The ${\mathrm{Ni}}^{12+}$ ion features an electronic transition with a natural width of only 8 mHz, allowing for a highly stable optical clock. We predict that the energy of this strongly forbidden $3s^{2}3p^4{{}^{3}P}_2\to 3s^{2}3p^4{{}^{3}P}_0$ electric quadrupole transition is $20081(10){\mathrm{cm}}^{-1}$. For this, we use both a hybrid approach combining configuration interaction with a coupled-cluster method and a pure configuration interaction calculation for the complete 16-electron system, ensuring convergence. The resulting very small theoretical uncertainty of only 0.05% allowed us to find the transition experimentally in a few hours, yielding an energy of $20078.984(10){\mathrm{cm}}^{-1}$. This level of agreement for a 16-electron system is unprecedented and qualifies our method for future calculations of many other complex atomic systems. While paving the way for a high-precision optical clock based on ${\mathrm{Ni}}^{12+}$, our theory and code development will also enable better predictions for other highly charged ions and other complex atomic systems.

We demonstrate ultrafast graphene spin-field-effect junctions, where gate-tunable superdiffusive spin currents across graphene-ferromagnet interfaces enable electric field control of magnetization dynamics in the ferromagnet. By electrostatically tuning the Fermi level in graphene underlying a cobalt thin film, we modulate the ultrafast spin transport across graphene-cobalt interfaces, reducing femtosecond laser-induced demagnetization time from 203 fs in bare cobalt thin films to 93 fs, a more than 100% increase in the rate of magnetization quenching. Supported by superdiffusive spin transport calculations, our findings unlock field-tunable magnetic speeds in devices, paving the way for innovations in subpicosecond spintronic memory-logic operations. Furthermore, this work creates new possibilities for electrical modulation of spin dynamics and ultrafast spin injection into two-dimensional quantum materials, with potential for next-generation quantum sensors and faster magnetic technologies.

We use nonequilibrium molecular dynamics simulations to verify recent tube-model predictions that associative polymer networks exhibit broad stretch fluctuations during elongational flow. Simulations further show that these fluctuating dynamics give rise to the rate-dependent extensional viscosity ${\eta }_E$ measured in filament stretching experiments on H-bonding networks. Simulations model bivalent associative networks with a reactive bead-spring model for varying association strength and extensional strain rate. We observe that stretch fluctuations are driven by a new form of chain tumbling, where chains continually collapse and elongate as their associations break and reform within the advecting network. This produces a broad, nearly uniform distribution of chain stretch over a wide range of strain rates, manifesting as a rate-independent plateau in the extensional stress. Our results show that the nonlinear viscoelasticity of associative networks is dominated by large fluctuations in molecular response, which cannot be captured by current mean-field models.

We present the first numerical relativity simulations including neutrino flavor transformations that could result from flavor instabilities, quantum many-body effects, or potential beyond standard model physics in neutron star mergers. We find that neutrino flavor transformations impact the composition and structure of the remnant, potentially leaving an imprint on the postmerger gravitational-wave signal. They also have a significant impact on the composition and nucleosynthesis yields of the ejecta.

Self-injection locking (SIL) of semiconductor lasers to on-chip microcavities enables significant laser noise purification and diverse nonlinear optical actions. Realizing nonlinear SIL in new material platforms is essential for advancing photonic integrated circuits. Here, we demonstrate nonlinear SIL in AlN microcavities that generates stimulated Raman lasers (SRLs) and microcombs. We achieve SRL emission with an output power exceeding 10 mW and a fundamental linewidth below 70 Hz in the 1750 nm band. The Kerr effect further mediates stimulated emissions at the second-Stokes and anti-Stokes frequencies. Multi-timescale thermal relaxations during turnkey SIL enable GHz-level frequency sweeps of the SRL and pump. Raman actions also render a Stokes platicon microcomb state with coemission in the pump and Stokes bands. Hybrid-integrated crystalline microresonators can be a versatile platform to investigate nonlinear photon-phonon interactions.

