Magneto-optical trapping of molecules has thus far been restricted to molecules with ${}^2\mathrm{\Sigma }$ electronic ground states. These species are chemically reactive and only support a simple laser cooling scheme from their first excited rotational level. Here, we demonstrate a magneto-optical trap (MOT) of aluminum monofluoride (AlF), a deeply bound and intrinsically stable diatomic molecule with a ${}^1{\mathrm{\Sigma }}^{+}$ electronic ground state. The MOT operates on the strong $A^1\mathrm{\Pi }\leftarrow X^1{\mathrm{\Sigma }}^{+}$ transition near 227.5 nm, whose $Q(J)$ lines are all rotationally closed. We demonstrate a MOT of about $6\times {10}^4$ molecules for the $J=1$ level of AlF, more than ${10}^4$ molecules for $J=2$ and 3, and with no fundamental limit in going to higher rotational levels. Laser cooling and trapping of AlF is conceptually similar to the introduction of alkaline-earth atoms into cold atom physics, and is key to leveraging its spin-forbidden $a^3\mathrm{\Pi }\leftarrow X^1{\mathrm{\Sigma }}^{+}$ transition for precision spectroscopy and narrow-line cooling.

A key challenge in ultrafast science has been to directly track the coupled motions of electrons and nuclei in real space and real time. This Letter presents a significant step toward this goal by demonstrating the feasibility of time-resolved real-space tracking of valence electron and hydrogen dynamics during the photodissociation of ammonia (${\mathrm{NH}}_3$) using MeV ultrafast electron diffraction. It is demonstrated that the enhanced temporal resolution, in conjunction with the analysis of the charge-pair distribution function, enables the disentanglement of the correlated motion of valence electrons and hydrogens in a photoexcited ammonia molecule. The methodology employed in this Letter, which utilizes the charge-pair distribution function from ultrafast electron scattering to retrieve intertwined electron and nucleus dynamics, may open up new opportunities in the study of quantum dynamics for a wide range of molecules.

In the early days of laser spectroscopy, Letokhov and Chebotayev proposed a scheme for measuring narrow spectral lines where the resolution is not restricted to Doppler effects because the molecules are entrained in a standing-wave light field. Now, such one-dimensional trapping in the intensity maxima of an intracavity field, slightly detuned from resonance, is experimentally demonstrated in the measurement of the very weak S(0) (2-0) quadrupole overtone transition in ${\mathrm{H}}_2$ at 1189 nm. The trapping manifests as an extremely narrow absorption feature at the predicted zero-recoil position, a 70 kHz shift from the blue-recoil component observed in Lamb-dip spectroscopy. A quantitative analysis of the saturation and trapping conditions supports the findings.

When embedded in superfluid helium nanodroplets, the dopant molecule exhibits either localization or delocalization behaviors that are critically determined by the spatial spread of its wave function, which in turn are influenced by the confining potential of the nanodroplets. Here, we examine the spatial extent of a molecule’s wave function with respect to the size of the nanodroplet by diagnosing the angular nodal structures in the photoelectron momentum distributions (PMDs). We find that, for the lightest ${\mathrm{H}}_2$ molecules or heavier ${\mathrm{D}}_2$ and ${\mathrm{O}}_2$ molecules, with extents of wave function distributions comparable to or smaller than the nanodroplet size, the emitted electrons undergo either minimal or significant scattering with the liquid helium surroundings, leading to either preserved or blurred nodal structures in the PMDs. These observations reveal the underlying delocalization or localization of a molecule within the nanodroplet, which is governed by its wave functions as determined by the confining potential of the droplet. Our findings provide insight into the quantum solvation dynamics of light impurities in nanoscale and confined systems.

We report the creation of ultracold samples of ${{}^{39}\mathrm{K}}^{133}\mathrm{Cs}$ molecules in their rovibrational ground state. By investigating potentially suitable excited states using one- and two-photon spectroscopy, we have identified a pathway to the ground state via an exceptionally narrow intermediate state. Using stimulated Raman adiabatic passage, we create trapped samples of up to 3500 molecules at temperatures of $1\mathrm{\mu }\mathrm{K}$ with one-way efficiencies of 71%. The lifetime of these samples is limited by a near-universal two-body loss process, which could shed new light on similar loss mechanisms in other molecular species. Our results are a step toward establishing an alternative molecular species as a platform for the study of bosonic and fermionic quantum matter with strong dipolar interactions.

