We observe thermalization in the production of a degenerate Fermi gas of polar ${}^{40}\mathrm{K}{}^{87}\mathrm{Rb}$ molecules. By measuring the atom-dimer elastic scattering cross section near the Feshbach resonance, we show that Feshbach molecules rapidly reach thermal equilibrium with both parent atomic species. Equilibrium is essentially maintained through coherent transfer to the ground state. Sub-Poissonian density fluctuations in Feshbach and ground-state molecules are measured, giving an independent characterization of degeneracy and directly probing the molecular Fermi-Dirac distribution.

We demonstrate three-dimensional trapping of individual Rydberg atoms in holographic optical bottle beam traps. Starting with cold, ground-state ${}^{87}\mathrm{Rb}$ atoms held in standard optical tweezers, we excite them to $n{S}_{1/2}$, $n{P}_{1/2}$, or $n{D}_{3/2}$ Rydberg states and transfer them to a hollow trap at 850 nm. For principal quantum numbers $60\le n\le 90$, the measured trapping time coincides with the Rydberg state lifetime in a 300 K environment. We show that these traps are compatible with quantum information and simulation tasks by performing single qubit microwave Rabi flopping, as well as by measuring the interaction-induced, coherent spin-exchange dynamics between two trapped Rydberg atoms separated by $40\mu \mathrm{m}$. These results will find applications in the realization of high-fidelity quantum simulations and quantum logic operations with Rydberg atoms.

The high surface sensitivity and controlled surface charge state of submicron sized droplets is exploited to study low-energy electron transport through liquid interfaces using photoelectron imaging. Already a few charges on a droplet are found to modify the photoelectron images significantly. For narrow escape barriers, the comparison with an electron scattering model reveals pronounced quantum effects in the form of above-barrier reflections at electron kinetic energies below about 1 eV. The observed susceptibility to the characteristics of the electron escape barrier might provide access to these properties for liquid interfaces, which are generally difficult to investigate.

The hallmark of quantum physics is Planck’s constant $h$, whose finite value entails the quantization that gave the theory its name. The finite value of $h$ gives rise to inevitable zero-point fluctuations even at vanishing temperature. The zero-point fluctuation of mechanical motion becomes smaller with growing mass of an object, making it challenging to observe at macroscopic scales. Here, we transition a dielectric particle with a diameter of 136 nm from the classical realm to the regime where its zero-point motion emerges as a sizable contribution to its energy. To this end, we optically trap the particle at ambient temperature in ultrahigh vacuum and apply active feedback cooling to its center-of-mass motion. We measure an asymmetry between the Stokes and anti-Stokes sidebands of photons scattered by the levitated particle, which is a signature of the particle’s quantum ground state of motion.

Strong-field ionization is central to intense laser-matter interactions. However, standard ionization measurements have been limited to extremely low density gas samples, ignoring potential high density effects. Here, we measure strong-field ionization in atmospheric pressure range air, ${\mathrm{N}}_2$, and Ar over 14 decades of absolute yield, using mid-IR picosecond avalanche multiplication of single electrons. Our results are consistent with theoretical rates for isolated atoms and molecules and quantify the ubiquitous presence of ultralow concentration gas contaminants that can significantly affect laser-gas interactions.

We have measured cyclotron frequency ratios of ${\mathrm{H}}_2^{+}$ to ${\mathrm{D}}^{+}$ with sufficient precision to resolve the mass increase of ${\mathrm{H}}_2^{+}$ due to vibrational energy. Additional discrimination against excited vibrational levels was provided by increasing the rate of vibrational decay through Stark quenching. From our results we obtain a value for the deuteron-to-proton mass ratio, $m_d/m_p=1.999007501274(38)$, which has an uncertainty three times smaller than the current CODATA value.

