The real-space picture of electron recollision with the parent ion guides our understanding of the highly nonlinear response of atoms and molecules to intense low-frequency laser fields. It is also an important player in high harmonic generation (HHG) in solids. It is typically viewed in the momentum space, as the recombination of the conduction band electron with the valence band hole, competing with another HHG mechanism, the strong-field driven Bloch oscillations. In this work, we use numerical simulations to directly test and confirm the real-space recollision picture as the key mechanism of HHG in solids. Our tests take advantage of the well-known characteristic features in the molecular harmonic spectra, associated with the real-space structure of the molecular ion. We show the emergence of analogous spectral features when similar real-space structures are present in the periodic potential of the solid-state lattice. This work demonstrates the capability of HHG imaging of spatial structures of a unit cell in solids.

We study high-order harmonic generation (HHG) from oriented isotope molecules beyond the Born-Oppenheimer approximation numerically and analytically. We compare the symmetric cases of ${\mathrm{H}}_2$ and its isotopic variant ${\mathrm{T}}_2$ to the asymmetric ones of ${\mathrm{HeH}}^{+}$ and ${\mathrm{HeT}}^{+}$. Different from symmetric molecules, the laser-induced stretching for asymmetric molecules depends strongly on the molecular orientation. In addition, the structures of the HHG spectra for oriented ${\mathrm{H}}_2$ versus ${\mathrm{T}}_2$ differ remarkably from each other, and one can trace the stretching of these symmetric isotope molecules directly from the spectra. However, the spectral structures for oriented ${\mathrm{HeH}}^{+}$ versus ${\mathrm{HeT}}^{+}$ are basically the same. In this situation, we propose a procedure for probing the stretching of these asymmetric isotope molecules using odd-even HHG, with a high time-space resolution.

We report on investigations of nonlinear magneto-optical rotation (NMOR) in rubidium vapor subjected to a modulated magnetic field and continuous-wave (cw) laser-light illumination. By superimposing modulation and a static (dc) magnetic field, we demonstrate the appearance of resonances at both small and large (compared to the ground-state relaxation rate) values of the static field. Since in conventional NMOR there is no rotation at high fields, this suggests an existence of an alternative mechanism generating anisotropy in the considered case, which we identify as parametric resonance. The experiments are performed using light of small ellipticity, and rotation signals are significantly enhanced by combining atom-induced polarization rotation with a passive rotation induced with a wave plate. All the observations are supported with theoretical simulations. The density-matrix formalism and angular-momentum probability surfaces are used to provide an intuitive explanation of the observed signals.

The authors of this paper [M. Pandit et al., Phys. Rev. A 100, 012131 (2019)] proposed an alternative formulation of the uncertainty relation in quantum mechanics in terms of the Lipschitz constants. They evaluated these constants in various cases and decided that the product of the square roots of Lipschitz constants for position and momentum probability distributions is bounded below by $0.3/\hslash$. In this comment, I invalidate this claim by giving an example of wave functions for which the value of the product of the Lipschitz constants for position and momentum does not obey this bound and tends to zero.

We show that the dynamics of any open quantum system that is initially correlated with its environment can be described by a set of $d^2$ (or less) completely positive maps, where $d$ is the dimension of the system. Only one such map is required for the special case of no initial correlations. The same maps describe the dynamics of any system-environment state obtained from the initial state by a local operation on the system. The reduction of the system dynamics to a set of completely positive maps allows known numerical and analytic tools for uncorrelated initial states to be applied to the general case of initially correlated states, which we exemplify by solving the qubit dephasing model for such states, and provides a natural approach to quantum Markovianity for this case. We show that this set of completely positive maps can be experimentally characterized using only local operations on the system, via a generalization of noise spectroscopy protocols. As further applications, we first consider the problem of retrodicting the dynamics of an open quantum system which is in an arbitrary state when it becomes accessible to the experimenter and explore the conditions under which retrodiction is possible. We also introduce a related one-sided or limited-access tomography protocol for determining an arbitrary bipartite state, evolving under a sufficiently rich Hamiltonian, via local operations and measurements on just one component. We simulate this protocol for a physical model of particular relevance to nitrogen-vacancy centers and in particular show how to reconstruct the density matrix of a set of three qubits, interacting via dipolar coupling and in the presence of local magnetic fields, by measuring and controlling only one of them.

