Optoelectronic metadevices control and enhance light-matter interactions by modulating the local optical field that interacts with emitters. However, the optical property of the highly localized mode in the plasmonic metasurface is sensitive to local disorders arising from experimental fluctuations. Recently, the nonlocal mode of metasurfaces has been attracting growing interest because the coupling within the array of plasmonics results in long-range ordering and collective resonances. Here we demonstrate the topological edge states and the nonlocal modes of Au bow ties in a photonic crystal (PHC) lattice. The strong coupling between plasmonic modes and PHC bands opens the topological band gap at the edge of the Brillouin zone and forms the nonlocal mode having a resonant energy insensitive to fluctuations in Au length, resulting in a robust Purcell effect for quantum dots (QDs) coupling with the nonlocal mode in metadevices. The topological band gap and robust enhancement indicate great potential for the nonlocal plasmonic metasurface in quantum photonic devices with novel functionalities.

Here we report the experimental demonstration of the nonlinear non-Hermitian skin effect (NHSE) in an effective Kerr nonlinear temporal photonic lattice, where the high-power requirements and lack of tunability intrinsic to optical materials are overcome by an artificial nonlinearity arising from optoelectronic feedforward. Thanks to Kerr self-trapping, the nonlinear NHSE is demonstrated to possess much better localization strength and robustness at the preferred boundary compared to the linear case. Away from the preferred boundary, Kerr self-trapping can even inhibit NHSE-induced transport and form stable skin solitons. Harnessing the nonlinearity-controlled NHSE, we judiciously design an optical router with a flexibly tuned output port. Our findings promise great applications in robust signal transmission, routing, and processing.

A highly charged muonic ion is a unique few-body atomic system where a negatively charged muon and a few electrons are simultaneously bound to a single nucleus. We report the first state-selective observation of highly charged muonic Ar ($\mu \mathrm{Ar}$) by electronic $K$ x-ray spectroscopy using an array of transition-edge sensor microcalorimeters. The high-precision $K$ x-ray spectra provide a clear signature of the presence of muonic atoms with one, two, and three electrons, i.e., H-like, He-like, and Li-like $\mu \mathrm{Ar}$. With the aid of theoretical calculations, we confirmed that the peak positions are consistent with the x-ray energies from highly charged Cl ions, and the intensities reflect deexcitation dynamics of highly charged $\mu \mathrm{Ar}$.

We investigate the formation and dynamics of Jones-Roberts solitons in a smoothly inhomogeneous quantum fluid. To do so, we create a superfluid of light using a paraxial, near-resonant laser beam propagating through a hot rubidium vapor. We excite a bounded vortex-antivortex dipole in the superfluid and observe its transition to a rarefaction pulse and back, in agreement with the seminal predictions of Jones and Roberts. Employing an analogy with ray optics, we calculate the trajectory of the interacting vortices, deriving an effective refractive index from the inhomogeneous fluid density. Finally, we examine analytically and experimentally the superfluid velocity correlations, observing a transfer of coherence from incompressible to compressible velocity of the quantum fluid, a direct signature of the dynamical conversion between vortices and the rarefaction pulse.

Nonlinearities in King plots (KP) of isotope shifts (IS) can reveal the existence of beyond-standard-model (BSM) interactions that couple electrons and neutrons. However, it is crucial to distinguish higher-order standard model (SM) effects from BSM physics. We measure the IS of the transitions ${{}^{3}P}_0\to {{}^{3}P}_1$ in ${\mathrm{Ca}}^{14+}$ and ${{}^{2}S}_{1/2}\to {}^2{D}_{5/2}$ in ${\mathrm{Ca}}^{+}$ with sub-Hz precision as well as the nuclear mass ratios with relative uncertainties below $4\times {10}^{-11}$ for the five stable, even isotopes of calcium (${}^{40,42,44,46,48}\mathrm{Ca}$). Combined, these measurements yield a calcium KP nonlinearity with a significance of $\sim {10}^3\sigma$. Precision calculations show that the nonlinearity cannot be fully accounted for by the expected largest higher-order SM effect, the second-order mass shift, and identify the little-studied nuclear polarization as the only remaining SM contribution that may be large enough to explain it. Despite the observed nonlinearity, we improve existing KP-based constraints on a hypothetical Yukawa interaction for most of the new boson masses between $10\mathrm{eV}/{\mathrm{c}}^2$ and ${10}^7\mathrm{eV}/{\mathrm{c}}^2$.

