Tunneling ionization, a fascinating quantum phenomenon, has played the key role in the development of attosecond physics. Upon absorption of a few tens of photons, tunneling ionization creates ions in different excited states and even enables the formation of population inversion between ionic states. However, the underlying physics is still being debated. Here, we demonstrate a significant enhancement in the efficiency of multiphoton excitation when ionization of neutral molecules and resonant excitation of ions coexist in strong laser fields. It facilitates the dramatic increase in population inversion and lasing radiation in ${\mathrm{N}}_2^{+}$ around 1000 nm pump wavelength. Utilizing the ionization-coupling theory, we discover that the synergistic interplay between tunneling ionization and multiphoton excitation enables the ionic coherence to be maximized by phase locking of the periodically created ionic dipoles and consistently maintain an optimal phase for the follow-up photoexcitation. This Letter provides new insights into the photoexcitation mechanism of ions in strong laser fields and opens up a route for optimizing ionic lasing radiations.

The searches for $CP$ violating effects in diatomic molecules, such as ${\mathrm{HfF}}^{+}$ and ThO, are typically interpreted as a probe of the electron’s electric dipole moment ($e\mathrm{EDM}$), a new electron-nucleon interaction, and a new electron-electron interaction. However, in the case of a nonvanishing nuclear spin, a new $CP$ violating nucleon-nucleon long-range force will also affect the measurement, providing a new interpretation of the $e\mathrm{EDM}$ experimental results. Here, we use the ${\mathrm{HfF}}^{+}$ $e\mathrm{EDM}$ search and derive a new bound on this hypothetical interaction, which is the most stringent from terrestrial experiments in the 1 eV–10 keV mass range. These multiple new physics sources motivate independent searches in different molecular species for $CP$ violation at low energy that result in model independent bounds, which are insensitive to cancellation among them.

We extracted the molecular-frame elastic differential cross sections (MFDCSs) for electrons scattering from ${\mathrm{N}}_2^{+}$ based on elliptical laser-induced electron diffraction (ELIED), wherein the structural evolution is initialized by the same tunneling ionization and probed by incident angle-resolved laser-induced electron diffraction imaging. To establish ELIED, an intuitive interpretation of the ellipticity-dependent rescattering electron momentum distributions was first provided by analyzing the transverse momentum distribution. It was shown that the incident angle of the laser-induced returning electrons could be tuned within 20° by varying the ellipticity and handedness of the driving laser pulses. Accordingly, the incident angle-resolved DCSs of returning electrons for spherically symmetric targets (${\mathrm{Xe}}^{+}$ and ${\mathrm{Ar}}^{+}$) were successfully extracted as a proof-of-principle for ELIED. The MFDCSs for ${\mathrm{N}}_2^{+}$ were experimentally obtained at incident angles of 4° and 7°, which were well reproduced by the simulations. The ELIED approach is the only successful method so far for obtaining incident angle-resolved ionic MFDCS, which provides a new sensitive observable for the transient structure retrieval of ${\mathrm{N}}_2^{+}$. Our results suggest that the ELIED has the potential to extract the structural tomographic information of polyatomic molecules with femtosecond and subangstrom spatiotemporal resolutions that can enable the visualization of the nuclear motions in complex chemical reactions as well as chiral recognition.

We report the electromagnetically induced transparency (EIT) cooling of ${}^{137}{\mathrm{Ba}}^{+}$ ions with a nuclear spin of $I=3/2$, which are a good candidate of qubits for future large-scale trapped-ion quantum computing. EIT cooling of atoms or ions with a complex ground-state level structure is challenging due to the lack of an isolated $\mathrm{\Lambda }$ system, as the population can escape from the $\mathrm{\Lambda }$ system to reduce the cooling efficiency. We overcome this issue by leveraging an EIT pumping laser to repopulate the cooling subspace, ensuring continuous and effective EIT cooling. We cool the two radial modes of a single ${}^{137}{\mathrm{Ba}}^{+}$ ion to average motional occupations of 0.08(5) and 0.15(7), respectively. Using the same laser parameters, we also cool all the ten radial modes of a five-ion chain to near their ground states. Our approach can be adapted to atomic species possessing similar level structures. It allows engineering of the EIT Fano-like spectrum, which can be useful for simultaneous cooling of modes across a wide frequency range, aiding in large-scale trapped-ion quantum information processing.