Ultrafast light-driven strongly correlated antiferromagnetic insulators, such as prototypical NiO with a large Mott energy gap $\simeq 4\mathrm{eV}$, have recently attracted experimental attention using photons of both subgap [H. Qiu et al., Nat. Phys. 17, 388 (2021)] and above-gap energy [K. Gillmeister et al., Nat. Commun. 11, 4095 (2020)]. In the former context, which is also of great interest to applications, emission of terahertz (THz) radiation is observed from NiO/Pt bilayers, where heavy metal (HM) Pt introduces strong spin-orbit coupling (SOC) effects. However, in contrast to amply studied spintronic THz emitters using femtosecond laser pulse (fsLP)-driven FM/HM (where FM represents a ferromagnetic metal of the conventional type, such as Fe, Ni, or Co) bilayers, where ultrafast demagnetization takes place and is directly related to THz emission, microscopic mechanisms of electromagnetic (EM) radiation from NiO/HM bilayers remain obscure, as the total magnetization of NiO is zero before fsLP application. We employ the two-orbital Hubbard-Hund-Heisenberg model and study, via numerically exact quantum many-body methods, the dynamics of its Néel vector and nonequilibrium magnetization. This reveals nonclassical (i.e., not describable by the Landau-Lifshitz equation) dynamics of Néel vector and nonequilibrium magnetization, changing only in length while not rotating, where the former is substantially reduced in the case of above-gap fsLPs. Additionally, we compute EM radiation by time dependence of magnetization or of local charge currents, finding that both contributions are significant in the THz frequency range only in NiO with proximity SOC introduced by the HM layer. Outside the THz range, we find an integer high-harmonic generation, as well as unusual noninteger harmonics for the above-gap fsLP pump.

Controlling the spontaneous emission of nanoscale quantum emitters (QEs) is crucial for developing advanced photon sources required in many areas of modern nanophotonics, including quantum information technologies. Conventional approaches to shaping photon emission are based on using bulky configurations, while approaches recently developed in quantum metaphotonics suffer from limited capabilities in achieving desired polarization states and directionality, failing to provide on-demand photon sources tailored precisely to technological needs. Here, we propose a universal approach to designing versatile photon sources using on-chip QE-coupled meta-optics that enable direct transformations of QE-excited surface plasmon polaritons into spatially propagating photon streams with arbitrary polarization states, directionality, and amplitudes via both resonance and geometric phases supplied by scattering meta-atoms. Leveraging this platform, we experimentally demonstrate the independent engineering of polarization states, directionality, and relative amplitudes of photon emission in single and multiple radiation channels. The developed universal approach represents a major step toward the realization of versatile quantum sources, highlighting the potential of integrated photon sources with superior properties for both classical and quantum information technologies.

We present a method for probing the quantum capacitance associated with the Rydberg transition of surface electrons on liquid helium using radio-frequency (rf) reflectometry. Resonant microwave excitation of the Rydberg transition induces a redistribution of image charges on capacitively coupled electrodes, giving rise to a quantum capacitance originating from adiabatic state transitions and the finite curvature of the energy bands. By applying frequency-modulated resonant microwaves to drive the Rydberg transition, we systematically measured a capacitance sensitivity of $0.34\mathrm{aF}/\sqrt{\mathrm{Hz}}$ for our rf reflectometry scheme. This sensitivity is sufficient to detect the Rydberg transition of a single electron, offering a scalable pathway toward qubit readout schemes based on surface electrons on helium.

