Spontaneous decays of small, hot silver-cluster anions ${\text{Ag}}_n^{-}$, $n=4–7$, have been studied using one of the rings of the Double ElectroStatic Ion Ring ExpEriment (DESIREE). Observation of these decays over very long time scales is possible due to the very low residual gas pressure ($\sim {10}^{-14}$) and cryogenic (13 K) operation of DESIREE. The yield of neutral particles from stored beams of ${\text{Ag}}_6^{-}$ and ${\text{Ag}}_7^{-}$ anions were measured for 100 milliseconds and were found to follow single power-law behavior with millisecond time-scale exponential cutoffs. The ${\text{Ag}}_4^{-}$ and ${\text{Ag}}_5^{-}$ anions were stored for 60 s and the observed decays show two-component power-law behaviors. We present calculations of the rate constants for electron detachment from and fragmentation of ${\text{Ag}}_4^{-}$ and ${\text{Ag}}_5^{-}$. In these calculations, we assume that the internal energy distribution of the clusters are flat and with this we reproduce the early steep parts of the experimentally measured decay curves for ${\text{Ag}}_4^{-}$ and ${\text{Ag}}_5^{-}$, which extends to tens and hundreds of milliseconds, respectively. The fact that the calculations reproduce the early slopes of ${\text{Ag}}_4^{-}$ and ${\text{Ag}}_5^{-}$, which differ for the two cases, suggests that it is the changes in fragmentation rates with internal cluster energies of ${\text{Ag}}_4^{-}$ and ${\text{Ag}}_5^{-}$ rather than conditions in the ion source that determine this behavior. Comparisons with the measurements strongly suggest that the neutral particles detected in these time domains originate from ${\text{Ag}}_4^{-}\to {\text{Ag}}_3^{-}+\text{Ag}$ and ${\text{Ag}}_5^{-}\to {\text{Ag}}_3^{-}+{\mathrm{Ag}}_2$ fragmentation processes.

We perform a detailed experimental study of resistive cooling of large ensembles of highly charged ions such as ${\mathrm{Ar}}^{13+}$ in a cryogenic Penning trap. Different from the measurements reported by Vogel et al. [M. Vogel et al., Phys. Rev. A 90, 043412 (2014)], we observe purely exponential cooling behavior when conditions are chosen to allow collisional thermalization of the ions. We provide evidence that in this situation resistive cooling time constants and final temperatures are independent of the initial ion energy and that the cooling time constant of a thermalized ion ensemble is identical to the single-ion cooling time constant. For sufficiently high ion number densities, our measurements show discontinuities in the spectra of motional resonances which indicate a transition of the ion ensemble to a fluidlike state when cooled to temperatures below approximately 14 K. With the final ion temperature presently being 7.5 K, ions of the highest charge states are expected to form ion crystals by mere resistive cooling, in particular not requiring the use of laser cooling.

We introduce and demonstrate double-bright electromagnetically-induced-transparency (D-EIT) cooling as an extension to EIT cooling. By involving an additional ground state, two bright states can be shifted individually into resonance for cooling of motional modes of frequencies that may be separated by more than the width of a single EIT cooling resonance. This allows three-dimensional ground-state cooling of a ${}^{40}\mathrm{Ca}^{+}$ ion trapped in a linear Paul trap with a single cooling pulse. Measured cooling rates and steady-state mean motional quantum numbers for this D-EIT cooling are compared with those of standard EIT cooling as well as concatenated standard EIT cooling pulses for multimode cooling. Experimental results are compared to full-density matrix calculations. We observe a failure of the theoretical description within the Lamb-Dicke regime that can be overcome by a time-dependent rate theory. Limitations of the different cooling techniques and possible extensions to multi-ion crystals are discussed.

We apply optimal-control techniques to an inhomogeneous spin ensemble coupled to a cavity. A general procedure is proposed for designing the control strategies. We numerically show the extent to which optimal-control fields robust against system uncertainties help enhance the sensitivity of the detection process. The parameters of the numerical simulations are taken from recent electron spin resonance experiments. The low and high cooperativity regimes are explored.