Optical atomic clocks with unrivaled precision and accuracy have advanced the frontier of precision measurement science and opened new avenues for exploring fundamental physics. A fundamental limitation on clock precision is the standard quantum limit (SQL), which stems from the uncorrelated projection noise of each atom. State-of-the-art optical lattice clocks interrogate large ensembles to minimize the SQL, but density-dependent frequency shifts pose challenges to scaling the atom number. The SQL can be surpassed, however, by leveraging entanglement, though it remains an open problem to achieve quantum advantage from spin squeezing at state-of-the-art stability levels. Here, we demonstrate clock performance beyond the SQL, achieving a fractional frequency precision of $1.1\times {10}^{-18}$ for a single spin-squeezed clock. With cavity-based quantum nondemolition measurements, we prepare two spin-squeezed ensembles of $\sim 30000$ strontium atoms confined in a two-dimensional optical lattice. A synchronous clock comparison with an interrogation time of 61 ms achieves a metrological improvement of 2.0(2) dB beyond the SQL, after correcting for state preparation and measurement errors. These results establish the most precise entanglement-enhanced clock to date and offer a powerful platform for exploring the interplay of gravity and quantum entanglement.

We show that idealized Dicke superradiant decay from a fully inverted state can at all times be described by a positive statistical mixture of coherent spin states (CSS). Since CSS are separable, this implies that no entanglement is involved in Dicke decay. Based on this result, we introduce a new numerical quantum trajectory approach leading to low-entanglement unravelings. This opens up new possibilities for employing matrix product state (MPS) techniques for large-scale numerical simulations with collective decay processes.

Over the last few decades, the predominant strategies for controlling spontaneous emission have involved tailoring the spatial surroundings of quantum emitters or atoms to create resonant or spatially periodic photonic structures. However, the rise of time-varying photonics has prompted a reevaluation of spontaneous emission in dynamically changing environments, especially within photonic time crystals, where optical properties undergo time-periodic modulation. Here, we apply classical light-matter interaction theory together with Floquet analysis to reveal a substantial enhancement of the spontaneous emission decay rate at the momentum gap frequency in photonic time crystals. Moreover, our findings suggest that photonic time crystals enable a nonequilibrium light-matter interaction process: the spontaneous excitation of an atom from its ground state to an excited state, accompanied by the concurrent emission of a photon, referred to as spontaneous emission excitation.

Recent advances in laser excitation of the low-energy nuclear isomer transition in ${}^{229}\mathrm{Th}$ have opened avenues for developing nuclear clocks, a novel quantum technology with exceptional performance and sensitivity to exotic physics. Here, we explore the host dependence of the nuclear clock frequency, focusing on the isomer shift induced by the difference in the nuclear charge distribution between the ground and excited nuclear states. We combine relativistic many-body methods of atomic structure with periodic density functional theory to evaluate the isomer shifts in solid-state hosts. We elucidate the critical importance of the “relaxation” effect in evaluating the isomer shifts. Our analysis predicts nuclear clock frequencies for various solid-state and trapped ion platforms: ${\omega }_{\mathrm{clk}}(\text{solid state})=2020407384(40)\mathrm{MHz}$, ${\omega }_{\mathrm{clk}}({{}^{229}\mathrm{Th}}^{4+})=2020407648(70)\mathrm{MHz}$, and ${\omega }_{\mathrm{clk}}({{}^{229}\mathrm{Th}}^{3+})=2020407114(70)\mathrm{MHz}$. We also determine the nuclear transition energy for the bare ${}^{229}\mathrm{Th}$ nucleus to be ${\omega }_{\mathrm{nuc}}=8.272(22)\mathrm{eV}$. Our calculated valence band isomer shifts for different host materials constrain the nuclear transition frequencies to an 80-MHz-wide frequency window, aiding experimental searches for the ${}^{229}\mathrm{Th}$ nuclear transition in novel materials.

Understanding many body systems is a key challenge in physics. Single atom resolved imaging techniques have unlocked access to microscopic correlations in ultracold quantum gases. However they cannot be used when the relevant length scales are obscured by the resolution of the detection technique. We present a matter wave magnification scheme, based on evolutions in optical potentials, tailored to magnify the wave function of atoms, such that all length scales can be resolved. To showcase this method, we image atoms in the strongly interacting regime, establishing a new way to characterize correlated systems.

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.

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.

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.

Controlling the Stark perturbation from ambient thermal radiation is key to advancing the performance of many atomic frequency standards, including state-of-the-art optical lattice clocks (OLCs). We demonstrate a cryogenic OLC that utilizes a dynamically actuated radiation shield to control the perturbation at $1.7\times {10}^{-20}$ fractional frequency, a factor of $\sim 40$ beyond the best OLC to date. Our shield furnishes the atoms with a near-ideal cryogenic blackbody radiation (BBR) environment by rejecting external thermal radiation at the part-per-million level during clock spectroscopy, overcoming a key limitation with previous cryogenic BBR control solutions in OLCs. While the lowest BBR shift uncertainty is realized with cryogenic operation, we further exploit the radiation control that the shield offers over a wide range of temperatures to directly measure and verify the leading BBR Stark dynamic correction coefficient for ytterbium. This independent measurement reduces the literature-combined uncertainty of this coefficient by 30%, thus benefiting state-of-the-art Yb OLCs operated at room temperature. We verify the static BBR coefficient for Yb at the low ${10}^{-18}$ level.

