We report on the experimental observation of Feshbach resonances in large-mass-imbalance mixtures of erbium (Er) and lithium (Li). All combinations of ${}^{168}\mathrm{Er}, {}^{166}\mathrm{Er}$, and ${}^7\mathrm{Li}, {}^6\mathrm{Li}$ are cooled to temperatures of a few microkelvin, partially by means of sympathetic cooling together with ytterbium (Yb) as a third mixture component. The Er-Li inelastic interspecies collisional properties are studied for magnetic fields up to $680\mathrm{G}$. In all cases, resonant interspecies loss features, indicative of Feshbach resonances, have been observed. While most resonances have sub-Gauss widths, a few of them are broad and feature widths of several Gauss. Those broad resonances are a key to the realization of ultracold Er-Li quantum gas mixtures with tunable interactions.

Atomic-scale imaging offers a reliable tool to directly measure the movement of microscopic particles. We present a scheme for achieving a nondestructive and ultrasensitive imaging of Rydberg atoms within an ensemble of cold probe atoms. This is made possible by the interaction-enhanced electromagnetically induced transparency at off resonance which enables an extremely narrow absorption dip for an enhanced transmission. Through the transmission of a probe beam, we obtain the distribution of Rydberg atoms with both high spatial resolution and fast response, which ensures a more precise real-time imaging. Increased resolution compared to the prior interaction-enhanced imaging technique allows us to accurately locate the atoms by adjusting the probe detuning only. This type of interaction-enhanced transmission imaging can be utilized in other impure systems containing strong many-body interactions, and is promising to develop superresolution microscopy of cold atoms.

Phase lag associated with coherent control where an excited system decays into more than one product channel has been subjected to numerous investigations. Although previous theoretical studies have treated the phase lag across resonances in model calculations, quantitative agreement has never been achieved between the theoretical model and an experimental measurement of the phase lag from the $\omega -2\omega$ ionization of atomic barium. Yamazaki and Elliot, Phys. Rev. Lett. 98, 053001 (2007); Phys. Rev. A 76, 053401 (2007), suggesting that a toy model with phenomenological parameters is inadequate to describe the observed phase lag behavior. Here the phase lag is treated quantitatively in a multichannel coupling formulation, and our calculation based on a multichannel quantum defect and $R$-matrix treatment achieves good agreement with the experimental observations. Our treatment also develops formulas to describe the effects of hyperfine depolarization on multiphoton ionization processes. Moreover, we identify resonances between ${\text{Ba}}^{+}5d_{3/2}$ and $5d_{5/2}$ thresholds that have apparently never been experimentally observed and classified.

The photoelectron momentum distribution (PMD) in two-photon XUV-IR ionization of Ne is studied theoretically by numerical solution of the time-dependent Schrödinger equation and within the analytical soft photon approximation. The symmetry of the PMD, projected on the plane encompassing the joint XUV-IR linear polarization axis, reveals the angular momentum composition of the photoelectron wave packet. This composition, in turn, allows to examine the validity of the angular momentum propensity rules which govern the bound-continuum and continuum-continuum photon driven transitions. The Fano propensity rule for bound-continuum transitions that favors an increase in the angular momentum is confirmed. However, its extension to continuum-continuum transitions suggested by Busto et al. [Phys. Rev. Lett. 123, 133201 (2019)] is less straightforward. In addition, we break the time-reversal symmetry of the XUV field by combining the odd and even multiples of the fundamental frequency. By doing so, we change the time and angular structure of the photoelectron continuum.

High-order harmonics can be generated in the gas, solid, plasma, and liquid samples driven by intense ultrafast lasers. The microscopic mechanisms of the former three have been well studied. However, liquid high-order harmonic generation (HHG) has demonstrated many unexpected results compared to other materials. Due to the complexity of liquids, it is still unknown what the quantum origin of the liquid HHG is. Here we reveal the role of localized charge-resonance states in HHG from disordered liquids. A quantum theory based on statistical two-level resonance is developed, which explains well almost all the characters of liquid harmonics known so far, such as the cutoff energy independence of wavelength. It may shed light on the optimal control of harmonic generation in liquids.