Two-dimensional Dirac semimetals feature flatband edge states but a zero Chern number, while Chern insulators support one-way edge states associated with a nonzero Chern number. Here, we demonstrate a two-dimensional photonic crystal combining the response of a semimetal and of a Chern insulator, termed a Chern semimetal. This photonic semimetal is characterized by a nonzero Chern number, simultaneously hosting both a flatband edge state and a one-way edge state. We experimentally realize such a photonic Chern semimetal using a gyromagnetic photonic crystal with programmable magnetic bias, tailoring its second-neighbor coupling within each sublattice. We also verify the ability to control over the light speed while preserving topological protection. Our findings open new avenues toward exploring topological phases and their applications across fermionic and bosonic systems, providing an exciting platform for nanophotonic applications and a playground for topological physics discoveries.

We perform three-photon photoassociation to obtain high resolution spectra of ${}^{87}\mathrm{Rb}$ trilobite dimers for the principal quantum numbers $n=22$, 24, 25, 26, and 27. The large binding energy of the molecules in combination with a relative spectroscopic resolution of ${10}^{-4}$ provides a rigorous benchmark for existing theoretical models. A recently developed Green’s function framework, which circumvents the convergence issues that afflicted previous studies, is employed to theoretically reproduce the vibrational spectrum of the molecule with high accuracy. The relatively large molecular binding energy is primarily determined by the low-energy $S$-wave electron-atom scattering length, thereby allowing us to extract the ${}^3{S}_1$ scattering phase shift with unprecedented accuracy, at low-energy regimes inaccessible to free electrons.

Waves propagating through weakly disordered smooth linear media undergo a universal phenomenon called branched flow. Branched flow has been observed and studied experimentally in various systems by considering coherent waves. Recent experiments have reported the observation of optical branched flow by using an incoherent light source, thus revealing the key role of coherent phase-sensitive effects in the development of incoherent branched flow. By considering the paraxial wave equation as a generic representative model, we elaborate a stochastic theory of both coherent and incoherent branched flow. We derive closed-form equations that determine the evolution of the intensity correlation function, as well as the value and the propagation distance of the maximum of the scintillation index, which characterize the dynamical formation of incoherent branched flow. We report accurate numerical simulations that are found in quantitative agreement with the theory without free parameters. Our theory highlights the important impact of coherence and interference on branched flow, thereby providing a framework for exploring branched flow in nonlinear media, in relation to the formation of freak waves in oceans.

The current experimental and theoretical values of the ionization energy of metastable ${}^4\mathrm{He}$ [$(1s)(2s){{}^{3}S}_1$] differ by $9\sigma$, which prevents the determination of the alpha-particle charge radius from spectroscopic measurements in ${}^4\mathrm{He}$. To help clarify the origin of this discrepancy, we report on a precision measurement of hyperfine-resolved transitions from the $(1s)(2s){{}^{3}S}_1$ metastable state of ${}^3\mathrm{He}$ to high $np$ Rydberg states converging on the $F^{+}=0$, 1 hyperfine levels of the ${}^3{\mathrm{He}}^{+}$ $(1s{)}^2{S}_{1/2}$ ground state. Rydberg-series extrapolation using multichannel quantum-defect theory (MQDT) enabled the determination of the ionization energy of the $(1s)(2s){{}^{3}S}_1$ state of ${}^3\mathrm{He}$ [$E_{\mathrm{I}}({}^3\mathrm{He})/h=1152788844.6154(77{)}_{\mathrm{stat}}(25{)}_{\mathrm{sys}}\mathrm{MHz}$] and of the corresponding isotopic shift [$(E_{\mathrm{I}}({}^4\mathrm{He})-E_{\mathrm{I}}({}^3\mathrm{He}))/h=53898.093(9)\mathrm{MHz}$]. The MQDT analysis also permitted the quantification of singlet-triplet mixing in the $np$ series induced by the hyperfine interaction. From the isotopic shift of the ionization energy of He, the difference $\delta r^2$ between the squared charge radii of the helion and alpha particles is determined to be $1.060(10){\mathrm{fm}}^2$.

