We experimentally demonstrate background-free, hyperfine-level-selective measurements of individual Cs atoms by simultaneous cooling to $5.3\mathrm{\mu }\mathrm{K}$ and imaging on the $6s_{1/2}\to 5d_{5/2}$ electric-quadrupole transition. We achieve hyperfine-resolved detection with fidelity 0.9993(4) and atom retention of 0.9954(5), limited primarily by vacuum lifetime. Performing state measurements in a 3D cooling configuration enables repeated low loss measurements. A theoretical analysis of an extension of the demonstrated approach, based on quenching of the excited state with an auxiliary field, identifies parameters for hyperfine-resolved measurements with a projected fidelity of $\sim 0.9995$ in $\sim 60\mathrm{\mu }\mathrm{s}$.

We introduce the concept of giant superatoms (GSAs), where two or more interacting atoms are nonlocally coupled to a waveguide through one of them, and explore their unconventional quantum dynamics. For braided GSAs, this setup enables decoherence-free transfer and swapping of their internal entangled states. For separate GSAs, engineering coupling phases leads to state-dependent chiral emission, which enables selective, directional quantum information transfer. This mechanism further facilitates remote generation of $W$-class entangled states. Our results thereby open exciting possibilities for quantum networks and quantum information processing.

Entangled photons carry nontrivial quantum correlations that defy classical physics and provide new tools for monitoring quantum dynamics in molecules. The use of low-flux entangled photons in molecular spectroscopy has been proposed in the past to probe excited-state dynamics with enhanced temporal and spectral resolutions. However, these techniques have been challenging to implement experimentally since the resulting signal has a low signal-to-noise ratio. The signal strength can be boosted by using intense entangled-photon beams, where multiple pairs of entangled photons are present in the environment. This allows the molecule under study to interact with both entangled and unentangled photons, where the classical component of the spectroscopic signal stemming from the latter can mask the spectral features associated with the former. In this Letter, we show how the signal arising from unentangled photons can be eliminated when intense entangled-photon beams are used in two-dimensional electronic spectroscopy (2DES), and demonstrate the advantage of photonic entanglement.

Metal hydrides $M{\mathrm{H}}_x(x\le 2$) with low hydrogen content are not expected to show high-$T_{\mathrm{c}}$ superconductivity owing to the low hydrogen-derived electronic density of states at Fermi level and the limited hydrogen contribution to electron-phonon coupling strength. In this Letter, we report on the successful synthesis of a novel bismuth dihydride superconductor, Cmcm-${\text{BiH}}_2$, at approximately 150 GPa, and the discovery of superconductivity with $T_{\mathrm{c}}$ about 62 K at 163 GPa, marking the first instance of superconductor among the $M{\mathrm{H}}_2$-type metal dihydrides. Cmcm-${\text{BiH}}_2$ adopts a unique host-guest type structure, in which the Bi atoms via weak Bi-Bi covalent bonds form a three-dimensional open-channel framework that encapsulates ${\mathrm{H}}_2$-like molecules as guests, thereby broadening the structural diversity of hydrides under high pressures. The occurrence of superconductivity is evidenced by a sharp drop of resistivity to zero and the characteristic downward shift of $T_{\mathrm{c}}$ under applied magnetic fields. Notably, Cmcm-${\text{BiH}}_2$ remains stable down to at least 97 GPa during decompression, with the calculated lowest pressure for dynamic stability of 10 GPa. In-depth analysis reveals that the covalent bismuth open-channel structure forms metallic conduction channels, dominates the electronic states near the Fermi level, and contributes approximately 51% of the total $\lambda$ in Cmcm-${\text{BiH}}_2$, distinguishing it from known high-pressure hydride superconductors. These findings highlight the critical role of nonhydrogen elements in producing superconductivity and open new avenues for the design and optimization of high-$T_{\mathrm{c}}$ hydride superconductors.