We theoretically investigate the temporal double-slit interference (DSI) structures in photoelectron momentum distribution driven by an orthogonally polarized two-color (OTC) laser field and focus on the nonadiabatic effect on the subcycle electron dynamics. Distinct discrepancies in the interference structures simulated with a time-dependent Schrödinger equation (TDSE) and an adiabatic semiclassical Monte Carlo approach are observed. By adopting the initial conditions derived from the nonadiabatic tunneling picture into the Monte Carlo algorithm, the improved nonadiabatic model well reproduces the TDSE result. Further analysis shows that the improvement is mainly attributed to the nonzero electron initial longitudinal velocity from the nonadiabatic tunneling and moreover, its influence on the subsequent electron dynamics is revealed. Our work demonstrates the sensitivity of the temporal DSI structures in an OTC field to the exit momentum, providing a way for comprehension of the nonadiabatic tunneling electron dynamics.

We study theoretically the phase diagram of strongly coupled two-dimensional Bose-Fermi mixtures interacting with attractive short-range potentials as a function of the particle densities. We focus on the limit where the size of the bound state between a boson and a fermion is small compared to the average interboson separation and develop a functional-renormalization-group approach that accounts for the bound-state physics arising from the extended Fröhlich Hamiltonian. By including three-body correlations we are able to reproduce the polaron-to-molecule transition in two-dimensional Fermi gases in the extreme limit of vanishing boson density. We predict frequency- and momentum-resolved spectral functions and study the impact of three-body correlations on quasiparticle properties. At finite boson density, we find that when the bound-state energy exceeds the Fermi energy by a critical value, the fermions and bosons can form a fermionic composite with a well-defined Fermi surface. These composites constitute a Fermi sea of dressed Feshbach molecules in the case of ultracold atoms, while in the case of atomically thin semiconductors a trion liquid emerges. As the boson density is increased further, the effective energy gap of the composites decreases, leading to a transition into a strongly correlated phase where polarons are hybridized with molecular degrees of freedom. We highlight the universal connection between two-dimensional semiconductors and ultracold atoms, and we discuss perspectives for further exploring the rich structure of strongly coupled Bose-Fermi mixtures in these complementary systems.

Exceptional point (EP) degeneracies in coupled cavities with gain and loss provide on-chip photonic devices with unconventional features and performance. However, such systems with realistic structures often miss the exact EPs even in simulation, and the mechanism of this EP disruption has yet to be thoroughly identified. Here we extend the coupled-mode theory of one-dimensional non-Hermitian resonator arrays to study the effects of the imaginary part of the intercavity coupling, which is a second-order term and attributed to material amplification, absorption, and radiation. By taking an appropriate gauge for the model, we clarify that the imaginary coupling components have a symmetric form in the effective Hamiltonian and hence represent non-Hermiticity. These additional factors can lift the gain- and loss-based EP degeneracies. However, they are proportional to the sum of the imaginary permittivities for involved cavity pairs. Thus, when the amplification and absorption of adjacent cavities are balanced, their contribution to the imaginary coupling is canceled and the EP singularity can be restored. Radiation-induced imaginary couplings measure the change in net radiation loss by the interference between cavity modes. Their impact on the EP can also be counteracted by small cavity resonance detuning even in loss-biased cases. We show and analyze eligible simulation examples based on photonic crystal nanocavities and highlight the design of an ideal EP degeneracy that is protected by generalized parity-time symmetry and induced by radiation.

We describe an efficient numerical method for simulating the dynamics of interacting spin ensembles in the presence of dephasing and decay. The method builds on the discrete truncated Wigner approximation for isolated systems, which combines the mean-field dynamics of a spin ensemble with a Monte Carlo sampling of discrete initial spin values to account for quantum correlations. Here we show how this approach can be generalized for dissipative spin systems by replacing the deterministic mean-field evolution by a stochastic process, which describes the decay of coherences and populations while preserving the length of each spin. We demonstrate the application of this technique for simulating nonclassical spin-squeezing effects or the dynamics and steady states of cavity QED models with ${10}^5$ interacting two-level systems. This opens up the possibility to perform accurate real-scale simulations of a diverse range of experiments in quantum optics or with solid-state spin ensembles under realistic laboratory conditions.

