Excitons in monolayer transition metal dichalcogenides ($1L\text{−}\mathrm{TMDs}$) exhibit a Rydberg-like series of bound states, including optically dark levels. Using a three-pulse scheme, we directly track the dynamics of the $2p$ dark exciton in $1L\text{−}{\mathrm{WS}}_2$ through the Autler-Townes splitting of the bright $1s$ state. The splitting, induced by a midinfrared field resonant with the $1s\text{−}2p$ transition, reveals distinct $1s$ and $2p$ responses to photoexcitation. This approach provides new insight into many-body effects and offers a versatile experimental framework to probe driven excitonic dynamics in quantum materials.

We report on the successful trapping of micrometer-sized, multiply-charged helium droplets over periods of several seconds up to a minute—the first observation of trapped charged helium droplets. Within a meter-long multireflection mass spectrometer, the droplets complete thousands of revolutions, opening up new possibilities for precision measurements of these unique systems. Their confinement is limited by collisions with residual gas, which gradually reduce their kinetic energy through evaporation of helium atoms from the droplet and prevent extraction from the trap below a critical value. The rate of energy loss is strongly influenced not only by the residual gas density, but also by the type of species picked up by the droplets. In particular, infrared absorption by water cluster ions inside the droplets enhances energy transfer to the helium matrix, leading to accelerated evaporation of helium atoms.

Spontaneous breaking of time-reversal and spatial-inversion symmetries in solids triggers diverse intriguing phenomena. Although these phenomena have been extensively studied in insulators, similar investigations for metals remain limited. Herein, we report the observation and properties of spontaneous (i.e., zero-magnetic field) nonreciprocal charge transport in the zigzag intermetallic compound ${\mathrm{NdRu}}_2{\mathrm{Al}}_{10}$. This effect is attributed to the antiferromagnetic order, which can be interpreted as a magnetic toroidal dipole order. Our results reveal an excessively large nonreciprocal coefficient for this material, attributed to the strong effective magnetic field generated through $c-f$ exchange interactions. The results also suggest that the nonreciprocal response of this material depends on the spin configurations of the antiferromagnetic domains. Overall, our findings are distinct from those previously reported for field-induced nonreciprocal charge transport and contribute to a comprehensive understanding of cross-correlations in symmetry-broken metals.

Quantum anomalous Hall effect (QAHE) in graphene moiré superlattices has drawn considerable and sustained attention, owing to both fundamental interests of Chern insulators and promising applications in topological electronics. An empirical regularity suggests that QAHE manifests exclusively at odd-electron filling per moiré unit cell, whereas even-filling configurations—particularly at half-filled bands—invariably yield topologically trivial insulators. This dichotomy, originating from quantum states competition, has been the key issue in theoretical investigations of topology and correlation, but its universality remains an open question. Here we demonstrate a violation of this paradigm in rhombohedral heptalayer graphene/hexagonal boron nitride moiré superlattices, where a robust Chern insulator emerges at two-electron filling per moiré unit cell. It wins the competition with a trivial correlated insulator only above a critical displacement field as large as $\sim 1\mathrm{V}/\mathrm{nm}$. The odd Chern number $C=1$ for the filling of 2 is intriguing: theoretical calculation shows that electrons occupy two interaction-renormalized bands with distinct spin-valley flavors, whose charge densities arrange into dual honeycomb and triangular lattices, leading to a total Chern number of 1 and minimized Coulomb repulsion. Our findings broaden the landscape for engineering nontrivial topological states, while providing critical insights into the delicate competition between emergent quantum phases in strongly correlated moiré systems.

The construction of entangling gates with individual addressing capability represents a crucial approach for implementing quantum computation in trapped-ion crystals. Conventional entangling gate schemes typically rely on laser beam wave vectors to couple the ions’ spin and motional degrees of freedom. Here, we experimentally demonstrate an alternative method that employs a polarization gradient field generated by a tightly focused laser beam—an approach theoretically proposed as the Magnus effect for quantum logic gate design [Mazzanti et al., Phys. Rev. Res. 5, 033036 (2023)]. Using this technique, we perform Raman operations on hyperfine qubits encoded in ${{}^{171}\mathrm{Yb}}^{+}$ ions, generating spin-dependent forces along axial motional modes in a linear trap. By utilizing an acousto-optic deflector to create arbitrary spot pairs for individual ion addressing in two-ion (four-ion) chains, we achieve Mølmer-Sørensen gates with Bell-state fidelities exceeding 98.7(1)% [and 97.2(4)%]. Further improvements in numerical aperture (NA) and laser power could reduce gate durations while enhancing fidelity by orders of magnitude. This method is compatible with—and can significantly simplify—optical tweezer gate proposals, where motional mode engineering enables scalable trapped-ion quantum computation. The technique can be readily extended to two-dimensional ion crystals, representing a key advancement toward large-scale trapped-ion quantum processors.

