Direct-detection searches for dark matter are insensitive to dark matter particles that have large interactions with ordinary matter, which are stopped in the atmosphere or the Earth’s crust before reaching terrestrial detectors. We use “dark” calibration images taken with the HgCdTe detectors in the near-infrared spectrograph (NIRSpec) on the James Webb Space Telescope (JWST) to derive novel constraints on sub-GeV dark matter candidates that scatter off electrons. We supplement the JWST analysis pipeline with additional masks to remove pixels with high-energy background events. For a 0.4% subcomponent of dark matter that interacts with an ultralight dark photon, we disfavor all previously allowed parameter space at high cross sections, and constrain some parameter regions for subcomponent fractions as low as $\sim 0.01\%$.

In semiconducting-superconducting hybrid devices, Andreev bound states (ABSs) can mediate the coupling between quantum dots, allowing for the realization of artificial Kitaev chains. In order to engineer Majorana bound states (MBSs) in these systems, one must control the energy of the ABSs. In this Letter, we show how extended ABSs in a flux-tunable Josephson junction can be used to control the coupling between distant quantum dots separated by $\simeq 1\mathrm{\mu }\mathrm{m}$. In particular, we demonstrate that the combination of electrostatic control and phase control over the ABSs increases the parameter space in which MBSs are observed. Finally, by employing an additional spectroscopic probe in the hybrid region between the quantum dots, we gain information about the spatial distribution of the Majorana wave function in a two-site Kitaev chain.

The kagome metal ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ exhibits an unusual charge-density-wave (CDW) order, where the emergence of loop current order that breaks time-reversal symmetry (TRS) has been proposed. A key feature of this CDW phase is a nonmonotonic Hall effect at low fields, often attributed to TRS breaking. However, its origin remains unclear. Here, we conduct comprehensive magnetotransport measurements on ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ and, through mobility spectrum analysis, identify the formation of tiny Fermi pockets with extremely high mobility below the CDW transition. Furthermore, electron irradiation experiments reveal that the nonmonotonic Hall effect is significantly suppressed in samples with reduced mobility, despite no substantial change in the electronic structure. These results indicate that the nonmonotonic Hall effect originates from these tiny Fermi pockets with high mobility carriers rather than anomalous Hall mechanisms, providing new insights into understanding the Hall anomaly in this kagome system.

Compact free-electron lasers (FELs) based on plasma-based accelerators have been envisioned for many years. While recent milestone experiments have demonstrated feasibility, further progress is needed to establish laser-plasma-accelerator-driven FELs as reliable light sources. Demonstrating both full FEL saturation and reliable operation commensurate with conventional FEL facilities is critical. We report progress on both of these fronts. FEL gain exceeding 1000 at a wavelength of 420 nm in the self-amplified spontaneous emission regime has been measured. Lower and upper bounds for the characteristic exponential gain length were measured at 16.7–22.5 cm. Additionally, reliability of the laser-plasma-accelerator-driven FEL, defined here as the percentage of shots displaying FEL gain, reached an unprecedented level of greater than 90% over the course of an hour while operating at 1 Hz repetition rate.

We propose and analyze a family of approximately symmetric neural networks for quantum spin liquid problems. These tailored architectures are parameter efficient, scalable, and significantly outperform existing symmetry-unaware neural network architectures. Utilizing the mixed-field toric code and PXP Rydberg Hamiltonian models, we demonstrate that our approach is competitive with state-of-the-art tensor network and quantum Monte Carlo methods. Moreover, at the largest system sizes ($N=480$ for toric code, $N=1584$ for Rydberg PXP), our method allows us to explore Hamiltonians with sign problems beyond the reach of both quantum Monte Carlo and finite-size matrix-product states. The network comprises an exactly symmetric block following a nonsymmetric block, which we argue learns a transformation of the ground state analogous to quasiadiabatic continuation. Our Letter paves the way toward investigating quantum spin liquid problems within interpretable neural network architectures.

Entanglement purification is to distill high-quality entanglement from low-quality entanglement. Entanglement purification determines the efficiency and communication rates and plays a key role in quantum repeaters and long-distance quantum communications. Existing entanglement purification protocols and proof of principle experiments usually choose two identical input pairs. They are limited to focus on the successful purification case to obtain high-quality entangled states and discard the remained items if the purification fails. Recent entanglement purification theory [Opt. Express 28, 2291 (2020)] for arbitrary nonidentical input mixed states indicated that the discarded items may have residual entanglement and can be reused to increase the yield of entanglement purification, while the successful purification may not obtain a high-quality entangled state. In this Letter, using hyperentanglement, we design and experimentally demonstrate an entanglement purification protocol with nonidentical input mixed states. For the first time, we experimentally confirm the existence of residual entanglement in different error models. Furthermore, we illustrate that successful purification may decrease the fidelity of purified entangled states. We also demonstrate the approach of optimal entanglement purification. Our proof of principle results have potential applications in future high-efficiency and long-distance quantum communication.