We present a supervised learning model to calibrate the photon collection rate during the fluorescence imaging of cold atoms. The linear regression model finds the collection rate at each location on the sensor such that the atomic population difference equals that of a highly precise optical cavity measurement. This 192 variable regression results in a measurement variance 27% smaller than our previous single variable regression calibration. The measurement variance is now in agreement with the theoretical limit due to other known noise sources. This model efficiently trains in less than a minute on a standard personal computer's CPU and requires less than 10 min of data collection. Furthermore, the model is applicable across a large change in population difference and across data collected on different days.

We analyze the dynamics of one-dimensional quantum gases with strongly attractive contact interactions. We specify a class of initial states for which attractive forces effectively act as strongly repulsive ones during the time evolution. Our findings extend the theoretical results on the super-Tonks-Girardeau gas to a highly nonequilibrium dynamics. The mechanism is illustrated on the prototypical problem of the domain stability in a two-component Fermi gas. We also discuss finite-range interactions and analyze the universality of the presented results. Moreover, we use our conclusions to argue for the existence of metastable quantum droplets in the regime of strongly attractive contact and attractive dipolar interactions.

Universal quantum computation encoded over continuous variables can be achieved via Gaussian measurements acting on entangled non-Gaussian states. However, due to the weakness of available nonlinearities, generally these states can only be prepared conditionally, potentially with low probability. Here we show how universal quantum computation could be implemented unconditionally using an integrated platform able to sustain both linear and quadratic optomechanical-like interactions. Specifically, considering cavity opto- and electromechanical systems, we propose a realization of a driven-dissipative dynamics that deterministically prepares the required non-Gaussian cluster states—entangled squeezed states of multiple mechanical oscillators suitably interspersed with cubic-phase states. We next demonstrate how arbitrary Gaussian measurements on the cluster nodes can be performed by continuously monitoring the output cavity field. Finally, the feasibility requirements of this approach are analyzed in detail, suggesting that its building blocks are within reach of current technology.

Most of the traditional attosecond pulse retrieval algorithms are based on a so-called attosecond streak camera technique, in which the momentum of the electron is shifted by an amount depending on the relative time delay between the attosecond pulse and the streaking infrared pulse. Thus, temporal information of the attosecond pulse is encoded in the amount of momentum shift in the streaked photoelectron momentum spectrogram $S(p,\tau )$, where $p$ is the momentum of the electron along the polarization direction and $\tau$ is the time delay. An iterative algorithm is then employed to reconstruct the attosecond pulse from the streaking spectrogram. This method, however, cannot be applied to attosecond pulses generated from free-electron x-ray lasers where each single shot is different and stochastic in time. However, using a circularly polarized infrared laser as the streaking field, a two (or three)-dimensional angular streaking electron spectrum can be used to retrieve attosecond pulses for each shot, as well as the time delay with respect to the circularly polarized IR field. Here we show that a retrieval algorithm previously developed for the traditional streaking spectrogram can be modified to efficiently characterize single-shot attosecond pulses. The methods have been applied to retrieve 188 single shots from recent experiments. We analyze the statistical behavior of these 188 pulses in terms of pulse duration, bandwidth, pulse peak energy, and time delay with respect to the IR field. The retrieval algorithm is efficient and can be easily used to characterize a large number of shots in future experiments for attosecond pulses at free-electron x-ray laser facilities.

Large clouds of atoms in a magneto-optical trap (MOT) are known to exhibit spatiotemporal instabilities when the frequency of the trapping lasers comes close to the atomic resonance. Such instabilities possess similarities with stars and confined plasmas, where corresponding nonlinearities may give rise to spontaneous oscillations. In this paper, we describe the kinetic model that has recently been employed in three-dimensional (3D) simulations of spatiotemporal instabilities in a MOT, yielding qualitative agreements with experimentally observed instability thresholds and regimes. Details surrounding its implementation are included, and the impact of its physical effects on the instabilities is investigated to improve the understanding of the complex mechanism at work.

Precision tilt measurements are becoming more and more important in fundamental research and crustal deformation observations. A quantum tiltmeter based on atom interferometry could be among the most stable tiltmeters in theory but is currently constrained by the low resolution. Here, we demonstrate an extremely sensitive quantum tiltmeter with the configuration of a Mach-Zehnder-type atom interferometer for precisely measuring the crustal deformation. The scheme of two pairs of Raman beams sharing one retroreflection mirror effectively increases the scale factor, making this quantum tiltmeter achieve a sensitivity of $40\mathrm{nrad}/{\mathrm{Hz}}^{1/2}$ and a resolution of 1.8 nrad within an integration time of 640 s for tilt measurements, which represents an improvement on the sensitivity by a factor of 32 comparing to previous work. The Earth tilt tide lasting for 63 h has been observed with high signal-to-noise ratio by our quantum tiltmeter, which paves the way to study the crustal deformation with a quantum sensor.

