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.

Understanding the emergence of novel collective behaviors in strongly interacting systems lies at the heart of quantum many-body physics. Valuable insight comes from examining how few-body correlations manifest in many-body systems, embodying the “from few to many” philosophy. An intriguing example is the set of universal relations in ultracold atomic gases, which connect a wide range of observables to a single quantity known as the contact. In this Letter, we demonstrate that universal relations manifest in a distinct class of quantum many-body systems, long-range quantum spin chains, which belong to an alternative universality class. Using effective field theory and the operator product expansion, we establish connections between the asymptotic behavior of equal-time spin correlation functions, the dynamical structure factor, and the contact density. The theoretical predictions for equal-time correlators are explicitly verified through numerical simulations based on matrix product states. Our results could be readily tested in state-of-the-art trapped-ion systems.

Understanding the nonequilibrium dynamics of quantum many-body systems remains one of the grand challenges of modern physics. In particular, increasing attention has been devoted to the emergence of nonequilibrium universality classes that have no equilibrium counterparts. A prominent example is the Kardar-Parisi-Zhang universality class realized in dissipative Bose-Einstein condensates. In this Letter, motivated by recent experimental advances, we investigate the universal dynamics of dissipative quantum systems with dipole symmetry. We develop an effective-field-theory description, supported by a concrete quantum spin model, to capture the resulting universal behaviors. Our analysis unveils a novel strongly interacting nonequilibrium fixed point that governs the equal-time phase fluctuations in systems with either strong or weak dipole symmetries. Moreover, charge transport becomes subdiffusive in the presence of strong dipole symmetry, while it remains diffusive in the weakly symmetric case. Our results reveal the intricate interplay between kinetic constraints and dissipation in quantum many-body systems.

We present a unified description of attractive and repulsive polarons, formed in a one-dimensional Bose gas hosting an impurity particle, by obtaining all ground- and excited-state solutions to the Gross-Pitaevskii equation. Modeling the impurity with an attractive square-well potential, we characterize the excited-state energy branches as a function of interaction strength. As the impurity-bath coupling increases, the excited states change from distinct soliton configurations to hybridized soliton-polaron states, eventually crossing over from repulsive to attractive polarons at unitarity. We identify a universal regime near this crossover where the polaron properties are accurately characterized by the zero-energy scattering length.

We propose a dynamical protocol to probe the rigidity and phase coherence of dipolar supersolids by merging initially separated fragments in quasi-one-dimensional double-well potentials. Simulations based on the extended Gross-Pitaevskii equation reveal distinct dynamical signatures across phases. Supersolids exhibit damped crystal oscillations following barrier removal, with the damping rate reflecting superfluid connectivity. A phase-imprinted jump additionally triggers metastable dark solitary waves, which excites second sound, as revealed by an out-of-phase drift between the droplet lattice and the superfluid background. Our results show a realizable path to dynamically detect the second sound and rigidity of supersolids, as well as to realize and probe soliton formation.

We present a method for preparing various exotic modifications of Schrödinger cat states by coupling a semiclassical oscillator to a system of qubits. Varying the number of qubits and parameters of the protocol (involving quantum quench of the coupled system and a subsequent spin measurement), we bring the oscillator into a coherent superposition composed of an arbitrary number of wave packets in tunable proportions and motion relations. The method can be implemented with the aid of current experimental techniques and may find applications in quantum information and sensing protocols.

We report the sympathetic cooling and Coulomb crystallization of xenon highly charged ions (HCIs) with laser-cooled ${\mathrm{Ca}}^{+}$ ions. The HCIs are produced in a compact electron beam ion trap, then charge selected, decelerated, and finally injected into a cryogenic linear Paul trap. There, they are captured into ${}^{40}\mathrm{Ca}^{+}$ Coulomb crystals, and cocrystallized within them, causing dark voids in their fluorescence images. Fine control over the number of trapped ions and HCIs allows us to realize mixed-species crystals with arbitrary ordering patterns. By investigating ${\mathrm{Xe}}^{q+}-{\mathrm{Ca}}^{+}$ strings, we confirm the HCI charge states, measure their lifetime, and characterize the mixed-species motional modes. Our system effectively combines the established quantum control toolbox for ${\mathrm{Ca}}^{+}$ with the rich set of atomic properties of Xe highly charged ions, providing a resourceful platform for optical frequency metrology, searches for signatures of new physics, and quantum information science.

The stability of nonlinear waves on curved surfaces is a problem of widespread interest across physics. Here, we establish the stability criteria for dark solitons on a spherical Bose-Einstein condensate. We demonstrate a sharp instability threshold in the nonlinear parameter, beyond which solitons decay into vortex dipoles via snake instabilities. Analytically and numerically, we prove this decay is dictated by a single unstable mode for each angular momentum $m\ge 2$, which is a universal mechanism that controls the resulting vortex state. Unlike in the full three-dimensional case, where snake instabilities lead to vortex rings, a dark soliton confined to the surface of a bubble can only decay into vortex pairs.