We report the observation of dynamic fringe patterns in the diffuse scattering of extreme ultraviolet light from surfaces, following femtosecond optical excitation. At each point on the detector, the diffuse scattering intensity exhibits oscillations at well-defined frequencies that correspond to surface phonons with wave vectors determined by the scattering geometry, indicating that the optical excitation generates coherent surface phonons propagating in all directions and spanning a wavelength range from 60 to 300 nm. This phenomenon is observed on a variety of samples, including single-layer and multilayer metal films, as well as bulk semiconductors. The measured surface phonon dispersions show good agreement with theoretical calculations. By comparing signal amplitudes from samples with different surface morphologies, we find that the excitation mechanism is linked to the natural surface roughness of the samples. However, the signal is still detectable on extremely smooth surfaces with subnanometer roughness. Our findings demonstrate a simple and effective method for optically exciting coherent surface phonons with nanoscale wavelengths on a wide range of solid samples and establish a foundation for surface phonon spectroscopy in a wave vector range well beyond the limit of conventional surface Brillouin scattering.

We establish a sharp quantum advantage in determining the parity (even-odd) of an unknown permutation applied to any number $n\ge 3$ of particles. Classically, this is impossible with fewer than $n$ labels, being that the success is limited to random guessing. Quantum mechanics does it with certainty with as few as $\lceil \sqrt{n}\rceil$ distinguishable states per particle, thanks to entanglement. Below this threshold, not even quantum mechanics helps: both classical and quantum success are limited to random guessing. For small $n$, we provide explicit expressions for states that ensure perfect parity identification. We also assess the minimum entanglement these states need to carry, finding it to be close to maximal, and even maximal in some cases. The task requires no oracles or contrived setups and provides a simple, rigorous example of genuine quantum advantage.

Optical atomic clocks play a crucial role in fundamental physics, relativistic geodesy, and the future redefinition of the Systeme International second. Standard operation relies on cyclic interrogation sequences, which alternate between atomic interrogation and dead time used for state preparation and readout. This approach introduces the Dick effect, where laser frequency noise aliases onto the atomic transition frequency. Although reducing laser noise improves clock stability, the Dick effect remains a key limitation. In this Letter, we demonstrate a zero-dead-time optical clock based on two interleaved ensembles of cold ${}^{87}\mathrm{Sr}$ atoms. Our system significantly suppresses this noise and achieves a fractional frequency instability at the ${10}^{-19}$ level between 10 000 and 20 000 s over repeated measurements, with a best value of $2.9\times {10}^{-19}$ at $\tau =20000\mathrm{s}$. The estimated long-term stability based on the combined data of these measurements reaches $2.5\times {10}^{-19}$ at 1 day. These results represent a more than ninefold improvement over a conventional single-ensemble clock, highlighting its potential for next-generation timekeeping applications.

We report fast atom diffraction through single-layer graphene using hydrogen atoms at kinetic energies from 150 to 1200 eV. High-resolution images reveal overlapping hexagonal patterns from coexisting monocrystalline domains. Time-of-flight tagging confirms negligible energy loss, making the method suitable for matter-wave interferometry. The diffraction is well described by the eikonal approximation, with accurate modeling requiring the full 3D interaction potential from density functional theory. Simpler models fail to reproduce the data, highlighting the exceptional sensitivity of diffraction patterns to atom-surface interactions and their potential for spectroscopic applications.