The excited orbitals of color centers often show strong electric dipoles, which can serve as a resource for entanglement, emission tuning, or electric field sensing. Here, we use resonant laser excitation to examine the electric transitions in the excited state (ES) orbitals of the negatively charged nitrogen vacancy center in diamond. By applying microwave electric fields, we perform Rabi driving between ES orbitals, and show that the dressed states can be tuned in frequency and are protected against fluctuations of the transverse electric field. In contrast with previous results, we observe sharp microwave resonances between magnetic states of the ES orbitals, and find that they are broadened due to simultaneous electric dipole driving.

One of the challenges in diagrammatic simulations of nonequilibrium phenomena in lattice models is the large memory demand for storing momentum-dependent two-time correlation functions. This problem can be overcome with the recently introduced quantics tensor train (QTT) representation of multivariable functions. Here, we demonstrate nonequilibrium Green’s function simulations within the $GW$ and Migdal approximations with high momentum resolution, up to times which exceed the capabilities of standard implementations and are long enough to study, e.g., transient Floquet physics during multicycle electric field pulses and thermalization dynamics. The self-consistent calculation on the three-leg Kadanoff-Baym contour is fully self-contained, employing only QTT-compressed functions and input functions which are either generated directly in QTT form or obtained via quantics tensor cross interpolation.

In topological phases of matter for which the bulk and boundary support distinct electronic gaps, there exists the possibility of decoupled mobility gaps in the presence of disorder. This is in analogy with the well-studied problem of realizing separate or concomitant bulk-boundary criticality in conventional Landau theory. Using a three-dimensional axion insulator having clean, gapped surfaces with $e^2/2h$ quantized Hall conductance, we show that the bulk and surface mobility gap evolve differently in the presence of disorder. The decoupling of the bulk and surface topology yields a regime that realizes a two-dimensional, unquantized anomalous Hall metal in the Gaussian unitary ensemble on each surface, which shares some spectral and response properties akin to the surface states of a conventional 3D topological insulator. The generality of these results, as well as extensions to other insulators and superconductors, is discussed.

Nonlinear optical (NLO) effects in materials with band crossings have attracted significant research interests due to the divergent band geometric quantities around these crossings. Most current research has focused on band crossings between the valence and conduction bands. However, such crossings are absent in insulators, which are more relevant for NLO applications. In this Letter, we demonstrate that NLO effects can be significantly enhanced by band crossings within the valence or conduction bands, which we designate as “deep band crossings” (DBCs). As an example, in two dimensions, we show that shift conductivity can be substantially enhanced or even divergent due to a mirror-protected “deep Dirac nodal point.” In three dimensions, we propose GeTe as an ideal material where shift conductivity is enhanced by “deep Dirac nodal lines.” The ubiquity of this enhancement is further confirmed by high-throughput calculations. Other types of DBCs and NLO effects are also discussed. By engineering band crossings between arbitrary bands, our Letter offers a simple, practical, and universal way to enhance NLO effects.

Understanding the mechanical properties of active suspensions is crucial for their potential applications in materials engineering. Among active phenomena with no analog in equilibrium systems, motility-induced phase separation (MIPS) in active colloidal suspensions is one of the most extensively studied. However, the mechanical properties of MIPS remain poorly understood. This Letter investigates the rheology of a suspension of active colloidal particles under constant and oscillatory shear. Systems consisting of pseudohard active Brownian particles exhibiting coexistence of dense and dilute phases behave as a viscoelastic Maxwell-like fluid at low and high frequencies, displaying exclusively shear thinning across a wide range of densities and activities. Remarkably, the crossover frequency between the storage and loss moduli is nonmonotonic, increasing with activity before the MIPS transition but decreasing with activity after the transition, in contrast to a passive analog system of attractive particles in liquid-gas phase coexistence, revealing the subtleties of how active forces and intrinsically out-of-equilibrium phases affect the mechanical properties.

