\left(r\right)\sim r^{8/3}$, in agreement with Kraichnan-Kolmogorov predictions for an inverse energy cascade in the inertial range, in a driven-open setting. Moreover, in high-order moments, intermittent deviations from linear scaling ${\mathrm{\Gamma }}^{2p}\left(r\right)\sim {\left[{\mathrm{\Gamma }}^2\left(r\right)\right]}^p$ are observed that are consistent with bifractal intermittency corrections previously measured in fully developed classical turbulence. These results establish a quantitative link between decaying quantum turbulence in a closed superfluid and universal dynamics near a nonthermal fixed point. Notably, the subdiffusive decay exponent $\beta \approx 1/5$ deviates significantly from values reported for classical systems.

We combine numerical and experimental approaches to study how impurities affect the maximum superflow in an annular Bose-Einstein condensate. By tuning the impurity density and radial position, we precisely control persistent-current stability: it increases with the number of impurities and when impurities are placed at larger radii. In the unstable regime, the complex vortex motion within the impurity landscape, characterized by pinning and unpinning events, governs the timescale of the current decay and its final value. Our work establishes atomic superfluids as a pristine platform for exploring universal mechanisms of superflow stabilization and decay, paving the way for atomtronic quantum technologies.

Optical switching remains a key outstanding challenge for scalable photonic quantum computing due to the trade-off between speed, bandwidth, and loss. Quantum photonics demands all three to enable high computational clock rates and resource-efficient scaling to large systems. We present a cavity-based optical switch that overcomes this limitation, demonstrating a rise time of 22 ns, an insertion loss of 2.4 dB, and an extinction ratio of 17.5, which bridges the gap between speed and efficiency in photonic switching. All-optical control is achieved via phase modulation of a signal field detuned from the near-degenerate two-photon absorption ladder in warm rubidium vapor. The ultimate performance of our switch will find applications in active multiplexing, loop-based quantum memory, and feed-forward for quantum error-correction protocols.

Realizing topological flatbands with tailored single-particle Hilbert spaces is a critical step toward exploring many-body phases, such as those featuring anyonic excitations. One prominent example is the Kapit-Mueller model, a variant of the Harper-Hofstadter model that stabilizes lattice analogs of the lowest Landau-level states. The Kapit-Mueller model is constructed based on the Poisson summation rule, an exact lattice sum rule for coherent states. In this work, we consider higher Landau-level generalizations of the Poisson summation rule, from which we derive families of parent Hamiltonians on a half-flux lattice which have exact flatbands whose flatband wave functions are lattice versions of higher Landau-level states. Focusing on generic Bravais lattices with only translation and inversion symmetries, we discuss how these symmetries enforce gaplessness and singular points for odd Landau-level series and how to achieve fully gapped parent Hamiltonians by mixing even and odd series. Our model points to a large class of tight-binding models with suitable energetic and quantum geometries that are potentially useful for realizing non-Abelian fractionalized states when interactions are included. The model exhibits rapidly decaying hopping amplitudes, making it potentially realizable with neutral atoms in optical lattices.

We show that interaction of an atomic target with a perturbative nonionizing ultraviolet (UV) pulse creates a short-lived virtual state (VS), which can be involved in the nonperturbative interaction of a target with an intense infrared (IR) field. In the example of the high-harmonic generation (HHG), it is shown that this VS provides an alternative HHG channel, which is coherently implicated in the IR-induced HHG amplitude and thereby leads to interference fringes in the HHG yield as a function of the time delay between UV and IR pulses. We discuss the VS-based enhancement of HHG yield and the use of interference fringes for retrieving the UV-pulse waveform.

Quantum token protocols enable unforgeable quantum tokens promising unconditional security beyond classical cryptographic assumptions. We show here that the three stages of the Quantum token protocols involving the preparation, storage, and verification can be made more secure when involving spin-photon interfaces that leverage high-fidelity hybrid tripartite (spin-photon-spin) entanglement, Bell state measurements, and highly coherent spin quantum memories. Further we describe the physical implementation of various stages of the protocol using the hybrid electron and nuclear spins in diamond interfaced with time-bin photons.

Geometric or Berry phases are fundamental manifestations that appear in many areas of physics. They arise from the geometry of the space describing the properties of multicomponent wave fields. An important example for electromagnetic waves is the spin-redirection Berry phase associated with the evolution of the spin direction. Because this effect has traditionally been studied in isotropic media where the spin is aligned with the ray trajectory, it has become commonly assumed that this spin-redirection Berry phase requires nonplanar rays. Here, we show that a spin-redirection phase can in fact arise along a planar ray if the spin evolves along the ray. We expose this effect through the singular example of a moving unmagnetized plasma, and demonstrate how this behavior can more generally arise from a finite transverse spin. In identifying this new spin-redirection mechanism, our work not only provides the tools to discover additional manifestations of spin-orbit interactions (SOIs) in nature, but also uncovers supplemental degrees of freedom to harness SOIs to control light.