The electronic band gap of a two-dimensional semiconductor within a device architecture is sensitive to variations in screening properties of adjacent materials in the device and to gate-controlled doping. Here, we employ microfocused angle-resolved photoemission spectroscopy to separate band gap renormalization effects stemming from environmental screening and electron doping during in situ gating of a single-layer ${\mathrm{WS}}_2$ device. The ${\mathrm{WS}}_2$ is supported on hexagonal boron nitride and contains a section that is exposed to vacuum and another section that is encapsulated by a graphene contact. We directly observe the doping-induced semiconductor-metal transition and band gap renormalization in the two sections of ${\mathrm{WS}}_2$. Surprisingly, a larger band gap renormalization is observed in the vacuum-exposed section than in the graphene-encapsulated—and thus ostensibly better screened—section of the ${\mathrm{WS}}_2$. Using $GW$ calculations, we determine that intrinsic screening due to stronger doping in vacuum-exposed ${\mathrm{WS}}_2$ exceeds the external environmental screening in graphene-encapsulated ${\mathrm{WS}}_2$.

The scattering matrix $S$ linearly relates the vector of incoming waves to outgoing wave excitations, and contains an enormous amount of information about the scattering system and its connections to the scattering channels. Time delay is one way to extract information from $S$, and the transmission time delay ${\tau }_T$ is a complex (even for Hermitian systems with unitary scattering matrices) measure of how long a wave excitation lingers before being transmitted. The real part of ${\tau }_T$ is a well-studied quantity, but the imaginary part of ${\tau }_T$ has not been systematically examined experimentally, and theoretical predictions for its behavior have not been tested. Here we experimentally test the predictions of Asano et al. [Nat. Commun. 7, 13488 (2016)] for the imaginary part of transmission time delay in a nonunitary scattering system. We utilize Gaussian time-domain pulses scattering from a two-port microwave graph supporting a series of well-isolated absorptive modes to show that the carrier frequency of the pulses is changed in the scattering process by an amount in agreement with the imaginary part of the independently determined complex transmission time delay, $\mathrm{Im}[{\tau }_T]$, from frequency-domain measurements of the subunitary $S$ matrix. Our results also generalize and extend those of Asano et al., establishing a means to predict pulse propagation properties of non-Hermitian systems over a broad range of conditions.

Moiré superlattices enable engineering of correlated quantum states through tunable periodic potentials, where twist angle controls periodicity but dynamic potential strength modulation remains challenging. Here, we develop a high-pressure quantum transport technique for van der Waals heterostructures, achieving the ultimate pressure limit ($\sim 9\mathrm{GPa}$) in encapsulated moiré devices. In aligned graphene/h-BN, we demonstrate that pressure induces a substantial enhancement of the moiré potential strength, evidenced by the suppression of the first valence bandwidth and the near-doubling of the primary band gap. Moreover, we report the first observation of a tertiary gap emerging above 6.4 GPa, verifying theoretical predictions. Our results establish hydrostatic pressure as a universal parameter to reshape moiré band structures. By enabling quantum transport studies at previously inaccessible pressure regimes, this Letter expands the accessible parameter space for exploring correlated phases in moiré systems.

Phosphorene has attracted extensive research interest due to its anisotropic properties with rich tunabilities such as strongly layer-dependent and field-tunable band gaps. Revealing how the electronic structure evolves down to monolayer is critical for understanding its anisotropic properties and field tunability. Here, we report the hitherto unexplored electronic structure of monolayer phosphorene, as well as field tunability in few-layer phosphorene. A strongly enhanced mass anisotropy is observed in monolayer phosphorene, which clearly distinguishes it from bilayer and thicker flakes. Moreover, by introducing an external electric field upon doping, modifications of the mass anisotropy and energy spacing between subbands in few-layer phosphorene are further revealed. Our work reveals the unique electronic structure of monolayer phosphorene, highlighting the critical roles of interlayer coupling as well as external electric field in tailoring the band parameters in phosphorene.

We study a one-dimensional system of two-component fermions in the limit of strong attractive particle-particle interactions. First, we analyze scattering in the corresponding few-body problem, which is analytically solvable via Bethe ansatz. This allows us to engineer effective interactions between the system’s effective degrees of freedom: fermions and bosonic dimers (tightly bound pairs of fermions). We argue that, although these interactions are strong, the resulting effective problem can be mapped onto a weakly interacting one, paving the way for the use of perturbation theory. This finding simplifies studies of many-fermion systems under confinement that are beyond reach of state-of-the-art numerical methods. We illustrate this statement by considering an impurity atom in a Fermi gas.