We report the observation of multiple ionization of argon through multi-XUV-photon absorption induced by an unprecedentedly powerful laser driven high-order harmonic generation source. Comparing the measured intensity dependence of the yield of the different argon charge states with numerical calculations we can infer the different channels—direct and sequential—underlying the interaction. While such studies were feasible so far only with free electron laser (FEL) sources, this paper connects highly nonlinear XUV processes with the ultrashort time scales inherent to the harmonic pulses and highlights the advanced perspectives of emerging large scale laser research infrastructures.

We investigate the properties of the superfluid phase in the three-dimensional disordered Bose-Hubbard model using quantum Monte Carlo simulations. The phase diagram is generated using Gaussian disorder on the on-site potential. Comparisons with box and speckle disorder show qualitative similarities leading to the reentrant behavior of the superfluid. Quantitative differences that arise are controlled by the specific shape of the disorder. Statistics pertaining to disorder distributions are studied for a range of interaction strengths and system sizes, where strong finite-size effects are observed. Despite this, both the superfluid fraction and compressibility remain self-averaging throughout the superfluid phase. Close to the superfluid–Bose-glass phase boundary, finite-size effects dominate but still suggest that self-averaging holds. Our results are pertinent to experiments with ultracold atomic gases where a systematic disorder averaging procedure is typically not possible.

We calculate quantum and classical Fisher information for gravity sensors based on matter-wave interference, and find that current Mach-Zehnder interferometry is not optimally extracting the full metrological potential of these sensors. We show that by making measurements that resolve either the momentum or the position we can considerably improve the sensitivity. We also provide a simple modification that is capable of more than doubling the sensitivity.

Quantum droplets have aroused much attention recently in view of their successful observations in ultracold homonuclear atoms. In this paper, we demonstrate an alternative mechanism for the formation of quantum droplets in heteronuclear atomic systems, i.e., by applying synthetic spin-orbit coupling (SOC). Taking the Bose-Fermi mixture, for example, we show that by imposing a Rashba SOC between the spin states of fermions such that all fermions occupy the lower helicity branch, the greatly suppressed Fermi pressure can enable the formation of Bose-Fermi droplets even for very weak boson-fermion attractions, which are insufficient to bound a droplet if without SOC. In such SOC-induced quantum droplets, the boson-fermion density ratio universally depends on the SOC strength, and they occur in the mean-field collapsing regime but with a negative fluctuation energy, distinct from the interaction-induced droplets found in literature. The accessibility of these Bose-Fermi droplets in ultracold Cs-Li and Rb-K mixtures is also discussed. Our results shed light on the droplet formation in a vast class of heteronuclear atomic systems through the manipulation of single-particle physics.

We study the interplay between the long- and short-range interaction of a one-dimensional optical lattice system of two-component dipolar fermions by using the density matrix renormalization group method. The atomic density profile, pairing-pairing correlation function, and compressibility are calculated in the ground state, from which we identify the parameter region of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing state, half-metal (HM) state, FFLO-HM state, and the normal polarized state, and thus the phase diagram in the coordinates of the long- and short-range interaction strength. The effect of the long-range dipolar interaction on the FFLO state is discussed in detail. We find that the long-range part of the dipole-dipole interaction does not sweep away the FFLO superconducting region that is driven by the short-range interaction in the Hubbard model, and thus the FFLO state survives in the wide parameter space of the long-range interaction, polarization, and filling.

The advanced-wave picture is “... an intuitive treatment of two-photon correlation with the help of the concept of an effective field acting upon one of the two detectors and formed by parametric conversion of the advanced wave emitted by the second detector ...” [A. V. Belinskii and D. N. Klyshko, Sov. Phys. JETP 78, 259 (1994)]. This quote from Belinskii and Klyshko nicely describes the concept of the advanced-wave picture: an intuitive tool for designing and predicting results from coincidence-based two-photon experiments. Up to now, the advanced-wave picture has been considered primarily for the case of an ideal plane-wave pump beam and only for design purposes. Here we study the advanced-wave picture for a structured pump beam and in the context of stimulated emission provoked by an auxiliary input laser beam. This suggests stimulated parametric down-conversion as a useful experimental tool for testing the experimental sets designed with the advanced-wave picture. We present experimental results demonstrating the strategy of designing the experiment with advanced-wave picture and testing with stimulated emission.