Strong coupling between an atom and an electromagnetic resonator is an important condition in cavity quantum electrodynamics. While strong coupling in various physical systems has been achieved so far, it remained elusive for single atomic ions. Here, we achieve a coupling strength of $2\pi \times (12.3\pm 0.1)\mathrm{MHz}$ between a single ${}^{40}{\mathrm{Ca}}^{+}$ ion and an optical cavity, exceeding both atomic and cavity decay rates which are $2\pi \times 11.5$ and $2\pi \times (4.1\pm 0.1)\mathrm{MHz}$, respectively. We use cavity assisted Raman spectroscopy to precisely characterize the ion-cavity coupling strength and observe a spectrum featuring the normal mode splitting in the cavity transmission due to the ion-cavity interaction. Our work paves the way towards new applications of cavity quantum electrodynamics utilizing single trapped ions in the strong coupling regime for quantum optics and quantum technologies.

We introduce multiplexed atom-cavity quantum electrodynamics with an atomic ensemble coupled to a single optical cavity mode. Multiple Raman dressing beams establish cavity-coupled spin-wave excitations with distinctive spatial profiles. Experimentally, we demonstrate the concept by observing spin-wave vacuum Rabi splittings, selective superradiance, and interference in the cavity-mediated interactions of two spin waves. We highlight that the current experimental configuration allows rapid, interchangeable cavity coupling to 4 profiles with an overlap parameter of less than 10%, enough to demonstrate, for example, a quantum repeater network simulation in the cavity. With further improvements to the optical multiplexing setup, we infer the ability to access more than $10^3$ independent spin-wave profiles.

We introduce and experimentally demonstrate a method where the two intrinsic timescales of a molecule, the slow nuclear motion and the fast electronic motion, are simultaneously measured in a photoelectron photoion coincidence experiment. In our experiment, elliptically polarized, 750 nm, 4.5 fs laser pulses were focused to an intensity of $9\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ onto ${\mathrm{H}}_2$. Using coincidence imaging, we directly observe the nuclear wave packet evolving on the $1s{\sigma }_g$ state of ${\mathrm{H}}_2^{+}$ during its first round-trip with attosecond temporal and picometer spatial resolution. The demonstrated method should enable insight into the first few femtoseconds of the vibronic dynamics of ionization-induced unimolecular reactions of larger molecules.

We extend state-to-state chemistry to a realm where besides vibrational, rotational, and hyperfine quantum states magnetic quantum numbers are also resolved. For this, we make use of the Zeeman effect, which energetically splits levels of different magnetic quantum numbers. The chemical reaction which we choose to study is three-body recombination in an ultracold quantum gas of ${}^{87}\mathrm{Rb}$ atoms forming weakly bound ${\mathrm{Rb}}_2$ molecules. Here, we find the propensity rule that the total $m_F$ quantum number of the two atoms forming the molecule is conserved. Our method can be employed for many other reactions and inelastic collisions and will allow for novel insights into few-body processes.

The propagation of two-color laser fields through optically thick atomic ensembles is studied. We demonstrate how the interaction between these two fields spawns the formation of copropagating, two-color solitonlike pulses akin to the simultons found by Konopnicki and Eberly [Phys. Rev. A 24, 2567 (1981)]. For the particular case of thermal Rb atoms exposed to a combination of a weak cw laser field resonant on the $D1$ transition and a strong sub-ns laser pulse resonant on the $D2$ transition, simulton formation is initiated by an interplay between the $5s_{1/2}-5p_{1/2}$ and $5s_{1/2}-5p_{3/2}$ coherences. The interplay amplifies the $D1$ field at the arrival of the $D2$ pulse, producing a sech-squared pulse with a length of less than $10\mu \mathrm{m}$. This amplification is demonstrated in a time-resolved measurement of the light transmitted through a thin thermal cell. We find good agreement between experiment and a model that includes the hyperfine structure of the relevant levels. With the addition of Rydberg dressing, quasisimultons may offer interesting prospects for strong photon-photon interactions in a robust environment.