High-order nonlinear optical processes beyond four-wave mixing (${\chi }^{(3)}$) are crucial for advancing ultraviolet (UV) light sources and quantum technologies, yet their practical implementation remains challenging due to inherently weak high-order nonlinear susceptibilities and stringent broadband phase-matching requirements—limitations that persist even in state-of-the-art high-Q microresonators. A breakthrough transverse multiwave mixing scheme is proposed in a high-Q lithium niobate microresonator, only under a single continuous-wave (cw) laser pump. Our approach leverages self-organized subwavelength photorefractive gratings (SPGs) generated through a bidirectional stimulated Raman scattering (SRS) process in the microresonator, without using two external counterpropagating lasers. Under single-wavelength pumping (1546 nm), bidirectional SRS (1713 nm) spontaneously generates dynamic SPGs that enable dual-function control: (1) broadband momentum compensation ($\mathrm{\Delta }k\approx 2\pi /\mathrm{\Lambda }$, $\mathrm{\Lambda }=\text{grating}$ period) to resolve broadband phase-matching challenges in high-order nonlinear interactions and (2) preservation of ultrahigh-quality factor ($Q>7\times {10}^6$) for enhanced nonlinear conversion. Moreover, unlike conventional longitudinal configurations requiring more rigid multiwavelength resonance alignment, our transverse architecture decouples nonlinear processes from cavity-mode constraints through SPG-mediated momentum engineering, enabling simultaneous support for sum-frequency generation to six-wave mixing processes across 500-nm bandwidth with high conversion efficiencies. Furthermore, the cascaded SRS process is simultaneously activated to generate a light signal for subsequent nonlinear interactions. Consequently, this novel approach enables the first demonstration of single-pump phase-matched transverse SFG with record conversion efficiency ($590\%/\mathrm{W}$), and transverse multiwave mixing processes from ${\chi }^{(3)}$ (four-wave) to ${\chi }^{(5)}$ (six-wave) processes are achieved for the first time using only the single cw pump, leading to efficient visible and UV light generation, representing a notable advance in nonlinear integration.

We propose and experimentally demonstrate a method for laser cooling an oscillator, using a two-level ancillary spin, based on sequences of spin-state-dependent displacements, followed by spin repumping. For a thermal state with mean occupation $\overline{n}\gg 1$, the method attains a reduction to 0.632 of the initial thermal oscillator occupation for two repumps of the spin state. This is within a factor of 2.53 of the optimum that is expected due to the reduction of the oscillator entropy by 2ln(2). We show that the method, which is based on encoding the value of the modular variable of the oscillator into the spin, has a simple semiclassical description in terms of a Bayesian update. We demonstrate the method experimentally using the internal and motional states of a single trapped ion.

