A measurement of the ratio of semileptonic branching fractions $\mathcal{R}(J/\psi )$, defined as $\mathcal{R}(J/\psi )\equiv \mathcal{B}(B_c^{+}\to J/\psi {\tau }^{+}{\nu }_{\tau })/\mathcal{B}(B_c^{+}\to J/\psi {\mu }^{+}{\nu }_{\mu })$, is reported using a sample of proton-proton collision data corresponding to an integrated luminosity of $5.4{\mathrm{fb}}^{-1}$ recorded by the LHCb experiment in 2016–2018 at a center-of-mass energy of 13 TeV. The measured value is found to be $\mathcal{R}(J/\psi )=0.51\pm 0.12(\mathrm{stat})\pm 0.08(\mathrm{syst})$, which is within 1.8 standard deviations of the predictions from the standard model assuming lepton flavor universality.

Cooperative emission is well understood for idealized symmetric systems, but its fundamental limits in spatially extended, free-space ensembles remain an open question. Here, we derive a universal law for the scaling of the maximum photon emission rate with system size that unifies both ordered arrays and disordered atomic clouds in arbitrary dimensions at fixed density. We demonstrate that, for a fixed atomic density, the maximum emission rate scales universally as the product of the atom number and the system’s geometric optical depth. The geometric optical depth differs from the result of the Beer-Lambert law at high density and encodes the dimensional scaling across all regimes from independent emission to the Dicke limit. Furthermore, we establish a scaling law for directional detection, revealing that the observed rate depends on the detector’s numerical aperture: small apertures yield Dicke-like quadratic scaling, whereas large apertures recover our integrated universal bound. Our results establish optical depth as the parameter governing many-body cooperative emission in both ordered and disordered ensembles, and reveal that directional and total-emission scalings must be carefully distinguished in experimental settings.

Unconventional superconductivity presents a defining challenge in physics. Prevailing theoretical frameworks have predominantly emphasized electrons, largely neglecting the rich physics inherent in the lattice. Conventional phonon theory omits quantum many-body effects of the nuclei, leading to misleading structural phase diagrams and an unsound foundation for superconducting theory. Here, by incorporating nuclear quantum many-body effects within first-principles calculations, we discover a lattice quantum disordered phase in superconductors ${\mathrm{H}}_3\mathrm{S}$ and ${\mathrm{L}\mathrm{a}}_3{\mathrm{N}\mathrm{i}}_2{\mathrm{O}}_7$. This phase occupies a triangular region in the $P\text{−}T$ phase diagram, whose left boundary aligns precisely with $T_c$ of the left flank of the superconducting dome. Its $T_c^{\max }$ coincides with the dome’s peak, identifying this phase as both the origin of superconducting transition on the left flank and a key ingredient of the pairing mechanism. Our findings advance the understanding of unconventional superconductivity and establish lattice quantum disorder as a unifying framework, both for predicting new superconductors and for elucidating phenomena in a broader context of condensed matter physics.

We perform large-scale density matrix renormalization group simulations of the SU(4) Heisenberg model on the honeycomb lattice to address the long-standing question of its ground state in an unbiased and quantitatively controlled manner. We find reliable numerical evidence that the ground state is a gapped spin-orbital liquid, presumably with a $Z_4$ topological order, characterized by a finite topological entanglement entropy close to ln(4), the absence of both SU(4) and lattice symmetry breaking, and a variationally optimized ground-state energy well below the previously proposed $\pi$-flux variational state. By exploiting full SU(4) symmetry and keeping up to 12 800 SU(4) multiplets, corresponding to more than $1\times {10}^6$ U(1) states, we achieve unprecedented accuracy for two-dimensional SU(4) quantum magnets. Finite-size scaling of energies and entanglement entropies supports a robust gapped phase in the two-dimensional limit, while a gapless critical state on narrow cylinders is identified as a proximate remnant of a Dirac spin-orbital liquid. Our results find the SU(4) honeycomb Heisenberg model a realization of a gapped topological spin-orbital liquid and provide convincing numerical evidence for topological order in a highly symmetric two-dimensional quantum magnet.

The all-electrical realization of highly spin-polarized currents and their efficient conversion into pure spin currents remains a fundamental challenge in spintronics. Here, we report a spin-axis dynamic locking (SADL) effect in altermagnets that pins the high and dynamically robust spin polarization to the crystalline axes: an in-plane electric field along one principal axis drives a highly spin-up-polarized current, whereas along the orthogonal axis, it generates a symmetry-enforced, equal-magnitude spin-down current. Consequently, applying an electric field diagonally yields a transverse pure spin current, reaching 100% charge-to-spin conversion in the ideal limit. Mechanistically, SADL originates from a spin-split tent-state band structure whose Lifshitz transitions delimit an open-Fermi-line regime. The momentum-separated Fermi lines carry orthogonal nonzero winding vectors, producing a pronounced velocity contrast while suppressing ordinary backscattering to dynamically stabilize the axial spin selectivity. High-throughput first-principles screening confirms SADL in broad materials. Notably, monolayer ${\mathrm{Cr}}_2{\mathrm{WSe}}_4$ and synthesized bulk ${(\text{BaF})}_2{\mathrm{Mn}}_2{\mathrm{Se}}_2\mathrm{O}$ exhibit efficiencies close to the ideal limit, paving the way for ultra-low-power, reconfigurable spintronic devices where the spin states are governed solely by electric field orientation.

