The realization of scalable quantum networks for distribution of entanglement over long distances hinges on quantum repeaters. To outperform the exponential transmission loss in optical fibers, quantum repeaters must employ multiplexing schemes in the temporal, spectral, or spatial domain. The performance of such a multiplexed scheme is contingent on efficient quantum memories offering both extended storage times and large multimode capacities. Toward this goal, we experimentally demonstrate a memory platform operating at telecom wavelength using an ${\mathrm{Er}}^{3+}:{\mathrm{Y}}_2{\mathrm{SiO}}_5$ crystal. We record on-demand storage and recall of weak coherent pulses for up to 1 ms, exceeding by an order of magnitude that of previously reported demonstrations based on ${\mathrm{Er}}^{3+}$. The memory exhibits an efficiency of 10.36% at $300\mathrm{\mu }\mathrm{s}$ storage time with a signal-to-noise ratio of 10.9, mainly limited by spontaneous emission noise. We also store two spectral modes representing a frequency-bin encoding and retrieve the state with an overlap of 81.8%. Furthermore, we demonstrate multimode capacity by simultaneously storing 20 temporal and 3 spectral modes with on-demand and selective recall capabilities, serving as a promising step toward a scalable quantum repeater architecture.

We solve Dicke superradiance with two or more competing collective decay channels of tunable rates using a symbolic quantum-trajectory construction. The method yields closed time-domain populations and observables as finite combinations of exponential terms for arbitrary numbers of emitters and arbitrary decay rates. For two channels, the behavior of the stationary ground-state distribution resembles a first-order phase transition at the point where the channel-rate ratio is equal to unity. For balanced $d$-channel decay, we obtain scaling laws for the superradiant peak time and intensity. These results unify and extend single-channel Dicke dynamics to multilevel emitters and provide a compact tool for cavity and waveguide experiments, where permutation-symmetric reservoirs engineer multiple collective decay paths.

In charge neutral fluids, such as the Dirac fluid in graphene at the Dirac point, charge transport remains diffusive despite the presence of ballistically propagating sound waves: sound waves “hydrodynamically decouple” from the slower charge fluctuations. For quasi-one-dimensional charge neutral fluids, we show that this convective charge diffusion is not smoothly connected to the normal diffusion that arises when momentum conservation is broken by noise (or static impurities). Instead, the charge diffusion constant is a discontinuous function of noise, which (in the weak-noise limit) depends only on the ratio of momentum and energy relaxation rates. In the special limit of momentum-conserving noise (e.g., spatially uniform fluctuations of the Hamiltonian), the diffusion constant diverges in the presence of noise. We describe the resulting superdiffusion in terms of coupled Burgers equations. We present a general mechanism—hydrodynamic recoupling—by which weak noise can induce singular changes in transport coefficients. Our results highlight the limits of zero-noise extrapolation for predicting dynamical quantities like diffusion constants.

The ultrastrong coupling regime of cavity photons and quantum materials has emerged as a pathway to modify materials properties. However, definitive signatures of ultrastrong coupling remain elusive. Focusing on the quantum photon statistics of light transmitted through a cavity-embedded superconductor, we show that a two-photon Higgs polariton at strong coupling realizes a photonic nonlinearity at the single terahertz photon level. We find that as light-matter coupling increases, the photon statistics show pronounced changes due to the formation of a hybrid photon-matter dark-cavity state with finite photon occupancy, producing testable signatures of ultrastrong coupling. We derive a non-Markovian input output relation and study the cavity-embedded superconductor $2{\mathrm{H}\text{−}\mathrm{NbSe}}_2$ as it approaches ultrastrong light-matter coupling. Our results reveal a diagnostic for ultrastrong coupling in the two-photon coincidence statistics that is absent in total counts.

We address the general problem of formulating the dynamical large deviations of non-Markovian systems in a closed form. Specifically, we consider a broad class of “self-interacting” jump processes whose dynamics depends on the past through a functional of a state-dependent empirical observable. Exploiting a natural separation of timescales, and under the corresponding mixing conditions, we obtain the exact (so-called “level 2.5”) large deviation joint statistics of the empirical measure over configurations and of the empirical flux of transitions. As an application of this general framework, we derive explicit general bounds on the fluctuations of trajectory observables, generalizing to the non-Markovian case both thermodynamic and kinetic uncertainty relations. We illustrate our theory with simple examples, and discuss potential applications of these results.