The Dark Energy Spectroscopic Instrument (DESI) is a massively parallel spectroscopic survey on the Mayall telescope at Kitt Peak, which has released measurements of baryon acoustic oscillations determined from over 14 million extragalactic targets. We combine DESI Data Release 2 with CMB datasets to search for evidence of matter conversion to dark energy (DE), focusing on a scenario mediated by stellar collapse to cosmologically coupled black holes (CCBHs). In this physical model, which has the same number of free parameters as $\mathrm{\Lambda }\mathrm{CDM}$, DE production is determined by the cosmic star formation rate density (SFRD), allowing for distinct early- and late-time cosmologies. Using two SFRDs to bracket current observations, we find that the CCBH model: accurately recovers the cosmological expansion history, agrees with early-time baryon abundance measured by BBN, reduces tension with the local distance ladder, and relaxes constraints on the summed neutrino mass $\sum m_{\nu }$. For these SFRDs, we find a peaked positive $\sum m_{\nu }<0.149\mathrm{eV}$ (95% confidence) and $\sum m_{\nu }={0.106}_{-0.069}^{+0.050}\mathrm{eV}$, respectively, in good agreement with lower limits from neutrino oscillation experiments. A peak in $\sum m_{\nu }>0$ results from late-time baryon consumption in the CCBH scenario and is expected to be a general feature of any model that converts sufficient matter to dark energy during and after reionization.

This Letter presents an investigation of low-energy electron-neutrino interactions in the Fermilab Booster Neutrino Beam by the MicroBooNE experiment, motivated by the excess of electron-neutrino-like events observed by the MiniBooNE experiment. This is the first measurement to use data from all five years of operation of the MicroBooNE experiment, corresponding to an exposure of $1.11\times {10}^{21}$ protons on target, a 70% increase on past results. Two samples of electron neutrino interactions without visible pions are used, one with visible protons and one without any visible protons. The MicroBooNE data show reasonable agreement with the nominal prediction, with $p$ values $\ge 26.7\%$ when the two ${\nu }_e$ samples are combined, though the prediction exceeds the data in limited regions of phase space. The data are further compared to two empirical models that modify the predicted rate of electron-neutrino interactions in different variables in the simulation to match the unfolded MiniBooNE low energy excess. In the first model, this unfolding is performed as a function of electron neutrino energy, while the second model aims to match the observed shower energy and angle distributions of the MiniBooNE excess. This measurement excludes an electronlike interpretation of the MiniBooNE excess based on these models at $>99\%{\mathrm{CL}}_{\mathrm{s}}$ in all kinematic variables.

Magnon-phonon hybridization in ordered materials is a crucial phenomenon with significant implications for spintronics, magnonics, and quantum materials research. We present direct experimental evidence and theoretical insights into magnon-phonon coupling in ${\mathrm{Mn}}_3\mathrm{Ge}$, a kagome antiferromagnet with noncollinear spin order. Using inelastic x-ray scattering and ab initio modeling, we uncover strong hybridization between planar spin fluctuations and transverse optical phonons, resulting in a large hybridization gap of $\sim 2\mathrm{meV}$. This coupling is driven by interlayer Heisenberg exchange interactions and is enhanced by the material’s symmetry and magnetic frustration. The simplicity of the ${\mathrm{Mn}}_3\mathrm{Ge}$ structure enables clear identification of the hybridized modes, bridging theoretical predictions and experimental observations. Our findings establish ${\mathrm{Mn}}_3\mathrm{Ge}$ as a model system for exploring magnon-phonon interactions and offer a pathway for designing materials with tunable magnetoelastic properties.

We present the first results from the Quantum Resolution-Optimized Cryogenic Observatory for Dark matter Incident at Low Energy (QROCODILE). The QROCODILE experiment uses a microwire-based superconducting nanowire single-photon detector (SNSPD) as a target and sensor for dark matter scattering and absorption, and is sensitive to energy deposits as low as 0.11 eV. We introduce the experimental configuration and report new world-leading constraints on the interactions of sub-MeV dark matter particles with masses as low as 30 keV. The thin-layer geometry of the system provides anisotropy in the interaction rate, enabling directional sensitivity. In addition, we leverage the coupling between phonons and quasiparticles in the detector to simultaneously constrain interactions with both electrons and nucleons. We discuss the potential for improvements to both the energy threshold and effective volume of the experiment in the coming years.