Ultrafast modulation of electron-phonon couplings via laser excitation has been experimentally demonstrated, yet a clear microscopic understanding remains elusive. Here, we establish the first ab initio framework combining time-dependent density-functional theory, density-functional perturbation theory, and Migdal-Eliashberg theory to track electron-phonon couplings and superconductivity in prototypical phonon-mediated superconductor ${\mathrm{MgB}}_2$ under laser illumination. We find that near-infrared laser pulses induce charge transfer from boron $\sigma$ to $\pi$ orbitals, effectively renormalizing the electron-phonon couplings and boosting the superconducting critical temperature to $\sim 52\mathrm{K}$. The simulated thermalized superconducting gaps match well with experimental measurements. We propose a generalizable route for ultrafast modulation of superconductivity in materials whose dominant electron-phonon coupling channel can be selectively addressed by light, with photoinduced changes in electronic coupling and screening playing a central role. This Letter offers fundamental insight into nonequilibrium optical control of electron-phonon interactions and superconductivity, and provides a predictive tool for studying photo-modulated electron-phonon coupling and macroscopically observable superconducting properties.

We study the time it takes for all states of a finite quantum system to return simultaneously to their original configuration. In particular, we define the recurrence time for a quantum system to be the time at which all time-evolved states are close to their initial configuration, and at least one state has deviated significantly during this interval. Considering finite-dimensional quantum systems evolving unitarily, we find bounds on this notion of recurrence time, for continuous time and discrete time, by using Dirichlet’s approximation theorem. We show how the problem of finding a bound on recurrence time can be related to approximating the difference of real numbers by rationals. We present a mathematical result on the latter, which we then use to obtain tighter bounds on recurrence time.

Quantum transport on structured networks is strongly influenced by interference effects, which can dramatically modify how information propagates through a system. We develop a quantum-information-theoretic framework for scattering on graphs in which a full network of connected scattering sites is treated as a quantum channel linking designated input and output ports. Using the Redheffer star product to construct global scattering matrices from local ones, we identify resonant concatenation, a nonlinear composition rule generated by internal back reflections. In contrast to ordinary channel concatenation, resonant concatenation can suppress noise and even produce superactivation of the quantum capacity, yielding positive capacity in configurations where each constituent channel individually has zero capacity. We illustrate these effects through models exhibiting resonant-tunneling-enhanced transport. Our approach provides a general methodology for analyzing coherent information flow in quantum graphs, with relevance for quantum communication, control, and simulation in structured environments.

Fully connected quantum networks enable scalable quantum communication, yet reliable entanglement characterization without trusting measurement devices remains challenging. Here we experimentally demonstrate measurement-device-independent (MDI) entanglement verification and quantification in a time-bin–encoded fully connected quantum network. Using a broadband periodically poled lithium niobate on insulator source combined with dense wavelength-division multiplexing, we distribute all six pairwise entangled links among four users over an end-to-end separation of 20 km. MDI measurements are realized by encoding the trusted input states in the polarization degree of freedom of the same photons carrying the distributed time-bin entanglement, thereby eliminating the need for independently prepared trusted photons. We show that both entanglement verification and quantification are obtained from the MDI measurements. Our results demonstrate the practical integration of resource-efficient MDI characterization into a fully connected time-bin quantum network, providing a scalable approach for reliable entanglement characterization in large-scale quantum networks.

Measurement-device-independent quantum key distribution (MDI-QKD) is a promising path toward practical quantum communication, but current high-performance demonstrations come at prohibitive overhead and isolated environments. Here, we present a high-speed MDI-QKD system operating at an unprecedented repetition frequency of 2.5 GHz and verify its strong noise tolerance by incorporating the advantage distillation method. The secure key is efficiently generated under 425 kHz noise counts, which is 4 orders of magnitude higher than isolated systems. A secure key rate with a kbps level is achieved under 30 dB channel attenuation without bulky, resource-intensive detection devices. Our Letter establishes a complete practical solution for QKD deployment and resolves the persistent difficulty of realizing practical quantum secure networks with advanced security, a high key rate, environmental robustness, and physical compactness.

Entanglement is a key resource in quantum information science, yet its properties and applications in nuclear systems remain largely unexplored. Here, using proton-proton scattering as a quantum laboratory, we report the emergence of a near-pure Bell-triplet state at a laboratory energy of 151 MeV and a center-of-mass scattering angle of 90°. In this unique kinematic regime, the scattering amplitude functions as a transition operator connecting distinct Bell states. Building upon this emergent resource, we propose a quantum teleportation protocol for proton spins, exploiting the intrinsic Hamiltonian of the strong interaction to perform the requisite Bell measurement effectively. These findings effectively bridge few-body nuclear physics and quantum technology, establishing proton-proton scattering as both a source of high-fidelity entanglement and a natural processor for quantum information.