Speeding up the relaxation dynamics of many-body quantum systems is important in a variety of contexts, including quantum computation and state preparation. We demonstrate that such acceleration can be universally achieved via transient stochastic resetting. This means that during an initial time interval of finite duration, the dynamics is interrupted by resets that take the system to a designated state at randomly selected times. We illustrate this idea for few-body open systems and also for a challenging many-body case, where a first-order phase transition leads to a divergence of relaxation time. In all scenarios, a significant and sometimes even exponential acceleration in reaching the stationary state is observed, similar to the so-called Mpemba effect. The universal nature of this speedup lies in the fact that the design of the resetting protocol only requires knowledge of a few macroscopic properties of the target state, such as the order parameter of the phase transition, while it does not necessitate any fine-tuned manipulation of the initial state.

While complex-pole hierarchical equations of motion (HEOM) have dramatically expanded the reach of numerically exact quantum dynamics simulations of open quantum systems, they suffer from numerical instabilities rooted in the non-Hermitian structure of their Liouvillian. Yet, the origin of this structure remains obscure. Here, we report a previously unknown gauge freedom in complex-pole HEOM: a continuous family of analytically equivalent Liouvillians, all encoding the same bath correlation function, whose numerical properties vary dramatically. This gauge controls both the eigenspectrum and non-normality of the hierarchy generator, revealing spectral divergence and non-normal error amplification as two distinct instability mechanisms. By optimizing this gauge, we introduce GO-HEOM, which eliminates divergences in strongly coupled Brownian oscillator environments and extends numerically exact simulations of sub-Ohmic dynamics—including through the delocalized-to-localized quantum phase transition—to coupling strengths inaccessible with the conventional HEOM. Because this gauge transformation is independent of the bath-correlation decomposition scheme, our GO-HEOM becomes a general, broadly compatible strategy for accessing numerically exact quantum dynamics of open quantum systems over arbitrary coupling and highly non-Markovian regimes.

Axions are a compelling dark matter candidate suggested by theories beyond the standard model. The predicted axion-electron coupling and the resulting pseudomagnetic field allow for laboratory axion searches. Here we report an experimental axion search via a superconductor-spin hybrid magnetometer. A state-of-the-art broadband magnetometer with a sensitivity of sub-$100\mathrm{a}\mathrm{T}/{\mathrm{Hz}}^{1/2}$ at tens of kHz has been realized by coupling a ferrite spin sensor to a readout system based on a superconducting quantum interference device. The most stringent terrestrial constraints on the axion-electron coupling have been established in the frequency range $5\sim 60\mathrm{kHz}$. In the future, an improvement on the constraints of more than 3 orders of magnitude is foreseeable through arrays of superconductor-spin hybrid magnetometers with optimized designs. This Letter demonstrates the potential of superconductor-spin hybrid magnetometers in high-precision magnetic field sensing and the study of fundamental physics.

We analyze the two-dimensional Palatini Gauss-Bonnet theory on an infinite strip (product of a finite interval with the infinite line, corresponding to “time”). The theory has only boundary degrees of freedom. Its phase space is the cotangent bundle to the group manifold of $SL(2,\mathbf{R})$, subject to a (first-class) constraint quadratic in the momenta. With the simplest choice of boundary Hamiltonian, namely $H=0$, the theory is shown to describe geodesics on the group manifold of $SL(2,\mathbf{R})$, with a “mass” determined by the Palatini Gauss-Bonnet coupling constant. Other choices of boundary Hamiltonians compatible with gauge invariance are also possible. The symmetry group contains (left and right) group translations on $SL(2,\mathbf{R})$. These are “boundary symmetries” from the bulk point of view, with one copy acting on one end of the interval and the other copy acting on the other end. Comments on the quantum theory are also given.

Quantum simulation offers a powerful approach to studying quantum field theories, particularly $(2+1)\mathrm{D}$ quantum electrodynamics (${\mathrm{QED}}_3$) with Wilson fermions, which hosts a rich landscape of physical phenomena. A key challenge in lattice formulations is the proper realization of topological phases and the Chern-Simons terms, where fermion discretization plays a crucial role. In this Letter, we highlight the differences between staggered and Wilson fermions coupled to U(1) gauge fields in the Hamiltonian formulation. We analyze why staggered fermions fail to induce $(2+1)\mathrm{D}$ topological phases, while Wilson fermions admit a variety of topological phases including Chern insulator and quantum spin Hall phases. Additionally, we uncover a rich phase diagram for the two-flavor Wilson fermion model in the presence of a chemical potential. Our findings resolve existing ambiguities in Hamiltonian formulations and provide a theoretical foundation for future quantum simulations of lattice field theories with topological phases. We further outline connections to experimental platforms, offering guidance for implementations on near-term quantum computing architectures. A complementary presentation of the analytical calculations, the identification of robust topological structure and response, and extensive numerical results is contained in a joint submission [S. Bharadwaj et al., Phys. Rev. D 114, 094502 (2026)].