We identify a three-dimensional system that exhibits long-range entanglement at sufficiently small but nonzero temperature—it therefore constitutes a quantum topological order at finite temperature. The model of interest is known as the fermionic toric code, a variant of the usual 3D toric code, which admits emergent fermionic pointlike excitations. The fermionic toric code, importantly, possesses an anomalous two-form symmetry, associated with the spacelike Wilson loops of the fermionic excitations. We argue that it is this symmetry that imbues low-temperature thermal states with a novel topological order and long-range entanglement. Based on the current classification of three-dimensional topological orders, we expect that the low-temperature thermal states of the fermionic toric code belong to a phase of matter that, in the context of equilibrium phases of matter, only exists at nonzero temperatures. We conjecture that further examples of topological orders at nonzero temperatures are given by discrete gauge theories with anomalous two-form symmetries. Our Letter therefore opens the door to studying quantum topological order at nonzero temperature in physically realistic dimensions.

Atomically thin hexagonal boron nitride (h-BN), especially monolayer, has emerged as a pivotal quantum material due to its intriguing optical and light-matter-interaction properties. Nevertheless, fundamental ambiguities persist regarding its intrinsic band structure and deep-UV optical responses. Here, a multispectroscopic approach—combining near-resonance deep-UV photoluminescence, Raman spectroscopy, and reflectance contrast measurements—is employed to systematically resolve the layer-dependent optoelectronic evolution of h-BN. It is revealed that the absence of band-edge luminescence in 1–3 layers h-BN is indicative of their indirect band gap nature, thereby rectifying longstanding misinterpretations of monolayer BN as a direct band gap semiconductor. Strikingly, band-edge luminescence signals and indirect band gap absorption start to appear in 4-layer, and the luminescence intensity increases with the number of layers, suggesting that interlayer interactions and periodicity along the $z$ axis enhance phonon-assisted indirect band gap transition, even in the 4-layer case, and furthermore indicating the formation process of flat bands at $K/M$ valleys as the periodicity along z direction increases. Moreover, the prominent resonance Raman signals in atomically thin h-BN reveals exceptionally strong electron-phonon coupling, a critical parameter for quantum optoelectronic applications. Our findings provide definitive experimental benchmarks for the long-debated monolayer BN’s band structure.

We report the first observation of a momentum-resolved superconducting gap in the Hg-based trilayer cuprate, which holds the highest record of superconducting transition temperature ($T_{\mathrm{c}}$) at ambient pressure. By angle-resolved photoemission spectroscopy utilizing a microfocused beam, clear quasiparticle dispersions originating from the inner and outer ${\mathrm{CuO}}_2$ planes (IP and OP, respectively) were separately identified. The magnitude of the superconducting gap for the IP was comparable to that of the Bi-based trilayer cuprate with a lower $T_{\mathrm{c}}$. In contrast, the superconducting gap for the OP was significantly larger than that of the Bi-based one. While strong pairing in the IP has been highlighted as the key element of trilayer cuprates, the present results suggest that the enhanced pairing energy in the OP is essential for the highest $T_{\mathrm{c}}$ at ambient pressure realized in the Hg-based trilayer cuprate.

More than 50 years ago, excitonic insulators formed by the pairing of electrons and holes due to Coulomb interactions were first predicted [A. N. Kozlov and L. A. Maksimov, Sov. J. Exp. Theor. Phys. 21, 790 (1965); L. V. Keldysh and Y. V. Kopaev, Sov. Phys. Solid State 6, 2219 (1965); D. Jérome, T. M. Rice, and W. Kohn, Phys. Rev. 158, 462 (1967)]. Since then, excitonic insulators have been observed in various classes of materials, including quantum Hall bilayers, graphite, transition metal chalcogenides, and more recently in moiré superlattices. In these excitonic insulators, an electron and a hole with the same spin bind together, and the resulting exciton is a spin singlet. Here, we report the experimental observation of a spin-triplet excitonic insulator in the ultra-quantum limit of a three-dimensional topological material ${\mathrm{HfTe}}_5$. We observe that the spin-polarized zeroth Landau bands dispersing along the field direction cross each other beyond a characteristic magnetic field in ${\mathrm{HfTe}}_5$, forming the one-dimensional Weyl mode. Transport measurements reveal the emergence of a gap of about $250\mathrm{\mu }\mathrm{eV}$ when the field surpasses a critical threshold. By performing the material-specific modeling, we identify this gap as a consequence of a spin-triplet exciton formation, where electrons and holes with opposite spin form bound states, and the translational symmetry is preserved. The system reaches charge neutrality following the gap opening, as evidenced by the zero Hall conductivity over a wide magnetic field range (10–72 T). Our finding of the spin-triplet excitonic insulator paves the way for studying novel spin transport including spin superfluidity, spin Josephson currents, and Coulomb drag of spin currents in analogy to the transport properties associated with the layer pseudospin in quantum Hall bilayers.

