In quantum many-body systems with kinetically constrained dynamics, the Hilbert space can split into exponentially many disconnected subsectors, a phenomenon known as Hilbert-space fragmentation. These subsectors can be viewed as protecting classical information about the initial state. We show that the interplay of fragmentation with symmetries enables the protection of quantum information. Focusing on charge conserving systems with charge conjugation and translation symmetries, we show that the nontrivial action of these symmetries on the fragmented subsectors generates exponentially many logical qubits encoded in degenerate pairs of eigenstates, which can be highly entangled. This symmetry action also provides necessary conditions for experimental signatures of Hilbert-space fragmentation, such as the persistence of density imbalances at late times.

In this Letter, we use path integral techniques to predict the switching rate in a single-mode bistable open quantum system. While analytical expressions are well-known to be accessible for systems subject to Gaussian noise obeying classical detailed balance, we extend this approach to a class of open quantum systems, those which satisfy the recently introduced notion of hidden time-reversal symmetry [D. Roberts et al., PRX Quantum 2, 020336 (2021)]. In particular, in the context of quantum computing, we obtain analytical estimates of bit-flip error rates in cat-qubit architectures. We confirm these findings by comparing our results to numerically exact diagonalization of the Lindbladian. Our results provide a path towards exploring switching phenomena in multistable open quantum systems.

Continuous-variable quantum key distribution (CVQKD) enables remote users to share high-rate and unconditionally secure secret keys while maintaining compatibility with classical optical communication networks and effective resistance against background noise. However, CVQKD experiments have been demonstrated only indoors or over short outdoor distances. Here, by developing channel-fluctuation-independent high-precision manipulation of continuous-variable quantum states, high-accuracy quantum signal acquisition and processing, and high-efficiency free-space acquisition, tracking, and pointing technology, we overcome the excess noise due to atmospheric effects especially in daylight without extra wavelength conversion and narrow-linewidth spectral filtering and demonstrate for the first time long-distance free-space quantum key distribution under the asymptotic condition over 7-km inland and 9.6-km maritime atmospheric channels with Gaussian-modulated coherent states. Given that the CVQKD system is naturally compatible with existing ground fiber telecommunication networks, it marks an essential step for realizing integrated air-ground quantum access networks with cross-domain applications.

We demonstrate that secondary ions sputtered from a macrocapillary’s inner surface by the primary beam induce premature saturation of the guiding field, hindering fast ion guiding. By suppressing secondary ion sputtering with grooved surfaces, we achieve a guiding-field potential difference exceeding 1 kV, 2 orders of magnitude larger than previously achieved in stable ion guiding. This enables stable guiding of a $20\text{−}\mathrm{keV}/q$ ${\mathrm{O}}^{5+}$ beam at offset angles up to 15°, with transmitted ions retaining their initial energy and charge state. A self-consistent field model accurately reproduces these results, revealing that guiding performance relies on managing secondary ions and the channel’s voltage tolerance, while remaining robust against beam fluctuations. This Letter paves the way for ionic fibers that, much like optical fibers for light, passively and adaptively control ion transport.

We report on the study of itinerant magnetism of lattice-trapped magnetic atoms, driven by magnetic dipole-dipole interactions, in the low-entropy and close-to-unit filling regime. We have used advanced dynamical decoupling techniques to efficiently suppress the sensitivity to magnetic field fluctuations. We have thus measured the spin coherence of an itinerant spin 3 Bose dipolar gas throughout a quantum phase transition from a superfluid phase to a Mott insulating phase. In the superfluid phase, a metastable ferromagnetic behavior is observed below a dynamical instability that occurs at lattice depths below the phase transition. In the insulating phase, the thermalization toward a paramagnetic state is driven by an interplay between intersite and superexchange interactions.

Weyl orbits are topologically protected cyclotron trajectories that connect Weyl nodes of opposite chiralities in momentum space, coupling bulk chiral Landau levels and surface Fermi arcs to produce quantum oscillations under static magnetic fields. While well-studied in condensed matter systems, their photonic analogs remain elusive due to the difficulty in generating global pseudomagnetic fields. Here, we overcome these limitations by engineering a pseudomagnetic field domain wall, enabling a novel photonic cyclic orbit (PCO) bound to a single Weyl point, unlike conventional Weyl orbits that link paired WPs. In the PCO, photons traversing the domain wall experience pseudomagnetic field reversal and flipping of momentum-space motion along Fermi arcs, forming a closed trajectory through a single Weyl point. Experimentally, we observe a photonic analog of quantum oscillations, manifested as discrete photonic resonances, and PCO states in the real space, comprising Fermi-arc surface states and chiral zero modes, providing definitive evidence of photonic Weyl orbits. Our Letter establishes a new paradigm for topological cyclic orbits and opens practical pathways for three-dimensional light manipulation.