We report the experimental observation of discrete bright matter-wave solitons with attractive interaction in an optical lattice. Using an accordion lattice with adjustable spacing, we prepare a Bose-Einstein condensate of cesium atoms across a defined number of lattice sites. By quenching the interaction strength and the trapping potential, we generate both single-site and multisite solitons. Our results reveal the existence and characteristics of these solitons across a range of lattice depths and spacings. We identify stable regions of the solitons based on interaction strength and lattice properties, and compare these findings with theoretical predictions. The experimental results qualitatively agree with a Gaussian variational model and match quantitatively with numerical simulations of the three-dimensional Gross-Pitaevskii equation extended with a quintic term to account for the loss of atoms. Our results provide insights into the quench dynamics and collapse mechanisms, paving the way for further studies of transport and dynamical properties of matter-wave solitons in lattices.

X-ray free-electron lasers (FELs) are powerful photon sources offering a wide wavelength range, subfemtosecond pulse duration, and high brightness. Most x-ray FELs are based on self-amplified spontaneous emission (SASE). SASE-FEL radiation has excellent transverse but only limited longitudinal coherence, with power and spectral profiles consisting of multiple randomly distributed spikes. In this Letter, we present the first experimental demonstration of mode-locked SASE, which generates periodic trains of phase-locked subfemtosecond pulses, thus providing an x-ray analog of the optical frequency comb. Our approach combines the mode-coupled SASE scheme, where magnetic chicanes between the undulator modules of the FEL increase the coherence of the output radiation, and an external optical laser that restricts the FEL amplification to periodic and short regions of the electron bunch. The work relies on evidence in the frequency and time domains for photons and electrons, respectively, and will benefit investigations of ultrafast dynamics as well as coherent spectroscopy, and enable new types of experiments requiring phase-correlated x-ray pulses.

As a quintessential quantum matter, realizing robust Wigner crystals and achieving their angstrom-scale imaging remain formidable challenges. Here, we devise an original charge-transfer-crystallization strategy to realize an intrinsic electron solid made of $4f$ electrons. The highly localized and correlated nature of $4f$ electrons gives rise to a heavy Wigner crystal. This unprecedented heaviness manifests as an electron solid with record-high density ($2.02\times {10}^{13}/{\mathrm{cm}}^{-2}$) and melting temperature ($>60\mathrm{K}$). $Q$-plus atomic force microscopy and Kelvin probe force microscopy are employed to directly image the Wigner crystal, by which the sub-unit-cell localization and electrostatic influence of Wigner electrons are accurately determined. Moreover, the collective tunneling is captured, underlining the quantum nature of a Wigner crystal. Our discovery presents the $4f$ system as a tunable platform with the charge-transfer degree of freedom to study the 2D many-body correlation of heavy electrons.

Quantum error correction (QEC) enables practical quantum computing by encoding logical qubits in many physical qubits, which can exponentially suppress the logical error rate with increasing code size provided that the physical error rate is below a critical threshold. However, the leakage of quantum information from the computational subspace presents a critical challenge to the development of scalable QEC, which creates long-lived, correlated errors that spread across space and time. Here, we demonstrate a quantum memory operating below the threshold by implementing an all-microwave leakage suppression architecture on a distance-7 surface code. We achieve a logical error suppression factor of $\mathrm{\Lambda }=1.40(6)$, definitively reversing the above-threshold scaling ($\mathrm{\Lambda }<1$) caused by unmitigated leakage. This scheme integrates a hardware-efficient leakage reduction unit for data qubits with a fast, unconditional reset for ancilla qubits, suppressing the average leakage population after 40 cycles by a factor of 72 to $6.4(5)\times {10}^{-4}$. Our results demonstrate the viability of all-microwave control architectures for suppressing critical errors at scale, paving the way for more advanced quantum error correction implementations.

The interface between semiconductors and ion-conducting electrolytes is characterized by charge distributions and potential drops that vary substantially with the evolution of surface states. These surface states at the very interface to the liquid can form or be passivated, depending on the applied potential between electrode and electrolyte, and hereby fundamentally impact properties such as charge transfer. Characterization and understanding of such potential-dependent surface states with high spatial and temporal resolution is a significant challenge for the understanding and control of semiconductor-electrolyte interfaces. Here, we show that the optical anisotropy of InP(100) can be used to detect the potential-dependent formation of highly ordered surface states under operating conditions. Upon formation of a surface state in the band gap of the semiconductor, the potential drop and hence the electric field is shifted away from the semiconductor to the Helmholtz layer of the electrolyte. This modifies the instantaneous response of the optical anisotropy to disturbances of the applied potential. We propose an electrochemical variant of the linear electro-optical effect and our findings open a novel route for understanding these interfaces. The results show how surface states from surface reconstructions at this reactive interface can be switched on or off with the applied potential.