We perform precise studies of two- and three-body interactions near an intermediate-strength Feshbach resonance in ${}^{39}\mathrm{K}$ at 33.5820(14) G. Precise measurement of dimer binding energies, spanning three orders of magnitude, enables the construction of a complete two-body coupled-channel model for determination of the scattering lengths with an unprecedented low uncertainty. Utilizing an accurate scattering length map, we measure the precise location of the Efimov ground state to test van der Waals universality. Precise control of the sample’s temperature and density ensures that systematic effects on the Efimov trimer state are well understood. We measure the ground Efimov resonance location to be at $-14.05(17)$ times the van der Waals length $r_{\mathrm{vdW}}$, significantly deviating from the value of $-9.7r_{\mathrm{vdW}}$ predicted by van der Waals universality. We find that a refined multichannel three-body model, built on our measurement of two-body physics, can account for this difference and even successfully predict the Efimov inelasticity parameter $\eta$.

Hanbury Brown–Twiss interference and stimulated emission, two fundamental processes in atomic physics, have been studied in a wide range of applications in science and technology. We study interference effects that occur when a weak probe is sent through a gas of two-level atoms that are prepared in a singly excited collective (Dicke or “superatom”) state and for atoms prepared in a factorized state. We measure the time-integrated second-order correlation function $g^{(2)}$ of the output field as a function of the delay $\tau$ between the input probe field and radiation emitted by the atoms and find that, for the Dicke state, $g^{(2)}$ is twice as large for $\tau =0$ as it is for ${\gamma }_e\tau \gg 1$ (${\gamma }_e$ is an excited state decay rate), while for the product state, this ratio is equal to $3/2$. The results agree with those of a theoretical model in which any effects related to stimulated emission are totally neglected—the coincidence counts measured in our experiment arise from Hanbury Brown–Twiss interference between the input field and the field radiated by the atoms.

Laser cooling is a well-established technique for the creation of ensembles of ultracold neutral atoms or positive ions. This ability has opened many exciting new research fields over the past 40 years. However, no negatively charged ions have been directly laser cooled because a cycling transition is very rare in atomic anions. Efforts of more than a decade currently have ${\mathrm{La}}^{-}$ as the most promising candidate. We report on experimental and theoretical studies supporting ${\mathrm{Th}}^{-}$ as a new promising candidate for laser cooling. The measured and calculated electron affinities of Th are, respectively, $4901.35(48){\mathrm{cm}}^{-1}$ and $4832{\mathrm{cm}}^{-1}$, or 0.607 690(60) and 0.599 eV, almost a factor of 2 larger than the previous theoretical value of 0.368 eV. The ground state of ${\mathrm{Th}}^{-}$ is determined to be $6d^{3}7s^2{{}^{4}F}_{3/2}^e$ rather than $6d^{2}7s^{2}7p{{}^{4}G}_{5/2}^o$. The consequence of this is that there are several strong electric dipole transitions between the bound levels arising from configurations $6d^{3}7s^2$ and $6d^{2}7s^{2}7p$ in ${\mathrm{Th}}^{-}$. The potential laser-cooling transition is ${}^2{S}_{1/2}^o\leftrightarrow {{}^{4}F}_{3/2}^e$ with a wavelength of $2.6\mu \mathrm{m}$. The zero nuclear spin and hence lack of hyperfine structure in ${\mathrm{Th}}^{-}$ reduces the potential complications in laser cooling as encountered in ${\mathrm{La}}^{-}$, making ${\mathrm{Th}}^{-}$ a new and exciting candidate for laser cooling.

We implement a microscopic spin filter for cold fermionic atoms in a quantum point contact (QPC) and create fully spin-polarized currents while retaining conductance quantization. Key to our scheme is a near-resonant optical tweezer inducing a large effective Zeeman shift inside the QPC while its local character limits dissipation. We observe a renormalization of this shift due to interactions of only a few atoms in the QPC. Our work represents the analog of an actual spintronic device and paves the way to studying the interplay between spin splitting and interactions far from equilibrium.