We show theoretically that an initially homogeneous planar Couette flow of a concentrated polymeric fluid is linearly unstable to the growth of two-dimensional (2D) perturbations, within two widely used constitutive models: the Johnson-Segalman model and the Rolie-Poly model. We perform 2D direct nonlinear simulations of both models to show that this instability leads to a state of elastic turbulence comprising several narrow shear bands that dynamically coalesce, split, and interact. Importantly, we show that this 2D instability arises not only in fluids that have a nonmonotonic constitutive curve and therefore show shear banding in 1D calculations, but also in shear thinning fluids with a monotonic constitutive curve, for which an initially homogeneous base state is stable to 1D perturbations. For the former category, the high shear branch of the constitutive curve is unstable to 2D instability in both models, so the high shear band may be turbulent. In the Rolie-Poly model, the low shear branch is also likewise unstable. Our Letter provides the first simulation evidence for elastic turbulence in highly entangled polymeric fluids. It also potentially explains rheo-chaotic states seen experimentally in shear banding wormlike micelles. We additionally demonstrate elastic turbulence within both models in the planar Poiseuille geometry.

A novel approach to constructing kinetic models is proposed on the basis of the separation of local and nonlocal contributions to particle interaction. The method results in a generic kinetic equation for complex fluid systems, amenable to efficient numerical realization. The main obstruction to causality caused by a coupling between the nonuniform temperature field and the nonideal equation of state is resolved based on gauge invariance of the hydrodynamic limit. A new lattice Boltzmann model is derived for compressible nonideal fluid simulation based on the Enskog-Vlasov kinetic theory as a case of practical importance.

Excitons underpin optical phenomena and can intertwine with correlated electronic states. Unlike bright excitons, dark excitons evade conventional optical detection, and their impact on quasiequilibrium electronic structures remains largely unexplored. Here, using angle-resolved photoemission spectroscopy, we report creating, detecting, and controlling dark excitons in the quasiequilibrium distribution of a doped semiconductor ${\mathrm{SnSe}}_2$. By tuning doping and temperature, we control the excitonic spectral weight and observe a concomitant anisotropic gapped state. Our results broaden the scope of dark excitons beyond ultrafast photoemission studies and highlight a correlation between dark excitons and correlated electrons under quasiequilibrium conditions.

Terahertz (THz) spectroscopy is a powerful probe of low-energy excitations in complex materials. Extending it into the nonlinear regime broadens its scope and can provide valuable insight into interactions among these modes. However, interpreting nonlinear spectra is challenging because resonant features in this case do not always reflect intrinsic material dynamics. Here, we study nonlinear THz-induced Kerr effect in a generic material ${\mathrm{LaAlO}}_3$. After detailed analysis of temporal oscillations of the Kerr signal, we identify an $E_g$ Raman mode at 1.1 THz excited through a two-photon process, while two additional peaks (0.86 and 0.36 THz) arise from phase matching of the near-infrared probe beam with co- and counterpropagating THz pump fields, mediated by off-resonant electronic hyperpolarizability. These results demonstrate the crucial role of kinematic effects in shaping THz-induced Kerr response and establish a framework for interpreting nonlinear spectroscopies in complex materials.

Second harmonic generation (SHG) has emerged as a powerful probe of fundamental geometric and topological attributes of the band structure of the Weyl semimetals (WSMs) through the photocurrent generation (shift, injection, and anomalous currents). Notably, circular photocurrents, such as injection and anomalous currents, are sensitive to the tilt of the Weyl cones, making circularly polarized light-induced SHG a tilt-inversion symmetry-specific probe of the Weyl topology. Here, we investigate the helicity-dependent nonlinear optical response in the type-2 Weyl semimetal ${\mathrm{WTe}}_2$ using equilibrium and ultrafast time-resolved SHG (TR-SHG). A large circular dichroism (CD) of 11% is observed in equilibrium SHG, attributed to the tilt of the Weyl cone. TR-SHG dynamics reveals a positive sign with biexponential decay that corresponds to the fast ($\sim 10\mathrm{ps}$) and long-lived ($\sim \mathrm{ns}$) components, along with an oscillation corresponding to the $A_1$ coherent optical phonon mode. Importantly, a giant helicity-dependent modulation ($\mathrm{CD}\sim 40\%$) is observed in the TR-SHG dynamics, corresponding to the asymmetric photocurrent generation in a tilted cone. A quantitative analysis of the helicity-dependent equilibrium and TR-SHG signals using quantum kinetic theory reveals that the injection current dominates the SHG response in an overtilted Weyl cone. Most importantly, control experiments on TaAs (type-1 WSM) show no helicity dependence, confirming that the observed effect in ${\mathrm{WTe}}_2$ is intrinsic to the overtilted Weyl cones. Altogether, these findings not only underscore the power of SHG to monitor photoinduced evolution of symmetry and topology in real time, but also lay the groundwork for helicity-controlled, ultrafast optoelectronic applications that harness topological band structures.