Geometry-gauge interplay constitutes a fundamental issue in quantum physics, with profound implications spanning from quantum gravity to topological matter. Here we investigate the dynamic effects of geometry-gauge interplay in Bose-Einstein condensates on a Haldane sphere with a magnetic monopole. We reveal an index theorem that links vortex configurations to the topology of the gauge field, enabling the construction of vortex-monopole composites. Furthermore, we derive the universal logarithmic interaction between composites, which governs the structure of the ground-state vortex lattice. By developing a kinetic theory, we predict scale-invariant vortex dynamics and an emergent dynamical correspondence between vortex-vortex and vortex-antivortex systems. Both are confirmed through numerical simulations. This work uncovers a dynamical coupling mechanism between spatial geometry and gauge fields, providing deep insights into superfluid systems with topological gauge structures in curved space.

The electromagnetic Green's function is a crucial ingredient for the theoretical study of modern photonic quantum devices, but is often difficult or even impossible to calculate directly. We present a numerically efficient framework for calculating the scattered electromagnetic Green's function of a multicavity system with spatially separated open cavities (with arbitrary shape, dispersion, and loss) and finite retardation times. The framework is based on a Dyson scattering equation that enables the construction of the Green's function from the quasinormal modes of the individual resonators within a few-mode approximation and a finite number of iteration steps without requiring nested integrals. The approach shows excellent agreement with the full numerical Green's function for the example of two coupled dipoles located in the gaps of two metal dimers serving as quasinormal mode cavities, and is easily extended to arbitrarily large separations and multiple cavities.

Quantum measurements cause an abrupt change, called measurement-induced phase transition, in the scaling behavior of entanglement entropy in a quantum many-body system. The phenomenon is often studied in random quantum circuits, with local measurements performed with a certain probability. We report here one such transition in an interacting spin-1/2 system where a global measurement is performed with certainty at every time step. We start with a pure state with all the spins up. Each time step consists of evolution under the transverse Ising Hamiltonian for a time $\tau$, followed by a measurement that provides a “yes or no” answer to the question “Are all spins up?”. For various $\tau$ values, we compute the survival probability of the inital state, entanglement in bipartition, and the generalized geometric measure (a genuine multiparty entanglement), for a chain of size $L\sim 28$, and identify the transition points ${\tau }_c$ for different field strengths. We then analytically derive a recursion relation that enables us to calculate the survival probability for system sizes up to 1000 and provides evidence of a scaling ${\tau }_c\sim 1/\sqrt{L}$. Therefore, the transition at finite ${\tau }_c$ for $L\sim 28$ seems to recede to ${\tau }_c=0$ in the thermodynamic limit. Our work prompts a widespread investigation on the existence of measurement-induced transitions at large system size.

Identifying properties in quantum dynamics, such as identity or time-reversal invariance, is a crucial challenge with profound implications for quantum technologies. We present a unified framework that integrates group representation theory with quantum hypothesis testing to optimally predict the fundamental properties of quantum dynamics. By exploiting the inherent symmetry of compact groups and their irreducible representations, we derive an exact characterization of the optimal type-II error (failure probability to detect a symmetry), offering an operational interpretation for the quantum maximum-relative entropy. In particular, we prove that parallel strategies achieve the same performance as adaptive or indefinite-causal-order protocols, resolving debates about the necessity of complex control sequences. Examples to the singleton group, maximal commutative group, and orthogonal group yield explicit results: for predicting the identity, Z-symmetry, and T-symmetry properties of unknown qubit unitaries, with zero type-I error and type-II error bounded by $\delta$, we establish the explicit optimal sample complexity which scales as $\mathcal{O}\left({\delta }^{-1/3}\right)$ for identity testing and $\mathcal{O}\left({\delta }^{-1/2}\right)$ for $T/Z$-symmetry testing. These findings offer theoretical insights and practical guidelines for efficient unitary property testing and symmetry-driven protocols in quantum information processing.

We study interference effects in the dynamics of a spin-$\frac{1}{2}$ particle propagating in two dimensions in a disordered potential and subject to a generalized spin-orbit coupling. With the particle initially in a spin-polarized plane-wave state, in the short-time regime, before the spin and momentum distributions reach their steady states, we observe a transient backscattering peak offset from the exact backscattering direction, coexisting with a coherent backscattering dip. We present an intuitive explanation of this momentum offset using a non-Abelian gauge transformation. We also describe the full time evolution of the transient peak, from its buildup to its decay with a precise prediction of the dephasing time within a perturbative framework for multiple scattering. Our results can be applied to general spatially uniform SU(2) gauge fields, including the synthetic gauge field in ultracold atoms.