The coherent interaction between free electrons and optical fields can produce free-electron compression and push the temporal resolution of ultrafast electron microscopy to the attosecond regime. However, a large electron-light interaction is required to attain a strong compression, generally necessitating short light and electron pulses combined with optical scattering at nanostructures. Here, we theoretically investigate an alternative configuration based on stimulated Compton scattering, whereby two counterpropagating Gaussian light beams induce energy jumps in a collinear electron beam by multiples of their photon-energy difference. Strong recoil effects are produced by extending the electron-light interaction over millimetric distances, enabling a dramatic increase in temporal compression and substantially reshaping the electron spectra for affordable laser powers. Beyond its fundamental interest, our Letter introduces a practical scheme to achieve a large temporal compression of continuous electron beams without involving optical scattering by material structures.

In the Alphatrap experiment, the $g$ factor of boronlike ${}^{118}{\mathrm{Sn}}^{45+}$ has been measured with a 0.5 ppb uncertainty. This is the first high-precision measurement of the $g$ factor of a heavy boronlike ion. The measured value of 0.6 447 038 265(4) is consistent with the presented ab initio state-of-the-art theory calculations, which predict a value of 0.64 470 29(8). So far, the only boronlike ion of which the $g$ factor was measured with high precision has been ${}^{40}{\mathrm{Ar}}^{13+}$. The measurement presented here therefore tests quantum electrodynamics as well as many-electron interactions at much higher $Z$. Furthermore, we discuss the potential for an independent determination of the fine-structure constant $\alpha$, which can be achieved with a specific difference of $g$ factors, combining the presented results with the recent electron $g$-factor measurement of hydrogenlike tin.

Ion-molecule reactions proceed with highly rotational-state-specific rate coefficients and are amenable to stereodynamical control at low temperatures. We introduce a scheme to control cold ion-molecule chemistry with microwaves based on the coherent transfer of molecular rotational-state populations. We use a merged-beam approach to reach collision energies in the range from $\sim 0$ to $k_{\mathrm{B}}\times 10\mathrm{K}$ for the reaction between ${\mathrm{He}}^{+}$ and rotationally cold CO molecules ($T_{\mathrm{rot}}\approx 3\mathrm{K}$) and manipulate the rotational-state population of CO using microwave pulses. We achieve reaction inhibition by up to $\sim 40\%$ with microwave pulses resonant with pure-rotational transitions in CO, unambiguously demonstrating a nonthermal mechanism for microwave-assisted chemistry.

Here, a novel photonic spin Hall effect is demonstrated based on the spatially varying Pancharatnam-Berry (PB) phases, aroused naturally in an $X$-cut ${\mathrm{LiTaO}}_3$ crystal via the Pockels effect. Applied with an electric field, the principal axes of crystal will rotate around the $x$-axis with the rotation angle varying linearly with the $y$ component of the applied field. By depositing an asymmetric electrode pair on the crystal to induce inhomogeneously distributed electric fields, spatially varying rotations of the principal axes of crystal are generated, resulting in spatially varying PB phases. These spatially varying PB phases are redistributable by the applied voltage, different from those unchangeable PB phases aroused by the spatially varying orientations of anisotropic structure units such as patterned liquid crystals and metasurfaces. The redistributable PB phases can cause tunable separations of spin photons in the momentum space. The separation speed is measured as $\sim 0.16\mathrm{GHz}$. These findings deepen the understanding of the geometric phase and pave the way for high-speed control of spin photons.

The intrinsic chirality of natural materials is known to be weak, prompting extensive efforts to enhance chiral light-matter interactions on the metasurface platform. Chiral metasurfaces are typically created by manipulating the geometry of nanostructures, such as three-dimensional helical structures and slanted structures. However, these approaches are generally challenging to implement experimentally in optical frequency ranges. Here, we present the achievement of significant chirality on planar lithium niobate metasurfaces. We theoretically demonstrate that the birefringence of lithium niobate enables a strong coupling between two nearly degenerate resonant modes when rotating the optical axis. Despite the achiral geometric morphology of the lithium niobate structure, we show that these mixed modes exhibit chirality and can produce nearly full circular dichroism signals. Moreover, we experimentally validate our theoretical proposal using our advanced process technique for lithium niobate nanostructures and the measured circular dichroism signal reaches $-0.53$. These findings offer new possibilities for chiral metaphotonics.