Quantum mechanics places noise limits and sensitivity restrictions on physical measurements. The balance between unwanted backaction and the precision of optical measurements imposes a standard quantum limit (SQL) on interferometric systems. In order to realize a sensitivity below the SQL, it is necessary to leverage a backaction evading measurement technique, reduce thermal noise to below the level of backaction, and exploit cancellations of any excess noise contributions at the detector. Many proof of principle experiments have been performed, but only recently has an experiment achieved sensitivity below the SQL. In this work, we extend that initial demonstration and realize sub-SQL sensitivity nearly two times better than previous measurements, and with an architecture applicable to interferometric gravitational wave detectors. In fact, this technique is directly applicable to Advanced LIGO, which could observe similar effects with a detuned signal recycling cavity. We measure a total sensitivity below the SQL by 2.8 dB, corresponding to a reduction in the noise power by $72\pm 5.1\%$ below the quantum limit. Through the use of a detuned cavity and the optical spring effect, this noise reduction is tunable, allowing us to choose the desired range of frequencies that fall below the SQL. This result demonstrates access to sensitivities well below the SQL at frequencies applicable to LIGO, with the potential to extend the reach of gravitational wave detectors further into the Universe.

Many active systems display nematic order, while interacting with their environment. In this Letter, we show theoretically how environment-stored memory acts an effective external field that aligns active nematics. The coupling to the environment leads to substantial modifications of the known phase diagram and dynamics of active nematics, including nematic order at arbitrarily low densities and arrested domain coarsening. We are motivated mainly by cells that remodel fibers in their extra-cellular matrix (ECM), while being directed by the fibers during migration. Our predictions indicate that remodeling promotes cellular and ECM alignment, and possibly limits the range of ordered ECM domains, in accordance with recent experiments.

We develop a generic geometric formalism that incorporates both $T\overline{T}$-like and root-$T\overline{T}$-like deformations in arbitrary dimensions. This framework applies to a wide family of stress-energy tensor perturbations and encompasses various well-known field theories. Building upon the recently proposed correspondence between Ricci-based gravity and $T\overline{T}$-like deformations, we further extend this duality to include root-$T\overline{T}$-like perturbations. This refinement extends the potential applications of our approach and contributes to a deeper exploration of the interplay between stress tensor perturbations and gravitational dynamics. Among the various original outcomes detailed in this Letter, we have also obtained a deformation of the flat Jackiw-Teitelboim gravity action.

One-dimensional Bose gases with contact repulsive interactions are characterized by the presence of infinite-lifetime quasiparticles whose momenta are called the “rapidities.” Here, we develop a probe of the local rapidity distribution, based on the fact that rapidities are the asymptotic momenta of the particles after a long one-dimensional expansion. This is done by performing an expansion of a selected slice of the gas. We first apply this idea to a cloud in the quasicondensate regime at equilibrium in a trap. We obtain an experimental picture of the position-dependent rapidity distribution which is in fair agreement with the theory prediction. The asymptotic regime is barely reached, but we show that finite expansion time can be taken into account using the generalized hydrodynamics theory. We then apply this local probe to an out-of-equilibrium situation where the local rapidity distribution is expected to be doubly peaked—a hallmark of a nonthermal state—even though the global rapidity distribution would possess no such distinctive feature. We observe the doubly peaked local rapidity distribution.

Near nonreciprocal media at finite temperature, fluctuating near fields exhibit imbalanced thermal populations in opposite directions, generating equilibrium topological thermal fluxes that circulate the media. While the existence of these fluxes remains unconfirmed, we propose exploiting their interaction with free electron beams for detection. We establish a general framework to quantify thermal flux at any location near an object of arbitrary shape. This reveals unexplored properties of thermal flux spectra depending on their orientation. Further, we connect the electron scattering rate to the equilibrium thermal flux. As a specific example, electrons encountering a planar surface’s perpendicular thermal flux preferentially scatter transversely. This measurable scattering distribution, i.e., via angle-resolved electron microscopy, allows us to recover the thermal flux spectrum. Additionally, electron interactions with equilibrium thermal fluxes surrounding local structures offer a novel approach to generating electron vortex beams.