Recently, it has been shown numerically and by a semiclassical approach [Phys. Rev. Lett. 120, 093902 (2018)] that second-order exceptional points can appear in the spectrum of weakly deformed microdisk resonators by coupling of optical modes with different angular momentum. Here, we use a perturbation theory for weak boundary deformations to derive an effective non-Hermitian Hamiltonian from which we obtain analytical formulas to describe the formation of second-order exceptional points in such systems. The theory is extended to third-order exceptional points and confirmed by full numerical calculations.

We present a detailed theoretical description of a recently proposed atomic scanning microscope in a cavity QED setup [D. Yang et al., Phys. Rev. Lett. 120, 133601 (2018)]. The microscope continuously observes atomic densities with optical subwavelength resolution in a nondestructive way. The superresolution is achieved by engineering an internal atomic dark state with a sharp spatial variation of population of a ground level dispersively coupled to the cavity field. Thus, the atomic position encoded in the internal state is revealed as a phase shift of the light reflected from the cavity in a homodyne experiment. Our theoretical description of the microscope operation is based on the stochastic master equation describing the conditional time evolution of the atomic system under continuous observation as a competition between dynamics induced by the Hamiltonian of the system, decoherence effects due to atomic spontaneous decay, and the measurement backaction. Within our approach we relate the observed homodyne current with a local atomic density and discuss the emergence of a quantum nondemolition measurement regime allowing continuous observation of spatial densities of quantum motional eigenstates without measurement backaction in a single experimental run.

We investigate the radiation emitted by an ultrarelativistic electron traveling in a one-dimensional parabolic potential. Having in mind a simplified model for beamstrahlung, we consider the realistic case of the electron motion being highly directional, with the transverse momentum being much smaller than the longitudinal one. In this case, we can find approximate solutions of the Dirac equation and we calculate the radiation emission using first-order perturbation theory. We compare our results to those obtained via the semiclassical method of Baier and Katkov, which includes quantum effects due to photon recoil in the radiation emission but ignores the quantum nature of the electron motion. On the one hand, we confirm a prediction of the semiclassical method that the emission spectrum should coincide with that in the case of a linearly polarized monochromatic wave. On the other hand, however, we find that the semiclassical method does not yield the exact result when the quantum number describing the transverse motion becomes small. In this way, we address quantitatively the problem of the limits of validity of the semiclassical method, which is known, generally speaking, to be applicable for large quantum numbers. Finally, we also discuss which beam conditions would be necessary to enter the studied regime where the motion of the particles must also be considered quantum mechanically to yield the correct spectrum.

Motivated by the recent proposal for the quantification of the Bell nonlocality [Phys. Rev. A 92, 030101(R) (2015)], we provide an extensive analysis of this concept, namely, the volume of violation. This new measure of nonlocality has been proposed to deliver an evidence that the anomaly between maximally entangled states and states that maximally violate a Bell inequality is caused by the method, which has been applied to quantify nonlocality and the anomaly disappears when the volume of violation in used. We prove that such conclusion is not true for all bipartite quantum systems with dimension $d\otimes d$. In fact, if one assumes the same condition as in Phys. Rev. A 92, 030101(R) (2015), it is limited to $d\le 7$. Furthermore, we discuss several type of local measurements and their influence on the quantification of nonlocality by mean of volume of violation. In particular, we propose a set of local observers that significantly enhance the volume of violation, which may have important meaning in a real-world application. Finally, we present a comparison between the volume of violation and the maximal violation of a Bell inequality.

We show that two related measures of $k$-coherence, called the standard and generalized robustness of $k$-coherence, are equal to each other when restricted to pure states. As a direct application of the result, we establish an equivalence between two analogous measures of Schmidt rank $k$-entanglement for all pure states. This answers conjectures raised in the literature regarding the evaluation of the quantifiers and facilitates an efficient quantification of pure-state resources by introducing computable closed-form expressions for the two measures.

X-ray transition energies and isotope shifts in heavy atoms are evaluated. The energy levels with vacancies in the inner shells are calculated within the approximation of the average of a nonrelativistic configuration employing the Dirac-Fock-Sturm method. The obtained results are compared with other configuration-interaction theoretical calculations and with experimental data.