Quantum state-resolved spectroscopy of the low energy thorium-229 nuclear transition was recently achieved. The five allowed transitions within the electric quadrupole structure were measured to the kilohertz level in a calcium fluoride host crystal, opening many new areas of research using nuclear clocks. Central to the performance of solid-state clock operation is an understanding of systematic shifts such as the temperature dependence of the clock transitions. In this work, we measure the four strongest transitions of thorium-229 in the same crystal at three temperature values: 150 K, 229 K, and 293 K. We find shifts of the unsplit frequency and the electric quadrupole splittings, corresponding to decreases in the electron density, electric field gradient, and field gradient asymmetry at the nucleus as temperature increases. The $m=\pm 5/2\to \pm 3/2$ line shifts only 62(6) kHz over the temperature range, i.e., approximately $0.4\mathrm{kHz}/\mathrm{K}$, representing a promising candidate for a future solid-state optical clock. Achieving ${10}^{-18}$ precision requires crystal temperature stability of $5\mathrm{\mu }\mathrm{K}$.

We report on the formation of multiple quantum droplets in a heteronuclear ${}^{41}\mathrm{K}\text{−}{}^{87}\mathrm{Rb}$ mixture released in an optical waveguide. By a sudden change of the interspecies interaction from the noninteracting to the strongly attractive regime, we initially form a single droplet in an excited compression-elongation mode. The latter axially expands up to a critical length and then splits into two or more smaller fragments, recognizable as quantum droplets. We find that the number of formed droplets increases with decreasing interspecies attraction and increasing atom number. We show, by combining theory and experiment, that this behavior is consistent with capillary instability, which causes the breakup of the stretching droplet due to the surface tension. Our results open new possibilities to explore the properties of quantum liquids and systems of multiple quantum droplets in two-component bosonic mixtures.

When two particles are spin entangled, one may exhibit behavior that suggests its spin can nonlocally influence the spin of the other. This implies that the energy of the latter particle can be altered by a nonlocal quantum correlation, without enabling any form of superluminal communication. This energy alteration, however, remains to be experimentally testified. To test this phenomenon, we construct an incomplete Mach-Zehnder interferometer utilizing two quantum memories. The Stokes photon generated by the quantum memories serves as the first particle, while the atomic excitation stored within the memories acts as the second particle. Our results demonstrate that if the de Broglie–Bohm interpretation is realistic, the trajectory of the Stokes photon can be nonlocally influenced by a slight probe of the atomic excitation. Conversely, the position of the atomic excitation, as well as the energy $\hslash {\omega }_{\mathrm{hf}}$ it carries, can also be nonlocally altered by controlling the Stokes photon. These findings underscore the continued relevance of nonlocality in hidden variable theories, consistent with Bell’s theorem and recent experimental validations, while making clear that such phenomena do not violate the principles of relativity.

We report on the imaging of the in situ spatial distribution of deterministically prepared single-atom wave packets as they expand in a plane, finding excellent agreement with the scaling dynamics predicted by the Schrödinger equation. Our measurement provides a direct and quantitative observation of the textbook free expansion of a one-particle Gaussian wave packet, which we believe has no equivalent in the existing literature. Second, we utilize these expanding wave packets as a benchmark to develop a protocol for the controlled projection of a spatially extended wave function from continuous space onto the sites of a deep optical lattice and subsequent single-atom imaging using quantum gas microscopy techniques. By probing the square modulus of the wave function for various lattice ramp-up times, we show how to obtain a near-perfect projection onto lattice sites. Establishing this protocol represents a crucial prerequisite to the realization of a quantum gas microscope for continuum physics. The method demonstrated here for imaging a wave packet whose initial extent greatly exceeds the pinning lattice spacing, is designed to be applicable to the many-body wave function of interacting systems in continuous space, promising a direct access to their microscopic properties, including spatial correlation functions up to high order and large distances.