Experiments have shown that over 10% of proteins are degraded nonexponentially. Gene expression models for nonexponentially degraded proteins are notoriously difficult to solve since the underlying stochastic dynamics is non-Markovian. Here we develop an effective Markovian dynamics (EMD) method which converts a large class of non-Markovian models into effective Markovian ones so that they have the same mRNA and protein distributions at any fixed time. Using the EMD approach, we analytically solve some classical gene expression models with nonexponential or delayed protein decay, whose exact distributions are previously unknown and fail to be obtained using conventional queueing theory. Our theory successfully explains why nonexponentially degraded proteins on average have smaller mRNA-protein correlation than exponentially degraded proteins, and it predicts that bimodality is significantly enhanced in the presence of delayed protein degradation.

Sliding ferroelectrics constructed from stacked nonpolar monolayers enable out-of-plane polarization in two dimensions with exceptional properties. However, the widely accepted switching mechanism, involving synchronized long-distance in-plane translation of entire atomic layers, contradicts experimental findings. We demonstrate that this spinodal decompositionlike homogeneous switching process violates Neumann’s principle and is therefore highly improbable. Instead, polarization reversal relies on symmetry-breaking domain walls (DWs) and the tensorial nature of Born effective charges, highlighting the quantum nature of sliding ferroelectrics. We reveal that wide, wavelike DWs coherently propagate, a stark departure from the narrow DW mechanism recognized for decades. These wavelike DWs exhibit superlubric dynamics, achieving ultrahigh velocities of approximately $4000\mathrm{m}/\mathrm{s}$ at room temperature and displaying an anomalous cooling-promoted switching speed. The unexpected emergence of DW superlubricity presents new avenues for enhancing key performance metrics and offers exciting opportunities for applications in cryogenic environments.

Understanding which parts of a dynamical system cause each other is extremely relevant in fundamental and applied sciences. However, inferring causal links from observational data, namely, without direct manipulations of the system, is still computationally challenging, especially if the data are high dimensional. In this Letter we introduce a framework for constructing causal graphs from high-dimensional time series, whose computational cost scales linearly with the number of variables. The approach is based on the automatic identification of dynamical communities, groups of variables which mutually influence each other and can therefore be described as a single node in a causal graph. These communities are efficiently identified by optimizing the information imbalance, a statistical quantity that assigns a weight to each putative causal variable based on its information content relative to a target variable. The communities are then ordered starting from the fully autonomous ones, whose evolution is independent from all the others, to those that are progressively dependent on other communities, building in this manner a community causal graph. We demonstrate the computational efficiency and the accuracy of our approach on discrete-time and continuous-time dynamical systems including up to 80 variables.

Perhaps the most basic question we can ask about cosmological correlations is how their strength changes as we smoothly vary kinematic parameters. The answer is encoded in differential equations that govern this evolution in kinematic space. In this Letter, we introduce a new perspective on these differential equations. We show that, in the simplified setting of conformally coupled scalars in power-law Friedmann-Robertson-Walker spacetimes, the equations for arbitrary tree-level processes can be obtained from a small number of simple combinatorial rules. While this “kinematic flow” is defined purely in terms of boundary data, it reflects the physics of bulk time evolution. The unexpected regularity of the equations suggests the existence of an autonomously defined mathematical structure from which cosmological correlations and the time evolution of the associated spacetime emerge.

Seeded free-electron lasers (FELs) capable of operating at repetition rates up to the MHz level are in high demand for advanced time-resolved spectroscopies, which require both full longitudinal coherence and high average photon flux in the extreme ultraviolet (EUV) and x-ray regimes. However, conventional external-seeding methods require ultraviolet seed lasers with peak powers on the order of 100 MW, constraining them to lower repetition rates. Here, we report the first lasing and stable operation of a direct-amplification enabled harmonic generation FEL driven by a weak seed laser with only MW-level peak power. Beginning with an ultraviolet seed laser with only $0.75\mathrm{\mu }\mathrm{J}$ pulse energy, we demonstrate its direct amplification to over $10\mathrm{\mu }\mathrm{J}$ within an 8-m-long modulator. We observe coherent harmonic generation up to the $12th$ harmonic of the seed and achieve saturation of the $7th$ harmonic in the radiator. These results represent a crucial milestone toward the realization of MHz-class, fully coherent EUV and x-ray light sources.