Quantum walk serves as a versatile tool for universal quantum computing and algorithmic research. However, the implementation of discrete-time quantum walks (DTQWs) with superconducting circuits is still constrained by some limitations such as operation precision, circuit depth, and connectivity. With improved hardware efficiency by using superconducting qutrits (three-level systems), we experimentally demonstrate a scalable DTQW in a superconducting circuit, observing the ballistic spreading of quantum walk in a qutrit chain. The usage of qutrits in our implementation allows hardware efficiently encoding of the walker position and the coin degree of freedom. By exploiting the flexibility and intrinsic symmetries of qutrit-based DTQWs, we successfully prepare two topological phases in the chain. For the first time, particle-hole-symmetry-protected edge states, bounded at the interface between these two topological phases, are observed in the superconducting platform. Measured parameter dependencies further validate the properties of edge states. The scalability and gate-control compatibility of the demonstrated DTQWs enable a versatile tool for superconducting quantum computing and quantum simulation.

Collisionless dissipation of turbulence is important for heating plasmas in astrophysical, space physics, and laboratory environments, controlling energy, momentum, and particle transport. We analyze Parker Solar Probe observations to understand the collisionless heating of the sub-Alfvénic solar wind, which is connected to the solar corona. Our results show that linear resonant heating through parallel-propagating cyclotron waves cannot account for turbulent dissipation in the sub-Alfvénic region, which observations suggest may dissipate turbulence at distances further from the Sun. Instead, we find that stochastic heating can account for the observed ion energization; however, because the dominant contributions arise from infrequent, large-amplitude events, turbulent intermittency must be explicitly incorporated. These observations directly connect stochastic heating via breaking of the proton magnetic moment with the intermittent and inhomogeneous heating of turbulence reported in many previous studies. Our identification of stochastic heating as a dynamic mechanism responsible for intermittent heating of the solar wind has significant implications for turbulent dissipation in the lower corona, other astrophysical environments, and laboratory plasmas.

Bragg coherent diffraction imaging struggles to phase data from nanocrystals exhibiting large strain inhomogeneities, which significantly limits its applicability across many scientific scenarios. Here, we demonstrate the experimental realization of Bragg coherent modulation imaging, an approach that incorporates wavefront modulation into the diffracted beam, enabling unambiguous structure recovery and resolving highly strained crystals. This method provides a more robust tool for three-dimensional lattice strain measurements in nanocrystals.

The spectrum of Markov generators encodes physical information beyond simple decay and oscillation, which reflects irreversibility and governs the structure of correlation functions. In this Letter, we prove an ellipse theorem that provides a universal thermodynamic geometric constraint on the spectrum of Markov rate matrices. The theorem states that all eigenvalues lie within a specific ellipse in the complex plane. In particular, the imaginary parts of the spectrum, which indicate oscillatory modes, are bounded by the maximum thermodynamic force associated with individual transitions. This spectral bound further constrains the initial short-time behavior of correlation functions between two arbitrary observables. Finally, we compare our result with a previously proposed conjecture, which remains an open problem and warrants further investigation.

Interferometers are known for precise measurements of the effects inside of them. Here we show that placing a Dove prism in one of the interferometer arms allows for also detecting minute effects happening outside the device. We derive how the additional reflection in one arm can amplify small tilts or displacements of the input beam in the dark output port of the interferometer, in analogy to weak value amplification. The experiment demonstrates a clear improvement of the measurement signal. Moreover, in the bright output port, small disturbances in the input beam get largely filtered, thereby enabling beam stabilization with a fully passive device.

We introduce new classes of gapped topological phases characterized by quantized crystalline-electromagnetic responses, termed multipolar Chern insulators. These systems are characterized by nonsymmorphic momentum-space symmetries and mirror symmetries, leading to quantization of momentum-weighted Berry curvature multipole moments. We construct lattice models for such phases and numerically confirm their quantized responses. These systems exhibit bound charge and momentum densities at lattice and magnetic defects, as well as currents induced by electric or time-varying strain fields. Our Letter extends the classification of topological matter by uncovering novel symmetry-protected topological phases with quantized responses.