Electronic orders such as charge density wave (CDW) and superconductivity raise exotic physics and phenomena as evidenced in recently discovered kagome superconductors and transition metal chalcogenides. In most materials, CDW induces a weak, perturbative effect, manifested as shadow bands, minigaps, resistivity kinks, etc. Here we demonstrate a unique example—transition metal tetratellurides ${\mathrm{TaTe}}_4$, in which the CDW order dominates the electronic structure and transport properties. Using angle-resolved photoemission spectroscopy, we found that the band structure of CDW ${\mathrm{TaTe}}_4$ is characterized by small, bulk electron pockets. Density functional theory analyses reveal their CDW origin from the folding of the original, large Fermi pockets. Importantly, the CDW induced pockets result in prominent frequencies in the quantum oscillation of the magnetoresistance. Satisfactory agreements are reached between results from photoemission spectroscopy, density functional theory, and quantum oscillation, concerning the shape, size, location, and angle dependence of the CDW pockets. Our results underline transition metal tetratellurides as an outstanding example for exploring the interplay between CDW, pressure induced superconductivity, and potential topological states under strong field.

Despite enormous efforts devoted to the study of the many-body localization (MBL) phenomenon, the nature of the high-energy behavior of the Heisenberg spin chain in a strong random magnetic field is lacking consensus. Here, we take a step back by exploring the weak interaction limit starting from the Anderson localized (AL) insulator. Through shift-invert diagonalization, we find that, below a certain disorder threshold $h^{*}$, weak interactions necessarily lead to an ergodic instability, whereas at strong disorder the AL insulator directly turns into MBL, in agreement with a simple interpretation of the avalanche theory for restoration of ergodicity. We further map the phase diagram for the generic $XXZ$ model in the disorder $h$–interaction $\mathrm{\Delta }$ plane. Taking advantage of the total magnetization conservation, our results unveil the remarkable behavior of the spin-spin correlation functions: in the regime indicated as MBL by standard observables, their exponential decay undergoes an inversion of orientation ${\xi }_z>{\xi }_x$. We find that the longitudinal length ${\xi }_z$ is a key quantity for capturing ergodic instabilities, as it increases with system size near the thermal phase, in sharp contrast to its transverse counterpart.

Topological modes (TMs) are typically localized at boundaries, interfaces and dislocations, and exponentially decay into the bulk of a large enough lattice. Recently, the non-Hermitian skin effect has been leveraged to delocalize the wave functions of TMs from the boundary and thus to increase the capacity of TMs dramatically. Here, we explore the capability of nonlinearity in designing and configuring the wave functions of TMs. With growing intensity, wave functions of these in-gap nonlinear TMs undergo an initial deviation from exponential decay, gradually merge into arbitrarily designable plateaus, then encompass the entire nonlinear domain, and eventually concentrate at the nonlinear boundary. Intriguingly, such extended nonlinear TMs are still robust against defects and disorders, and stable in dynamics under external excitation. Advancing the conceptual understanding of the nonlinear TMs, our results open new avenues for increasing the capacity of TMs and developing compact and configurable topological devices.

Within the Landau-Ginzburg picture of phase transitions, scalar field theories develop phase separation because of a spontaneous symmetry-breaking mechanism. This picture works in thermodynamics but also in the dynamics of phase separation. Here we show that scalar nonequilibrium field theories undergo phase separation just because of nonequilibrium fluctuations driven by a persistent noise. The mechanism is similar to what happens in motility-induced phase separation where persistent motion introduces an effective attractive force. We observe that noise-induced phase separation occurs in a region of the phase diagram where disordered field configurations would otherwise be stable at equilibrium. Measuring the local entropy production rate to quantify the time-reversal symmetry breaking, we find that such breaking is concentrated on the boundary between the two phases.