We present an efficient scheme to implement a gray optical molasses for sub-Doppler cooling of ${}^6\mathrm{Li}$ atoms with minimum experimental overhead. To integrate the $D_1$ light for the gray molasses cooling into the same optical setup that is used for the $D_2$ light for a standard magneto-optical trap (MOT), we rapidly switch the injection seeding of a slave laser between the $D_2$ and the $D_1$ light sources. Switching times as short as 30 $\mu \mathrm{s}$ can be achieved, inferred from monitor optical beat signals. The resulting low-intensity molasses cools a sample of $N=9\times {10}^8$ atoms to about 60 $\mu \mathrm{K}$. A maximum phase-space density of $\rho =1.2\times {10}^{-5}$ is observed. On the same setup, the performance of the GM is compared to that of narrow-line cooling in an ultraviolet (UV) MOT, following the procedure in Sebastian et al. [J. Sebastian, Ch. Gross, K. Li, H. C. J. Gan, W. Li, and K. Dieckmann, Phys. Rev. A 90, 033417 (2014)]. Further, we compare the production of a degenerate Fermi gas using both methods. Loading an optical dipole trap from the gray molasses yields a quantum degenerate sample with $3.3\times {10}^5$ atoms, while loading from the denser UV MOT yields $2.4\times {10}^6$ atoms. Where the highest atom numbers are not a priority this implementation of the gray molasses technique yields sufficiently large samples for a comparatively low technical effort.

Experiments on the nonequilibrium dynamics of an isolated Bose-Einstein condensate (BEC) in a magnetic double-well trap exhibit a puzzling divergence: While some show dissipation-free Josephson oscillations, others find strong damping. Such damping in isolated BECs cannot be understood on the level of the coherent Gross-Pitaevskii dynamics. Using the Keldysh functional-integral formalism, we describe the time-dependent system dynamics by means of a multimode BEC coupled to fluctuations (single-particle excitations) beyond the Gross-Pitaevskii saddle point. We find that the Josephson oscillations excite an excess of fluctuations when the effective Josephson frequency ${\tilde{\omega }}_J$ is in resonance with the effective fluctuation energy ${\tilde{\varepsilon }}_m$, where both ${\tilde{\omega }}_J$ and ${\tilde{\varepsilon }}_m$ are strongly renormalized with respect to their noninteracting values. By evaluating and using the model parameters for the respective experiments, we describe quantitatively the presence or absence of damping.

We theoretically investigate the behavior of a mobile impurity immersed in a one-dimensional quasiperiodic Fermi system with topological $p$-wave superfluidity. This polaron problem is solved by using a standard variational approach, the so-called Chevy ansatz. The polaron states are found to be strongly affected by the strength of the quasidisorder and the amplitude of the $p$-wave pairing. We analyze the phase diagram of the polaron ground state and find four phases: two extended phases, a weakly localized phase, and a strongly localized phase. It is remarkable that these polaron phases are directly corresponding to the four distinct phases experienced by the underlying background Fermi system. In particular, the weakly localized polaron phase corresponds to an intriguing critical phase of the Fermi system. Therefore, the different phases of the background system can be unambiguously probed by measuring the polaron properties via radio-frequency spectroscopy. We also investigate the high-lying polaron excited states at an infinite temperature and address the possibility of studying many-body localization of these states. We find that the introduction of $p$-wave pairing may delocalize the many-body localized states and make the system easier to thermalize. Our results could be observed in current state-of-the-art cold-atom experiments.

We present a unified picture of dispersive readout of quantum systems in and out of equilibrium. A cornerstone of the approach is the backaction of the measured system to the cavity obtained with nonequilibrium linear-response theory. It provides the dispersive shift of the cavity frequency in terms of a system susceptibility. It turns out that already effortless computations of the susceptibility allow one to generalize former results beyond a rotating-wave approximation. Examples are the readout of detuned qubits and thermally excited multilevel systems. For ac-driven quantum systems, we identify the relevant Fourier component of the susceptibility and introduce a computational scheme based on Floquet theory. The usefulness is demonstrated for two-tone spectroscopy and interference effects in driven two-level systems. This also reveals that dispersive readout does not necessarily measure excitation probabilities.