Solid-state quantum sensors based on ensembles of nitrogen-vacancy (NV) centers in diamond have emerged as powerful tools for precise sensing applications. Nuclear spin sensors are particularly well suited for applications requiring long coherence times, such as inertial sensing, but remain underexplored due to control complexity and limited optical readout efficiency. In this work, we propose cooperative cavity quantum electrodynamic (cQED) coupling to achieve efficient nuclear spin readout. Unlike previous cQED methods used to enhance electron spin readout, here we employ two-field interference in the NV hyperfine subspace to directly probe the nuclear spin transitions. We model the nuclear spin NV-cQED system (nNV-cQED) and observe several distinct regimes, including electromagnetically induced transparency, masing without inversion, and oscillatory behavior. We then evaluate the nNV-cQED system as an inertial sensor, indicating a rotation sensitivity improved by 3 orders of magnitude compared to previous solid-state spin demonstrations. Furthermore, we show that the NV electron spin can be simultaneously used as a comagnetometer, and the four crystallographic axes of NVs can be employed for vector resolution in a single nNV-cQED system. These results showcase the applications of two-field interference using the nNV-cQED platform, providing critical insights into the manipulation and control of quantum states in hybrid NV systems and unlocking new possibilities for high-performance quantum sensing.

We study plasmonic time crystals, an extension of dielectric-based photonic time crystals to plasmonic media. We demonstrate that such systems may amplify both longitudinal and transverse modes. In particular, we show that plasmonic time crystals support “collective resonances” of longitudinal modes, which occur independent of the wave vector $k$, even in the presence of significant dissipation. These resonances originate from the coupling between the positive- and negative-frequency branches of the plasmonic dispersion relation of the unmodulated system and from the divergence of the density of states near the plasma ($ϵ$-near-zero) frequency ${\omega }_p$. The strongest resonance arises at a modulation frequency $\mathrm{\Omega }=2{\omega }_p$, corresponding to a direct interband transition. We demonstrate these resonances for various periodic modulation profiles and provide a generic perturbative formula for resonance widths in the weak modulation limit. Furthermore, we propose transparent conducting oxides as promising platforms for realizing plasmonic time crystals, as they enable significant modulation of the electron effective mass while maintaining moderate dissipation levels. Our findings provide new insights into leveraging time-modulated plasmonic media to enhance optical gain and control wave dynamics at the nanoscale.

We report the first direct in situ observation of density fluctuations on the scale of the thermal de Broglie wavelength in an ultracold gas of bosons. Bunching of ${}^{87}\mathrm{Rb}$ atoms in a quasi-two-dimensional system is observed by single-atom imaging using a quantum gas microscope. Compared to a classical ensemble, we observe a 30% enhancement of the second-order correlation function. We show the spatial and thermal dependence of these correlations. The reported method of detecting in situ correlations can be applied to interacting many-body systems and to the study of critical phenomena near phase transitions.

We demonstrate atom-resolved detection of itinerant bosonic ${}^{23}\mathrm{Na}$ and fermionic ${}^6\mathrm{Li}$ quantum gases, enabling the direct in situ measurement of interparticle correlations. In contrast to prior work on lattice-trapped gases, here we realize microscopy of quantum gases in the continuum. We reveal Bose-Einstein condensation with single-atom resolution, measure the enhancement of two-particle $g^{(2)}$ correlations of thermal bosons, and observe the suppression of $g^{(2)}$ for fermions; the Fermi or exchange hole. For strongly interacting Fermi gases confined to two dimensions, we directly observe nonlocal fermion pairs in the BEC-BCS crossover. We obtain the pair size and the short-range contact directly from the pair correlations. In situ thermometry is enabled via the fluctuation-dissipation theorem. Our technique opens the door to the atom-resolved study of strongly correlated quantum gases of bosons, fermions, and their mixtures.

Atom-based quantum simulators offer a unique platform that enables the imaging of each particle in a many-body system. Until now, however, this capability has been limited to quantum systems in discretized space such as optical lattices and tweezers, where spatial degrees of freedom are quantized. Here, we introduce a novel method for imaging atomic quantum many-body systems in the continuum, allowing for in situ resolution of every particle. We demonstrate the capabilities of our approach on a two-dimensional atomic Fermi gas. We probe the density correlation functions, resolving their full spatial functional form, and reveal the shape of the Fermi hole arising from Pauli exclusion as a function of temperature. Our method opens the door to probing strongly correlated quantum gases in the continuum with unprecedented spatial resolution, providing in situ access to spatially resolved correlation functions of arbitrarily high order across the entire system.