The scattering matrix $S$ linearly relates the vector of incoming waves to outgoing wave excitations, and contains an enormous amount of information about the scattering system and its connections to the scattering channels. Time delay is one way to extract information from $S$, and the transmission time delay ${\tau }_T$ is a complex (even for Hermitian systems with unitary scattering matrices) measure of how long a wave excitation lingers before being transmitted. The real part of ${\tau }_T$ is a well-studied quantity, but the imaginary part of ${\tau }_T$ has not been systematically examined experimentally, and theoretical predictions for its behavior have not been tested. Here we experimentally test the predictions of Asano et al. [Nat. Commun. 7, 13488 (2016)] for the imaginary part of transmission time delay in a nonunitary scattering system. We utilize Gaussian time-domain pulses scattering from a two-port microwave graph supporting a series of well-isolated absorptive modes to show that the carrier frequency of the pulses is changed in the scattering process by an amount in agreement with the imaginary part of the independently determined complex transmission time delay, $\mathrm{Im}[{\tau }_T]$, from frequency-domain measurements of the subunitary $S$ matrix. Our results also generalize and extend those of Asano et al., establishing a means to predict pulse propagation properties of non-Hermitian systems over a broad range of conditions.

We report a single-ion optical atomic clock with a fractional frequency uncertainty of $5.5\times {10}^{-19}$ and fractional frequency stability of $3.5\times {10}^{-16}/\sqrt{\tau /\mathrm{s}}$, based on quantum logic spectroscopy of a single ${}^{27}{\mathrm{Al}}^{+}$ ion. A cotrapped ${}^{25}{\mathrm{Mg}}^{+}$ ion provides sympathetic cooling and quantum logic readout of the ${}^{27}{\mathrm{Al}}^{+}{{}^{1}S}_0\leftrightarrow {{}^{3}P}_0$ clock transition. A Rabi probe duration of 1 s, enabled by laser stability transfer from a remote cryogenic silicon cavity across a 3.6 km fiber link, results in a threefold reduction in instability compared to previous ${}^{27}{\mathrm{Al}}^{+}$ clocks. Systematic uncertainties are lower due to an improved ion trap electrical design, which reduces excess micromotion, and a new vacuum system, which reduces collisional shifts. We also perform a direction-sensitive measurement of the ac magnetic field due to the rf ion trap, eliminating systematic uncertainty due to field orientation.

Quantum field theory (QFT) in curved spacetimes predicts the amplification of field excitations and the occurrence of classical and quantum correlations, as in the Hawking effect for example. This raises interest in experiments in which the curvature of spacetime can be controlled and amplification measured, as in fluids going from subsonic to supersonic speeds where acoustic excitations are effectively trapped inside an acoustic horizon. Quantum fluctuations of the acoustic field are predicted to yield entangled emission across the horizon, as in black holes. Here, we introduce such a QFT simulator in a one-dimensional polaritonic fluid of light. We demonstrate the unique tunability of our system by engineering smooth and steep horizons, which respectively have quasithermal, but weak, and strong Hawking radiation. We measure the spectrum on either side of the horizon and evidence the excitation of negative energy waves in fluids of light for the first time. Notably, we explicitly show that, beyond phononic excitations as in other systems, our simulator also supports excitations with a massive, relativistic dispersion. In the future, quantum optics techniques offer the possibility to measure entanglement in unexplored regimes, giving insight in this outstanding prediction of relativistic QFT.

Fermionic superfluid junctions typically exhibit suppressed thermal and spin transport due to the presence of a pairing gap but allow coherent particle transport. While dissipation generally weakens coherent transport, it can also induce excitations that open other transport channels. In this work, we experimentally study a one-dimensional superfluid junction of strongly interacting fermions with local particle loss and observe dissipation-induced thermal and spin transport that appear to saturate at strong dissipation. Notably, in this regime, the measured thermal and spin conductances are comparable to the universal quantized conductance of one-dimensional ideal Fermi gas. Qualitatively similar behavior is observed for two dissipation mechanisms, either spin-imbalanced or pairwise losses. Our findings provide new insights into transport in interacting open quantum systems and suggest possibilities of dissipative control of spin and thermoelectric transport.

Laser excitation of ${}^{229}\mathrm{Th}$ nuclei in doped wide band gap crystals has been demonstrated recently, opening the possibility of developing ultrastable solid-state clocks and sensitive searches for new physics. We develop a quantitative theory of the internal conversion (IC) of isomeric ${}^{229}\mathrm{Th}$ in solid-state hosts and elucidate a crucial requirement in choosing the solid-state hosts in nuclear clock applications. The IC of the isomer proceeds by resonantly exciting a valence band electron to a defect state, accompanied by multiphonon emission. We demonstrate that, if the process is energetically allowed, it generally quenches the isomer on a millisecond timescales, much faster than the isomer’s radiative lifetime, despite ${}^{229}\mathrm{Th}$ being in the $+4$ charge state in the valence band.