We have observed the signatures of valence electron rearrangement in photoexcited ammonia using ultrafast hard x-ray scattering. Time-resolved x-ray scattering is a powerful tool for imaging structural dynamics in molecules because of the strong scattering from the core electrons localized near each nucleus. Such core-electron contributions generally dominate the differential scattering signal, masking any signatures of rearrangement in the chemically important valence electrons. Ammonia represents an exception to the typically high core-to-valence electron ratio. We measured 9.8 keV x-ray scattering from gas-phase deuterated ammonia following photoexcitation via a 200 nm pump pulse to the 3s Rydberg state. We observed changes in the recorded scattering patterns due to the initial photoexcitation and subsequent deuterium dissociation. Ab initio calculations confirm that the observed signal is sensitive to the rearrangement of the single photoexcited valence electron as well as the interplay between adiabatic and nonadiabatic dissociation channels. The use of ultrafast hard x-ray scattering to image the structural rearrangement of single valence electrons constitutes an important advance in tracking valence electronic structure in photoexcited atoms and molecules.

Matter waves have been observed in double-slit experiments with microscopic objects, such as atoms or molecules. The wave function describing the motion of these objects must extend over a distance comparable to the slit separation, much larger than the characteristic size of the objects. Preparing such states for more massive objects, such as mechanical oscillators, remains an outstanding challenge. Here we delocalize the quantum ground state of an optically levitated nanosphere by modulating the stiffness of the confining potential. We show a more than threefold increase of the initial coherence length, which corresponds to mechanical momentum squeezing of more than 7 dB. Our work is a stepping stone toward the generation of coherence lengths comparable to the object size, a crucial regime for macroscopic quantum experiments.

Magnetic heat capacity measurements of a high-quality single crystal of the dipole-octupole pyrochlore ${\mathrm{Ce}}_2{\text{Hf}}_2{\mathrm{O}}_7$ down to a temperature of $T=0.02\mathrm{K}$ are reported. These show a two-peaked structure, with a Schottky-like peak at $T_1\sim 0.065\mathrm{K}$, similar to what is observed in its sister Ce pyrochlores ${\mathrm{Ce}}_2{\text{Zr}}_2{\mathrm{O}}_7$ and ${\mathrm{Ce}}_2{\text{Sn}}_2{\mathrm{O}}_7$. However, a second sharper peak is observed at $T_2\sim 0.025\mathrm{K}$, signifying the entrance to the ground state. The ground state appears to have gapped excitations, as even the most abrupt extrapolation to $C_P=0$ at $T=0\mathrm{K}$ fully accounts for the $R\ln (2)$ entropy associated with the pseudospin-$1/2$ doublet for ${\mathrm{Ce}}^{3+}$ in this environment. The ground state could be conventionally ordered, although theory predicts a much larger anomaly in $C_P$ at much higher temperatures than the measured $T_2$ for expectations from an all-in, all-out ground state of the XYZ Hamiltonian for ${\mathrm{Ce}}_2{\text{Hf}}_2{\mathrm{O}}_7$. The sharp low-temperature peak could also signify a crossover from a classical spin liquid to a quantum spin liquid (QSL). For both scenarios, comparison of the measured $C_P$ with NLC calculations suggests that weak interactions beyond the nearest-neighbor XYZ Hamiltonian become relevant below $T\sim 0.25\mathrm{K}$. The diffuse magnetic neutron scattering observed from ${\mathrm{Ce}}_2{\text{Hf}}_2{\mathrm{O}}_7$ at low temperatures between $T_2$ and $T_1$ resembles that observed from ${\mathrm{Ce}}_2{\text{Zr}}_2{\mathrm{O}}_7$, which is well established as a $\pi$-flux quantum spin ice (QSI). Together with the peak in the heat capacity at $T_2$, this diffuse scattering from ${\mathrm{Ce}}_2{\text{Hf}}_2{\mathrm{O}}_7$ is suggestive of a classical spin liquid regime above $T_2$ that is distinct from the zero-entropy quantum ground state below $T_2$.