The laws of thermodynamics are a cornerstone of physics. At the nanoscale, where fluctuations and quantum effects matter, there is no unique thermodynamic framework because thermodynamic quantities such as heat and work depend on the accessibility of the degrees of freedom. We derive a thermodynamic framework for coherently driven systems, where the output light is assumed to be accessible. The resulting second law of thermodynamics is strictly tighter than the conventional one, and it demands the output light to be more noisy than the input light. We illustrate our framework across several well-established models, and we show how the three-level maser can be understood as an engine that reduces the noise of a coherent drive. Our framework opens a new avenue for investigating the noise properties of driven-dissipative quantum systems.

The precision advantages offered by harnessing the quantum states of sensors can be readily compromised by noise. However, when the noise has a different spatial function than the signal of interest, recent theoretical work shows how the advantage can be maintained and even significantly improved. In this Letter, we experimentally demonstrate the associated sensing protocol, using trapped-ion sensors. An entangled state of multidimensional sensors is created that isolates and optimally detects a signal, while being insensitive to otherwise overwhelming noise fields with different spatial profiles over the sensor locations. The quantum protocol is found to outperform a perfect implementation of the best comparable strategy without sensor entanglement. While our demonstration is carried out for magnetic and electromagnetic fields over a few microns, the technique is readily applicable over arbitrary distances and for arbitrary fields, thus present a promising application for emerging quantum sensor networks.

Negatively charged boron vacancy ($V_{\mathrm{B}}^{-}$) in hexagonal boron nitride is the most extensively studied room-temperature quantum spin system in two-dimensional materials. Nevertheless, the current effective readout of $V_{\mathrm{B}}^{-}$ spin states is carried out by systematically optical methods. This limits their exploitation in compact and miniaturized quantum devices, which would otherwise hold substantial promise to address quantum sensing and quantum information tasks. In this Letter, we demonstrate a photoelectric spin readout technique for $V_{\mathrm{B}}^{-}$ spins in hexagonal boron nitride. The observed photocurrent signals stem from the spin-dependent ionization dynamics of boron vacancies, mediated by spin-dependent nonradiative transitions to a metastable state. We further extend this electrical detection technique to enable the readout of dynamical decoupling sequences, including the Carr-Purcell-Meiboom-Gill protocols, and of nuclear spins via electron-nuclear double resonance. These results provide a pathway toward on-chip integration and real-field exploitation of quantum functionalities based on two-dimensional material platforms.

We show that the gravothermal collapse of self-interacting dark matter (SIDM) halos can deviate from local thermodynamic equilibrium. As a consequence, the self-similar evolution predicted by the commonly adopted conducting fluid model can be altered or broken. Our results are obtained using a novel, efficient kinetic solver called kiss-sidm for tracing the gravothermal evolution based on the direct simulation Monte Carlo framework. In the long mean free path stage, the code is a viable alternative to the fluid model, yet requires no calibration parameters. Further, this method enables a fully kinetic treatment well into the late, short mean free path, stage of the collapse. We apply the method to a canonical case with isotropic, velocity independent scattering. We find that although a fluid treatment is appropriate deep in the short mean free path core, departures from local thermodynamic equilibrium develop in the intermediate mean free path region bounding the core, which modify the late-time evolution.

We reexamine the case for dark matter (DM) produced by ultrarelativistic freeze-out (UFO). UFO is the mechanism by which standard model neutrinos decouple from the radiation bath in the early Universe at a temperature $T_d\approx 1\mathrm{MeV}$. This corresponds to chemical freeze-out without Boltzmann suppression, such that the freeze-out (decoupling) temperature $T_d$ is much greater than $m_{\nu }$ and the neutrinos are therefore ultrarelativistic at freeze-out. While UFO has historically been rejected as a viable mechanism for DM production due to its association with hot DM and the accompanying incompatibility with $\mathrm{\Lambda }\mathrm{CDM}$, we show that when the approximation of instantaneous reheating after inflation is lifted, UFO can produce cold DM and account for the entire observed relic density in large regions of parameter space. In fact, DM with masses ranging from sub-eV to PeV scales can undergo UFO and be cold before structure formation, given only a simple perturbative, postinflationary reheating period prior to radiation domination. For some interactions, such as a contact interaction between the Higgs and DM scalars, there is a seamless transition between the weakly interacting massive particle and feebly interacting massive particle regimes that excludes UFO. However, for many other interactions, such as standard model fermions producing fermionic DM via a heavy scalar or vector mediator, the weakly interacting massive particle to feebly interacting massive particle transition occurs necessarily via a large intermediate regime corresponding to UFO. We characterize the general features of UFO in this Letter, while we supply a more detailed analysis in a companion paper. These results render the conventional mechanism for producing hot DM broadly compatible with cold DM.