In this work, we demonstrate a new method for quantum state control and nondestructive internal quantum state readout of a single ${\mathrm{HD}}^{+}$ ion in the rovibrational ground state in a Penning trap. Furthermore, we demonstrate a measurement of the magnetic trapping field with the same ion to sub-ppb uncertainty. This could enable nondestructive, high-precision spectroscopy of the hyperfine, Zeeman, and rovibrational level structure of single molecular ions. In particular, this technique can be applied to ${\mathrm{H}}_2^{+}$ as well as to its antimatter equivalent, ${\overline{\mathrm{H}}}_2^{-}$, with the potential for high-precision tests of charge-parity-time reversal symmetry beyond current sensitivities [Phys. Scr. T 1995, 423 (1995), Phys. Rev. A 98, 010101 (2018)].

Asymmetric energy exchange interactions, also known as Hatano-Nelson type couplings, enable the study of non-Hermitian physics and associated phenomena like the non-Hermitian skin effect and exceptional points (EPs). Since these interactions are by definition nonreciprocal, there have been very few options for implementations in integrated photonics. In this work, we show that asymmetric couplings are readily achievable in integrated photonic systems through time-domain dynamic modulation. We experimentally study this concept using a two-resonator photonic molecule produced in a lithium niobate on insulator platform that is electro-optically modulated by rf stimuli. We demonstrate the dynamic tuning of the Hatano-Nelson coupling between the resonators, surpassing the asymmetry that has been achieved in previous work, to reach an EP for the first time. We are additionally able to flip the relative sign of the couplings for opposite directions by going past the EP. Using this capability, we show that the through-chain transport can be configured to exhibit both giant ($\sim 60\mathrm{dB}$) optical contrast as well as photonic gyration or nonreciprocal $\pi$ phase contrast.

The energy-level degeneracies, also known as exceptional points (EPs), are crucial for comprehending emerging phenomena in materials and enabling innovative functionalities for devices. Since EPs were proposed over half a century ago, only two types of EPs have been experimentally discovered, revealing intriguing phases of materials such as Dirac and Weyl semimetals. These discoveries have showcased numerous exotic topological properties and novel applications, such as unidirectional energy transfer. Here, we report the observation of a novel type of EP, named the Dirac EP, utilizing a nitrogen-vacancy center in diamond. Two of the eigenvalues are measured to be degenerate at the Dirac EP and remain real in its vicinity. This exotic band topology associated with the Dirac EP enables the preservation of the symmetry when passing through, and makes it possible to achieve adiabatic evolution in non-Hermitian systems. We examined the degeneracy between the two eigenstates by quantum state tomography, confirming that the degenerate point is a Dirac EP rather than a Hermitian degeneracy. Our research of the distinct type of EP contributes a fresh perspective on dynamics in non-Hermitian systems and is potentially valuable for applications in quantum control in non-Hermitian systems and the study of the topological properties of EPs.

Exceptional points (EPs) are singularities in the parameter space of a non-Hermitian system where eigenenergies and eigenstates coincide. They hold promise for enhancing sensing applications, but this is limited by the divergence of shot noise near EPs. According to recent studies, EP sensors operating in the nonlinear regime may avoid these limitations. By analyzing an exemplary nonlinear system, we show that the interplay of noise and nonlinearity introduces previously unidentified obstacles to enhanced sensing. The noise effectively displaces the EP in parameter space and reduces its order, thereby eliminating the sought-for divergence in the signal-to-noise ratio. Moreover, the noise near the nonlinear EP experiences a stronger divergence than predicted by standard calculations of the Petermann noise factor, due to the properties of the Bogoliubov–de Gennes Hamiltonian governing the fluctuations. Our semianalytical estimates for the noise level agree quantitatively with the results of stochastic numerical simulations.