Self-propulsion plays a crucial role in biological processes and nanorobotics, enabling small systems to move autonomously in noisy environments. Here, we theoretically demonstrate that a bound skyrmion-skyrmion pair in a synthetic antiferromagnetic bilayer can function as a self-propelled topological object, reaching speeds of up to a hundred million body lengths per second—far exceeding those of any known synthetic or biological self-propelled particles. The propulsion mechanism is triggered by the excitation of back-and-forth relative motion of the skyrmions, which generates nonreciprocal gyrotropic forces, driving the skyrmion pair in a direction perpendicular to their bond. Remarkably, thermal noise induces spontaneous reorientations of the pair and momentary reversals of the propulsion, mimicking behaviors observed in motile bacteria and microalgae.

Coupling with a resonant optical cavity is well known to modify the coherence of molecular vibrations. However, in the case of molecules coupled to a plasmonic nanocavity mode, the local mechanisms of vibrational coherence decay remain unclear. Here, the dynamics of a few hundred molecules of nitrothiophenol (NTP) within a single plasmonic nanocavity are studied by sum-frequency generation. Time-resolved experiments reveal a coherence lifetime sensitive to intermolecular dipole coupling strength, and is found to compete with cavity-induced vibrational dephasing above a threshold $\sim 10{\mathrm{cm}}^{-1}$. Tuning of the system dipole strength presents a quantitative means to explore the energy landscape of coherent interactions within nanoscale vibrational platforms, relevant for studies from vibrational chemistry to cavity quantum optomechanics.

Single crystal x-ray diffraction measurements have been carried out on epsilon oxygen up to 30 GPa to examine the behavior of the constituent $({\mathrm{O}}_2{)}_4$ units. An isostructural phase transition is evidenced by lattice parameter and intracluster (${\mathrm{O}}_8$) distance discontinuities and clear changes in the equation of state at $18.1\pm 0.5\mathrm{GPa}$ on compression. This transition corresponds well to the predicted collapse of the molecular magnetic moment from spin-liquid, ${\epsilon }_1$, to a spinless, ${\epsilon }_0$, states of the ${\mathrm{O}}_8$ structure. The collapse of the molecular magnetic moment has to date only been supported by spectroscopic data and, until now, has not been substantiated structurally.

Femtosecond laser excitation of nanometer thin heterostructures comprising a heavy metal and a magnetically ordered material is known to result in the emission of terahertz radiation. However, the nature of the emitted radiation from heavy metal/antiferromagnet heterostructures has sparked debates and controversies in the literature. Here, we unambiguously separate spin and charge contributions from Pt/NiO heterostructures by introducing an unprecedented methodology combining high external magnetic fields with a symmetry analysis of the emitted terahertz polarization. We observe two distinct mechanisms of terahertz emission, which we identify as optical difference frequency generation and ultrafast laser-induced quenching of the magnetization. We emphasize the absence of spin transport effects and signatures of coherent magnons. Overall, our Letter provides a general experimental methodology to separate spin and charge contributions to the laser-induced terahertz emission from heterostructures comprising a magnetically ordered material, thus holding great potential for advancing terahertz spintronics and establishing terahertz orbitronics.

Assembling increasingly larger-scale defect-free optical-tweezer-trapped atom arrays is essential for quantum computation and quantum simulations based on atoms. Here, we propose an AI-enabled, rapid, constant-time-overhead rearrangement protocol, and we experimentally assemble defect-free 2D and 3D atom arrays with up to 2024 atoms with a constant-time cost of 60 ms. The AI model calculates the holograms for real-time atom rearrangement. With precise controls over both position and phase, a high-speed spatial light modulator moves all the atoms simultaneously. This protocol can be readily used to generate defect-free arrays of tens of thousands of atoms with current technologies and become a useful toolbox for quantum error correction.