We consider dynamics of a Rydberg impurity in a cloud of ultracold bosonic atoms in which the Rydberg electron undergoes spin-changing collisions with surrounding atoms. This system realizes a new type of quantum impurity problems that compounds essential features of the Kondo model, the Bose polaron, and the central spin model. To capture the interplay of the Rydberg-electron spin dynamics and the orbital motion of atoms, we employ a new variational method that combines an impurity-decoupling transformation with a Gaussian ansatz for the bath particles. We find several unexpected features of this model that are not present in traditional impurity problems, including interaction-induced renormalization of the absorption spectrum that eludes simple explanations from molecular bound states, and long-lasting oscillations of the Rydberg-electron spin. We discuss generalizations of our analysis to other systems in atomic physics and quantum chemistry, where an electron excitation of high orbital quantum number interacts with a spinful quantum bath.

Quantum networks require functional nodes consisting of stationary registers with the capability of high-fidelity quantum processing and storage, which efficiently interface with photons propagating in an optical fiber. We report a significant step towards realization of such nodes using a diamond nanocavity with an embedded silicon-vacancy (SiV) color center and a proximal nuclear spin. Specifically, we show that efficient SiV-cavity coupling (with cooperativity $C>30$) provides a nearly deterministic interface between photons and the electron spin memory, featuring coherence times exceeding 1 ms. Employing coherent microwave control, we demonstrate heralded single photon storage in the long-lived spin memory as well as a universal control over a cavity-coupled two-qubit register consisting of a SiV and a proximal ${}^{13}\mathrm{C}$ nuclear spin with nearly second-long coherence time, laying the groundwork for implementing quantum repeaters.

Through simultaneous but unequal electromechanical amplification and cooling processes, we create a method for a nearly noiseless pulsed measurement of mechanical motion. We use transient electromechanical amplification (TEA) to monitor a single motional quadrature with a total added noise $-8.5\pm 2.0\mathrm{dB}$ relative to the zero-point motion of the oscillator, or equivalently the quantum limit for simultaneous measurement of both mechanical quadratures. We demonstrate that TEA can be used to resolve fine structure in the phase space of a mechanical oscillator by tomographically reconstructing the density matrix of a squeezed state of motion. Without any inference or subtraction of noise, we directly observe a squeezed variance $2.8\pm 0.3\mathrm{dB}$ below the oscillator’s zero-point motion.

We report on the first timescale based entirely on optical technology. Existing timescales, including those incorporating optical frequency standards, rely exclusively on microwave local oscillators owing to the lack of an optical oscillator with the required frequency predictability and stability for reliable steering. We combine a cryogenic silicon cavity exhibiting improved long-term stability and an accurate ${}^{87}\mathrm{Sr}$ lattice clock to form a timescale that outperforms them all. Our timescale accumulates an estimated time error of only $48\pm 94\mathrm{ps}$ over 34 days of operation. Our analysis indicates that this timescale is capable of reaching a stability below $1\times {10}^{-17}$ after a few months of averaging, making timekeeping at the ${10}^{-18}$ level a realistic prospect.

Nuclear-spin-symmetry conservation makes the observation of transitions between quantum states of ortho- and para-${\mathrm{H}}_2$ extremely challenging. Consequently, the energy-level structure of ${\mathrm{H}}_2$ derived from experiment consists of two disjoint sets of level energies, one for para-${\mathrm{H}}_2$ and the other for ortho-${\mathrm{H}}_2$. We use a new measurement of the ionization energy of para-${\mathrm{H}}_2$ [$E_{\mathrm{I}}({\mathrm{H}}_2)/(hc)=124417.491098(31){\mathrm{cm}}^{-1}$] to determine the energy separation [$118.486770(50){\mathrm{cm}}^{-1}$] between the ground states of para- and ortho-${\mathrm{H}}_2$ and thus link the energy-level structure of the two nuclear-spin isomers of this fundamental molecule. Comparison with recent theoretical results [M. Puchalski et al., Phys. Rev. Lett. 122, 103003 (2019)] enables the derivation of an upper bound of 1.5 MHz for a hypothetical global shift of the energy-level structure of ortho-${\mathrm{H}}_2$ with respect to that of para-${\mathrm{H}}_2$.