We investigated turbulence in 2D atomic Bose-Einstein condensates (BECs) using a minimally destructive, impurity injection technique analogous to particle image velocimetry in conventional fluids. Our approach transfers small regions of the BEC into a different hyperfine state and tracks their displacement, ultimately yielding the velocity field. This allows us to quantify turbulence in the same way as is conventional in fluid dynamics in terms of velocity-velocity correlation functions, called velocity structure functions, that obey Kolmogorov scaling laws. Furthermore, the velocity increments show a clear fat-tail non-Gaussian distribution that results from intermittency corrections to the initial “K41” Kolmogorov theory. Our observations are fully consistent with the later “KO62” description. These results are validated by a 2D dissipative Gross-Pitaevskii simulation.

We demonstrate the existence of topological phase transitions in interacting, symmetry-protected quantum matter at finite temperatures. Using a combined numerical and analytical approach, we study a one-dimensional Su-Schrieffer-Heeger model with added Hubbard interactions, where no thermodynamic phase transition occurs at finite temperatures. The transition is signalled by a quantized, nonlocal bulk topological order parameter. It is driven by defects, which are enabled by the combination of interaction and thermal activation, with no counterpart in the noninteracting limit. The defects localize topological zero modes, which, when sufficiently abundant, cause the order parameter to vanish. This phenomenon, interpreted via bulk-boundary correspondence, reflects the loss of a topological edge mode at a well-defined critical temperature in the thermodynamic limit. Unlike zero-temperature topological transitions, these finite-temperature transitions lack thermodynamic signatures but remain observable in controlled quantum systems, such as ultracold fermionic atoms in optical lattices.

We demonstrate an optical clock based on Ramsey-Bordé interferometry in a laser-cooled ${}^{40}\mathrm{Ca}$ beam. The mean velocity is reduced by an order of magnitude relative to a thermal beam and the transverse temperature approaches the Doppler limit, enabling the measurement of sub-kHz linewidth fringes in a compact interferometer. Using tailored phase and intensity modulation of the spectroscopy laser to add uniform frequency sidebands, we interrogate atoms throughout the transverse velocity distribution, increasing the Ramsey-Bordé fringe amplitude by a factor of 14 and improving the Allan deviation to $3.4\times {10}^{-15}$ at one second averaging time.

We present a scalable mixed-species Coulomb crystal clock based on the ${}^1{S}_0\leftrightarrow {{}^{3}P}_0$ transition in ${}^{115}{\mathrm{In}}^{+}$. ${}^{172}{\mathrm{Yb}}^{+}$ ions are cotrapped and used for sympathetic cooling. Reproducible interrogation conditions for mixed-species Coulomb crystals are ensured by a conditional preparation sequence with permutation control. We demonstrate clock operation with a $1{\mathrm{In}}^{+}\text{−}3{\mathrm{Yb}}^{+}$ crystal, achieving a relative systematic uncertainty of $2.5\times {10}^{-18}$ and a relative frequency instability of $1.6\times {10}^{-15}/\sqrt{\tau /1\mathrm{s}}$. We report on absolute frequency measurements with an uncertainty of $1.3\times {10}^{-16}$ and optical frequency comparisons with clocks based on ${}^{171}{\mathrm{Yb}}^{+}$ ($E3$) and ${}^{87}\mathrm{Sr}$. With a fractional uncertainty of $4.4\times {10}^{-18}$, the former is—to our knowledge—the most accurate frequency ratio value reported to date. For the ${}^{115}{\mathrm{In}}^{+}/{}^{87}\mathrm{Sr}$ ratio, we improve upon the best previous measurement by more than an order of magnitude. We also demonstrate operation with four ${}^{115}{\mathrm{In}}^{+}$ clock ions, which reduces the instability to $9.2\times {10}^{-16}/\sqrt{\tau /1\mathrm{s}}$.

Coupling is an essential mechanism that drives complexity in natural systems, transforming single, noninteracting elements into intricate networks with rich physical properties. Here, we demonstrate a chip-scale coupled laser system that exhibits complex optical states impossible to achieve in an uncoupled system. We show that a pair of coupled semiconductor ring lasers spontaneously forms a frequency comb consisting of the hybridized modes of its coupled cavity, exhibiting a large number of phase-locked tones that anticross with one another. Experimental coherent waveform reconstruction reveals that the hybridized frequency comb manifests itself as pairs of bright and dark picosecond-long solitons circulating simultaneously. The dark and bright solitons exit the coupled cavity at the same time, leading to breathing bright solitons temporally overlapped with their dark soliton counterparts—a state inaccessible for a single, free-running laser. Our results demonstrate that the rules that govern allowable states of light can be broken by simply coupling elements together, paving the way for the design of more complex networks of coupled on-chip lasers.