We calculate the dimension-six $\mathrm{\Delta }Q=0$ four-quark matrix elements describing heavy-meson lifetime ratios using the gradient flow with its short flow-time expansion (SFTX) as a renormalization procedure. On six RBC/UKQCD $2+1$-flavor domain-wall fermion ensembles, we determine flowed bag parameters for physical charm and strange quarks and match to the $\overline{\mathrm{MS}}$ scheme with perturbative SFTX coefficients through next-to-next-to-leading order (NNLO). A multiscale matching procedure using renormalization-group running improves the extrapolation to zero flow time. For the operators relevant to $\tau (D_s)/\tau (D^0)$ at the ${\mathrm{SU}(3)}_{\mathrm{F}}$ symmetric point, we obtain $B_1^{\overline{\mathrm{MS}}}(3\mathrm{GeV})=1.0524(97)$, $B_2^{\overline{\mathrm{MS}}}(3\mathrm{GeV})=0.9621(70)$, ${\epsilon }_1^{\overline{\mathrm{MS}}}(3\mathrm{GeV})=-0.2275(76)$, and ${\epsilon }_2^{\overline{\mathrm{MS}}}(3\mathrm{GeV})=-0.0005(8)$ using a specific choice of evanescent operators. This is the first lattice-QCD determination of $\mathrm{\Delta }Q=0$ four-quark operators with a full error budget. It opens the path toward higher-precision predictions of heavy-meson lifetimes and similar quantities exhibiting operator mixing under renormalization.

We find that heavy quark transport beyond leading logarithm at weak coupling is intrinsically non-Gaussian: the longitudinal momentum transfer distribution has asymmetric exponential tails that are crucial for equilibration dynamics. We show this by computing the leading-order momentum transfer kernel for relativistic heavy quarks in weakly coupled non-Abelian plasmas, matching perturbative momentum transfer on the thermal scale to hard-thermal-loop-resummed soft physics. This is the same structure previously found in strongly coupled holographic plasmas, showing that it is not peculiar to weak or strong coupling, conformality, or supersymmetry. We therefore expect that this is a robust feature that physical quark-gluon plasma should also exhibit.

We propose a classification scheme for rotational bands built on nuclear scissors vibrations, based on a systematic analysis of the competition among distinct collective modes. Our analysis within the proton-neutron interacting boson model reveals that scissors-mode bands exhibit robust odd-even staggering in excitation energies, leading to multiple energy-splitting patterns in scissors rotations. Remarkably, despite their distinct origins, these splitting patterns share a common spectroscopic fingerprint: pronounced clustering between the two lowest band members with spin-parity $1^{+}$ and $2^{+}$. This behavior naturally accounts for the puzzlingly small separation between the two levels observed in ${}^{156}\mathrm{Gd}$ and establishes a general mechanism applicable to axially asymmetric nuclei and, potentially, to other two-fluid systems exhibiting scissors-mode collectivity. We also present theoretical predictions for the systematic evolution of this splitting across the deformed Gd isotopic chain, further corroborating the proposed mechanism.

We report ultrafast dissociation of gas-phase water following photoexcitation above the O 1s ionization threshold. High-resolution resonant Auger spectroscopy reveals sharp constant-kinetic-energy features arising from nonradiative decay of neutral atomic oxygen formed within the O 1s core-hole lifetime. Comparison with ab initio Auger spectra identifies these features as transitions from the atomic $(1\mathrm{s}{)}^1(2\mathrm{s}{)}^2(2\mathrm{p}{)}^5$ ${}^1\mathrm{P}$ state, demonstrating that neutral fragments can form in well-defined electronic states before Auger decay.

We study the superradiant phase transition of a mesoscopic Fermi gas comprising between a few tens and a few thousand ${}^6\mathrm{Li}$ atoms in a high-finesse cavity across a wide range of densities. We observe a nonmonotonic variation of the superradiant threshold as a function of density, with a minimum reached when the Fermi and recoil wave vectors are comparable. The minimum corresponds to a crossover between Fermi pressure-assisted ordering and Pauli blocking of photon scattering, in good agreement with theory. This interpretation is confirmed by a study of the atom-number dependence of the ordering threshold and photon number scaling. Our system opens the perspective of studying few-fermion systems with strong and coherent light-matter coupling.