Interactions derived from effective field theories of quantum chromodynamics have thus far failed to bind calcium nuclei beyond neutron number $N=40$, while nuclear density functionals typically place the neutron drip line near ${}^{70}\mathrm{Ca}$, at $N=50$. We present the chiral interaction ${\mathrm{N}}^3{\mathrm{LO}}_{\mathrm{Texas}}$, a combination of two- and three-nucleon potentials at fourth and third chiral order, respectively, with low-energy constants optimized using emulator-accelerated fits to few- and many-body data. This interaction accurately reproduces binding energies and charge radii of key nuclei with mass number $A=3$ to 208, important excited states, and nuclear matter near saturation. Using ab initio methods, coupled cluster and valence-space in-medium similarity renormalization group, we find that the calcium two-neutron drip line extends to ${}^{71}\mathrm{Ca}$.

Yield-stress fluids only flow above a critical stress. Here, we observe the emergence of a distinct, elasticity-driven flow state in a yield-stress fluid in the absence of inertia. Numerical simulations show that this elastoplastic turbulent state is characterized by non-Gaussian bimodal velocity distribution and a broad spectrum of velocity and stress fluctuations. Results show a nonmonotonic relationship between the volume fraction of the unyielded flow and plasticity. Surprisingly, the system can fluidize near the emergence of the elastoplastic turbulent state. Our results reveal the complex interplay between elasticity and plasticity in simple shear flows, indicating that plasticity can mobilize the flow rather than arrest it. These findings are relevant for the flow and stability of muds, clays, and soft materials in general.

In compound semiconductors and insulators, the polar electron-phonon coupling diverges at long range, known as the Fröhlich interaction. Modern first-principles electron-phonon calculations treat the Fröhlich interaction in a semiclassical electrostatic formalism based on density-functional perturbation theory. Here, using many-body $GW$ perturbation theory, we reveal important electron correlation effects in the Fröhlich-type electron-phonon coupling, which are missed by the prevailing approaches and rooted in the fundamentally distinct behavior between quasiparticle self-energy and semilocal exchange-correlation functionals in the long-range limit. Going beyond the electrostatic treatment, we derive and implement the $GW$ self-energy contribution to the long-range polar electron-phonon coupling, and demonstrate its critical role and nontrivial behaviors in properties such as electron linewidth and polaron formation in several prototype semiconductors. Remarkably, calculations on photoemission kink in electron-doped ${\mathrm{TiO}}_2$ achieve excellent agreement with experiment. Our work establishes the many-body generalization of the Fröhlich interaction that is essential for accurate electron-phonon calculations at the full $GW$ level combined with Wannier interpolation techniques.

Bulk–boundary correspondence guarantees the presence of robust, anomalous states on the boundary of topological matter. The edges of a two-dimensional Chern insulator harbor one-dimensional chiral states, which have a conductance $n{e}^2/h$, where $n$ is an integer that is solely determined by the bulk. In this Letter we show that step edges on the surface of three-dimensional topological metals have a robust conductance $K{e}^2/h$, where $K$ is also fixed by the bulk and assumes noninteger values. We explain this prediction on the basis of the topology of gapless systems, exemplify it on a lattice model, and connect to recent experimental observations of enhanced density of states at step edges in topological metals.

The Landauer principle establishes a fundamental lower bound on the energetic cost of the erasure for a one-bit memory in thermal equilibrium. Here, we experimentally demonstrate how this bound can be shifted by introducing a hysteretic bias $h$ in the feedback-generated virtual potential of a microresonator acting as the information bit. By tuning the hysteresis, we engineer a nonequilibrium steady state with an adjustable effective temperature different from the bath temperature $T_0$, enabling erasure processes that consume over 20% below the Landauer bound $k_B{T}_0\ln 2$. Unlike sub-Landauer protocols that rely on a deliberately prepared nonequilibrium initial state, the cost reduction observed here is generated during reset by an embedded feedback bias acting as an information engine. The same platform reproduces the standard quasistatic result at $h=0$ and shifts the asymptotic erasure cost above or below it for $h\ne 0$. Our results provide a clean experimental playground for theoretical descriptions based on feedback, information flow, hidden controller states, and generalized nonequilibrium Landauer relations.

Biological systems rely on autonomous modes of charge transport to transmit signals. Instead, conventional synthetic systems typically depend on external fields, such as voltage or pressure gradients, to induce transport, which limits their applicability. Here, we investigate nanochannels in which an electrolyte is confined by symmetric boundary patterns combining surface charge and active ionic fluxes. We show that the interplay between diffusion, electrostatics, and hydrodynamics in such nano-confined active-charged systems can trigger symmetry breaking above a critical active flux, leading to directed flow. Our results suggest that active-charged nanochannels can generate net flows of the order of hundreds of millimeters per second, opening pathways toward adaptable ionic devices and neuromorphic architectures.