We demonstrate continuous loading and strong collective coupling of atoms to a high finesse ring cavity. A series of spatially separated laser cooling stages delivers a continuous flux of $2.1(3)\times {10}^7$ ${}^{88}\mathrm{Sr}\mathrm{atoms}/\mathrm{s}$ into an intracavity magic wavelength lattice where the atoms are deterministically transported inside the ring cavity. We observe a continuous atom-cavity vacuum Rabi splitting, demonstrating the first steady-state strong collective coupling on a narrow linewidth atomic transition. This Letter opens the path to future continuous cold atom devices, such as a continuous wave superradiant laser with millihertz linewidth and low vibration sensitivity, enabling searches for new physics and technologies.

We realize a Laughlin state of two rapidly rotating fermionic atoms in an optical tweezer. By utilizing a single atom and spin resolved imaging technique, we sample the Laughlin wave function thereby revealing its distinctive features, including a vortex distribution in the relative motion, correlations in the particles’ relative angle, and suppression of the interparticle interactions. Our Letter lays the foundation for atom-by-atom assembly of fractional quantum Hall states in rotating atomic gases.

We present the first successful trapping of single erbium atoms in an array of optical tweezers. Using a single narrow-line optical transition, we achieve deep cooling for direct tweezer loading, pairwise ejection, and continuous imaging without additional recoil suppression techniques. Our tweezer wavelength choice enables us to reach the magic trapping condition by tuning the ellipticity of the trapping light. Additionally, we implement an ultrafast high-fidelity fluorescence imaging scheme using a broad transition, allowing time-resolved study of the tweezer population dynamics from many to single atoms during light-assisted collisions. In particular, we extract a pair-ejection rate that qualitatively agrees with the semiclassical predictions by the Gallagher-Pritchard model. This Letter represents a promising starting point for the exploration of erbium as a powerful resource for quantum simulation in optical tweezers.

Nuclear magnetic resonance (NMR) spans diverse fields from biology to quantum science. Employing NMR on a floating object could unveil novel possibilities beyond conventional operational paradigms. Here, we observe NMR within a levitating microdiamond using the nuclear spins of nitrogen-14 atoms. By tightly confining the angular degrees of freedom of the diamond in a Paul trap, we achieve efficient hyperfine interaction between optically polarized electronic spins of nitrogen-vacancy centers and the ${}^{14}\mathrm{N}$ nuclear spin, enabling nuclear spin polarization and quantum state readout revealing coherence times up to hundreds of microseconds. This represents the longest recorded spin coherence time in a levitated system, surpassing previous records by 3 orders of magnitude. Our results offer promise for various applications, including cooling macroscopic particles to their motional ground state and exploring geometric phases for gyroscopy.

Strongly driven nonlinear optical processes such as spontaneous parametric down-conversion and spontaneous four-wave mixing can produce multiphoton nonclassical beams of light which have applications in quantum information processing and sensing. In contrast to the low-gain regime, new physical effects arise in a high-gain regime due to the interactions between the nonclassical light and the strong pump driving the nonlinear process. Here, we describe and experimentally observe a gain-induced group delay between the multiphoton pulses generated in a high-gain type-II spontaneous parametric down-conversion source. Since the group delay introduces distinguishability between the generated photons, it will be important to compensate for it when designing quantum interference devices in which strong optical nonlinearities are required.

We show that strong molecular rotation drastically modifies the autodetachment of ${\mathrm{C}}_2^{-}$ ions in the lowest quartet electronic state $a{}^4{\mathrm{\Sigma }}_u^{+}$. In the strong-rotation regime, levels of this state only decay by a process termed “rotationally assisted” autodetachment, whose theoretical description is worked out based on the nonlocal resonance model. For autodetachment linked with the exchange of six rotational quanta, the results reproduce a prominent, hitherto unexplained electron emission signal with a mean decay time near 3 ms, observed on stored ${\mathrm{C}}_2^{-}$ ions from a hot ion source.