Interwoven frustrated parametric down-conversion (PDC) processes produce interference effects based on path identity [Krenn et al., Entanglement by path identity, Phys. Rev. Lett. 118, 080401 (2017)]. In this Letter we show Bell nonclassicality of the processes: a proper violation of the Clauser-Horne inequality when the local measurements are controlled by on-off switching of the final local PDC processes. Such a nonstandard approach is needed because if only the local phase shifts are used for the measurement settings, as done in the experiment reported in Wang et al. [Violation of Bell inequality with unentangled photons, Sci. Adv. 11, eadr1794 (2025)], there exists a local realistic model of the interference, which we present in this Letter. Nevertheless, the reported destructive interference is deep enough to violate the inequality when using on-off switching, so our result establishes a firm footing for nonclassicality of the new interferometry, in both theory and experiment. The on-off approach to Bell analysis of path-identity-based interference forms a new platform for seeking new highly counterintuitive quantum phenomena.

$CPT$ symmetry is at the heart of the standard model of particle physics and experimentally very well tested, but expected to be broken in some approaches to quantum gravity. It thus becomes pertinent to explore which of the two alternatives is realized: (i) $CPT$ symmetry is emergent, so that it is restored in the low-energy theory, even if it is broken beyond the Planck scale, (ii) $CPT$ symmetry cannot be emergent and must be fundamental, so that any approach to quantum gravity, in which $CPT$ is broken, is ruled out. We explore this by calculating the renormalization group flow of $CPT$-violating interactions under the impact of quantum fluctuations of the metric. We find that $CPT$ symmetry cannot be emergent and conclude that quantum-gravity approaches must avoid the breaking of $CPT$ symmetry. As a specific example, we discover that in asymptotically safe quantum gravity $CPT$ symmetry remains intact, if it is imposed as a fundamental symmetry, but it is badly broken at low energies if a tiny amount of $CPT$ violation is present in the transplanckian regime.

We study the impact of an unshielded dielectric—here, a bare optical fiber—on a ${}^{40}{\mathrm{Ca}}^{+}$ ion held several hundred microns away in a cryogenic surface electrode trap. We observe distance-dependent stray electric fields of up to a few $\mathrm{kV}/\mathrm{m}$ due to the dielectric, which drift on average less than 10% per month and can be fully compensated with reasonable voltages on the trap electrodes. We observe ion motional heating rates attributable to the dielectric of $\approx 30$ quanta per second at an ion-fiber distance of $215(4)\mathrm{\mu }\mathrm{m}$ and $\approx 1.5\mathrm{MHz}$ motional frequency. These results demonstrate the viability of using unshielded, trap-integrated dielectric objects such as miniature optical cavities or other optical elements in cryogenic surface electrode ion traps.

Bulk material properties such as strength and stiffness are highly sensitive to changes in the degree of interparticle cohesion or the introduction of elongated particles, effects that are both associated with an increase in the average number of constraints per particle. By performing an ensemble of isotropic compression experiments, all starting from the same initial particle configuration but with varying fraction of bonded particles, we use photoelastic force measurements to identify the causes of this phenomenon at the particle scale and mesoscale. By introducing a variable percentage of bonded dimers, we observe the expected decrease in the critical packing fraction at which jamming occurs. Above jamming, the local pressure increases predominantly for the bonded particles, measured relative to the unbonded case. The use of bonded dimers allows us to directly measure the relative contributions of tensile and compressive forces at the bond, which we observe occur with approximately equal probability. Histograms of the magnitude of the interparticle forces become broader for systems with more bonded particles. We measure both pressure and coordination number as a function of distance from a bonded dimer, both of which are locally enhanced for nearest neighbors, indicating that dimers appear to act as areas of concentrated force and connectivity that improve rigidity.

Superposed coherent states are central to quantum technologies, yet their reliable identification remains a challenge, especially in noisy or resource-constrained settings. We introduce a novel, directly measurable criterion for detecting catlike features in quantum states. The criterion is based on the concept of nonlinear squeezing and bypasses the need for full state tomography of the tested states. The numerical results confirm its robustness under loss and its potential for experimental implementation. The method naturally generalizes to more exotic superpositions, including multiheaded cat states.

One of the manifestations of the quantum Mpemba effect (QME) is that a tilted ferromagnet exhibits faster restoration of the spin-rotational symmetry after a quantum quench when starting from a larger tilt angle. This phenomenon has recently been observed experimentally in an ion trap that simulates a long-range spin chain. However, the underlying mechanism of the QME in the presence of long-range interactions remains unclear. Using the time-dependent spin-wave theory, we investigate the dynamical restoration of the spin-rotational symmetry and the QME in generic long-range spin systems. We show that quantum fluctuations of the magnetization drive the restoration of symmetry by melting the initial ferromagnetic order and are responsible for the QME. We find that this effect occurs across a wide parameter range in long-range systems, in contrast to its absence in some short-range counterparts.