An atom placed in an optical vortex close to the axis may, upon absorbing a photon, acquire a transverse momentum much larger than the transverse momentum of any plane-wave component of the vortex light field. This surprising phenomenon dubbed “superkick” has been clarified previously in terms of the atom wave-packet evolution in the field of an optical vortex treated classically. Here, we study this effect within the quantum field theoretical (QFT) framework. We consider collision of a Bessel twisted wave with a compact Gaussian beam focused to a small focal spot $\sigma$ located at distance $b$ from the twisted beam axis. Through a qualitative discussion supported by exact analytical and numerical calculations, we recover the superkick phenomenon for $\sigma \ll b$ and explore its limits when $\sigma$ becomes comparable to $b$. On the way to the final result within the QFT treatment, we encountered and resolved apparent paradoxes related to subtle issues of the formalism. These results open a way to a detailed QFT exploration of other superkick-related effects recently suggested to exist in high-energy collisions.

Shortcuts to adiabaticity are alternative fast processes which reproduce the same final state as the adiabatic process in a finite or even shorter time, which have been extended from Hermitian systems to non-Hermitian systems in recent years, but they are barely explored for general non-Hermitian systems where off-diagonal elements of the Hamiltonian are not Hermitian. In this paper, we propose a shortcuts to adiabaticity technique which is based on a transitionless quantum driving algorithm to realize population transfer for general two-level non-Hermitian systems and give both exact and approximate analytical solutions of the corresponding counteradiabatic driving Hamiltonian, where the latter can be extended to the zeroth-order and first-order terms by applying perturbative theory. We find that the first-order correction term is different from the previous results, which is caused by the non-Hermiticity of the off-diagonal elements. We work out an exact expression for the control function and present examples consisting of a general two-level system with gain and loss to show the theory. The results suggest that the high-fidelity population transfer can be implemented in general non-Hermitian systems by our method, which works even with strong non-Hermiticity and without rotating wave approximation. Furthermore, we show that the general Hamiltonian the off-diagonal elements of which are not conjugate to each other can be implemented in many physical systems with the present experimental technology, such as an atom-light interaction system and whispering-gallery microcavity, which might have potential applications in quantum information processing.

Thin-film cavities containing layers of Mössbauer nuclei have been demonstrated to be a rich platform for x-ray quantum optics. At low excitation, these systems can be described by effective few-level schemes, thereby providing tunable artificial quantum systems at hard x-ray energies. With the recent advent of an ab initio theory, a numerically efficient description of these systems is now possible. On this basis, we introduce the inverse design and develop a comprehensive optimization for an archetypal system with a single resonant layer, corresponding to an artificial two-level scheme. We discover a number of qualitative insights into x-ray photonic environments for nuclei that will likely impact the design of future x-ray cavities and thereby improve their performance. The methods presented readily generalize beyond the two-level case and thus provide a clear perspective towards the inverse design of more advanced tunable x-ray quantum optical level schemes.

We propose a quantum metrological protocol based on a Mach-Zehnder interferometer with a squeezed vacuum input state and an antisqueezing operation at one of its output channels. A simple and intuitive geometrical picture of the state evolution is provided by the marginal Wigner functions of the state at each interferometer output channel. The protocol allows us to detect the values of the sum $\beta =\frac{1}{2}({\phi }_1+{\phi }_2)+{\theta }_{\mathrm{in}}-{\theta }_{\mathrm{out}}$, of the relative phase ${\theta }_{\mathrm{in}}-{\theta }_{\mathrm{out}}$ between the two squeezers, and of the average of the phase delays ${\phi }_1$ and ${\phi }_2$ in the two arms of the interferometer. The detection sensitivity scales at the Heisenberg limit and, remarkably, is robust not only to detector inefficiencies but also to any photon losses occurring before the antisqueezing operation. Interestingly, we demonstrate that in the latter case an increase of sensitivity can even occur by increasing the losses in a suitable range.