From correlations in measurement outcomes alone, can two otherwise isolated parties establish whether such correlations are atemporal? That is, can they rule out that they have been given the same system at two different times? Classical statistics says no, yet quantum theory disagrees. Here, we introduce the necessary and sufficient conditions by which such quantum correlations can be identified as atemporal. We demonstrate the asymmetry of atemporality under time reversal and reveal it to be a measure of spatial quantum correlation distinct from entanglement. Our results indicate that certain quantum correlations possess an intrinsic arrow of time and enable classification of general quantum correlations across space-time based on their (in)compatibility with various underlying causal structures.

Axions and axionlike particles are strongly motivated dark-matter candidates that are the subject of many current ground based dark-matter searches. We present first results from the Axion Dark-Matter Birefringent Cavity (ADBC) experiment, which is an optical bow-tie cavity probing the axion-induced birefringence of electromagnetic waves. Our experiment is the first optical axion detector that is tunable and quantum noise limited, making it sensitive to a wide range of axion masses. We have iteratively probed the axion mass ranges $40.9–43.3\mathrm{neV}/{\mathrm{c}}^2$, $49.3–50.6\mathrm{neV}/{\mathrm{c}}^2$, and $54.4–56.7\mathrm{neV}/{\mathrm{c}}^2$, and found no dark-matter signal. On average, we constrain the axionlike particle and photon coupling at the level $g_{a\gamma \gamma }\le 1.9\times {10}^{-8}{\mathrm{GeV}}^{-1}$. We also present prospects for future axion dark-matter detection experiments using optical cavities.

We apply a formulation of Einstein’s general relativity with only cubic interactions for deriving the metric of a Schwarzschild black hole to all orders in perturbation theory. This cubic interactions formulation coupled to effective worldline action of a massive point particle allows to derive a recursion relation for the form factors of the off-shell graviton emission current. The unique solution to the recursion relation leads to the Schwarzschild black-hole solution in four dimensions. This provides the first derivation of the black hole metric from a matter source to all orders in perturbation theory from an amplitude approach.

The NA62 experiment at CERN, configured in beam-dump mode, has searched for dark photon decays in flight to electron-positron pairs using a sample of $1.4\times {10}^{17}$ protons on dump collected in 2021. No evidence for a dark photon signal is observed. The combined result for dark photon searches in lepton–antilepton final states is presented and a region of the parameter space is excluded at 90% confidence level, improving on previous experimental limits for dark photon mass values between 50 and $600\mathrm{MeV}/c^2$ and coupling values in the range ${10}^{-6}$ to $4\times {10}^{-5}$. An interpretation of the $e^{+}{e}^{-}$ search result in terms of the emission and decay of an axionlike particle is also presented.

Quasi-static transformations, or slow quenches, of many-body quantum systems across quantum critical points generate topological defects. The Kibble-Zurek mechanism regulates the appearance of defects in a local quantum system through a classical combinatorial process. However, long-range interactions disrupt the conventional Kibble-Zurek scaling and lead to a density of defects that is independent of the rate of the transformation. In this Letter, we analytically determine the complete full counting statistics of defects generated by slow annealing a strong long-range system across its quantum critical point. We demonstrate that the mechanism of defect generation in long-range systems is a purely quantum process with no classical equivalent. Furthermore, universality is not only observed in the defect density but also in all the moments of the distribution. Our findings can be tested on various experimental platforms, including Rydberg gases and trapped ions.

Single-photon nonlinearity, namely, the change in the response of the system as the result of the interaction with a single photon, is generally considered an inherent property of a single quantum emitter. Although the dependence on the number of emitters is well understood for the case of two-level systems, deterministic operations such as single-photon switching or photon-atom gates inherently require more complex level structures. Here, we theoretically consider single-photon switching in ensembles of emitters with a $\mathrm{\Lambda }$-level scheme and show that the switching efficiency vanishes with the number of emitters. Interestingly, the mechanism behind this behavior is the quantum Zeno effect, manifested in a slowdown of the photon-controlled dynamics of the atomic ground states.