We report on a search for weakly interacting massive particle (WIMP) dark matter (DM) via elastic DM-xenon-nucleus interactions in the XENONnT experiment. We combine datasets from the first and second science campaigns resulting in a total exposure of 3.1 tonne-years. In a blind analysis of nuclear recoil events with energies above $3.8{\mathrm{keV}}_{\mathrm{NR}}$, we find no significant excess above background. We set new upper limits on the spin-independent WIMP-nucleon scattering cross section for WIMP masses above $10\mathrm{GeV}/c^2$ with a minimum of $1.7\times {10}^{-47}{\mathrm{cm}}^2$ at 90% confidence level for a WIMP mass of $30\mathrm{GeV}/c^2$. We achieve a best median sensitivity of $1.4\times {10}^{-47}{\mathrm{cm}}^2$ for a $41\mathrm{GeV}/c^2$ WIMP. Compared to the result from the first XENONnT science dataset, we improve our sensitivity by a factor of up to 1.8.

The IKKT model (or the type IIB matrix model) has been investigated as a promising nonperturbative formulation of superstring theory. One of the recent developments concerning this model is the discovery of the dual supergravity solution corresponding to the model obtained after supersymmetry-preserving mass deformation, which is dubbed the polarized IKKT model. Here we perform Monte Carlo simulations of this model in the case of matrix size $N=2$ for a wide range of the deformation parameter $\mathrm{\Omega }$. While we reproduce precisely the known result for the partition function obtained by the localization method developed for supersymmetric theories, we also calculate the observables, which were not accessible by the previous work, in order to probe the spacetime structure emergent from the dominant matrix configurations. In particular, we find that the saddle point corresponding to the original IKKT model is smoothly connected to the saddle represented by the fuzzy sphere dominant at large $\mathrm{\Omega }$, whereas the dominant configurations become diverging commuting matrices at small $\mathrm{\Omega }$.

We argue that ${}^{23}\mathrm{Na}$ is a potentially significant source of galactic axions. For temperatures $\gtrsim 7\times {10}^8\mathrm{K}$—characteristic of carbon burning in the massive progenitors of supernovae and ONeMg white dwarfs—the 440 keV first excited state of ${}^{23}\mathrm{Na}$ is thermally populated, with its repeated decays pumping stellar energy into escaping axions. Odd-A nuclear abundances are typically very low in high-temperature stellar environments (or absent entirely due to burn-up). ${}^{23}\mathrm{Na}$ is an exception: $\approx 0.1M_{\odot }$ of the isotope is synthesized during carbon burning then maintained at $\approx {10}^9\mathrm{K}$ for times ranging up to $6\times {10}^4\mathrm{y}r$. Using MESA simulations, a galactic model, and sampling over progenitor masses, locations, and evolutionary stages, we find a continuous flux at Earth of $⟨{\varphi }_a⟩\approx 22/{\mathrm{cm}}^2\mathrm{s}$ for $g_{aNN}^{\mathrm{eff}}={10}^{-9}$. Some fraction of these axions converts to photons as they propagate through the galactic magnetic field, producing a distinctive 440 keV line $\gamma$-ray detectable by all-sky detectors like the Compton Spectrometer and Imager (COSI). Assuming a $1\mathrm{\mu }\mathrm{G}$ galactic magnetic field and a sufficiently light axion mass, we find that COSI will be able to probe $|g_{aNN}^{\mathrm{eff}}{g}_{a\gamma \gamma }|\gtrsim 1.8\times {10}^{-22}{\mathrm{GeV}}^{-1}$ at $3\sigma$ after two years of surveying.