We report a frequency measurement on the $5s5p{}^{3}P_2-5s4d{}^{3}D_3$ transition of ${}^{88}\mathrm{Sr}$ atoms at $2.9\mu \mathrm{m}$. The excitation spectra are obtained by measuring the photon momenta transferred to ultracold atoms. The transition frequency is determined to be 102 550 196 205(9) kHz using an erbium fiber-based optical frequency comb. We numerically investigate possible frequency corrections by taking an acceleration of atoms and attenuation of the excitation light into account. This measurement technique is particularly useful for transitions where laser-induced fluorescence is difficult to observe but the spatial imaging of atoms is possible.

We report that high-order harmonic generation (HHG) of atoms driven by $\omega \text{−}2\omega$ bi-elliptical pumps exhibits a substantial sensitivity to variation of the spatial extent of the participating valence orbitals and effective potential. We identify the physical mechanisms responsible for this enhanced sensitivity through theoretical investigations in atomic He, Ne, Ar, and Kr systems using several quantum models. We demonstrate this size sensitivity experimentally by measuring significant disparities in the bi-elliptical HHG spectra of atomic Ar and Kr, which have relatively similar ionization potentials, but substantially different $p$-shell sizes and effective potentials. This result may be useful for ultrafast spectroscopy which is sensitive to effective potentials and orbitals.

We study a two-dimensional Fermi gas with an attractive interaction subjected to synthetic magnetic fields, which are assumed to be mutually antiparallel for two different spin components with population imbalance. By employing the mean-field approximation, we show that the Fulde-Ferrell state is energetically favored over the Larkin-Ovchinnikov state in the weak-coupling limit. We then elucidate the zero-temperature phase diagram in the space of attraction and two chemical potentials analytically at weak coupling as well as numerically beyond it. Rich structures consisting of quantum Hall insulator, unpolarized superfluid, and Fulde-Ferrell phases separated by various second-order and first-order quantum phase transitions are found.

The general formula for probability to record a twisted photon produced by a charged particle moving in an inhomogeneous isotropic dispersive medium is derived. The explicit formulas for probability to record a twisted photon are obtained for the radiation of a charged particle traversing a dielectric plate or an ideally conducting foil. It is shown that, in the case when the charged particle moves along the detector axis, all the radiated twisted photons possess a zero projection of the total angular momentum and the probability of their radiation is independent of the photon helicity. The radiation produced by helically microbunched beams of charged particles is also considered. The fulfillment of the strong addition rule for the projection of the total angular momentum of radiated twisted photons is demonstrated. Thus the helical beams allow one to generate coherent transitions and Vavilov-Cherenkov radiation with large projections of the total angular momentum. The radiation produced by charged particles in a helical medium is studied. Typical examples of such a medium are metallic spirals and cholesteric liquid crystals. It is shown that the radiation of a charged particle moving along the helical axis of such a medium is a pure source of twisted photons.

In a recent publication [Phys. Rev. A 98, 023851 (2018)] the authors reported on a systematic approach based on perturbation theory to generate non-Hermitian degeneracies, so-called exceptional points, in weakly deformed microdisk cavities by coupling modes of different angular momentum. In the present paper we extent this approach to fully asymmetric boundary deformations. This allows us to increase the order of the exceptional point and create modes with interesting angular momentum patterns where a nonuniform clockwise and counterclockwise propagation is present. Furthermore, we combine the perturbation theory of weakly deformed microdisks with the coupled-mode theory to explain possible measurements of transmission and reflection spectra via a waveguide attached to the cavity.

We experimentally explore the validity of the Jarzynski and Wójcik quantum heat-exchange fluctuation relation by implementing an interferometric technique in liquid-state nuclear magnetic resonance setup and study the heat-exchange statistics between two coupled spin-1/2 quantum systems. We experimentally emulate two models—(i) the XY-coupling model, containing an energy conserving interaction between the qubits, and (ii) the XX-coupling model—and analyze the regimes of validity and violation of the fluctuation symmetry when the composite system is prepared in an uncorrelated initial state with individual spins prepared in local Gibbs thermal states at different temperatures. We further extend our analysis for heat exchange by incorporating correlation in the initial state. We support our experimental findings by providing exact analytical results. Our experimental approach is general and can be systematically extended to study heat statistics for more complex out-of-equilibrium many-body quantum systems.

We present a theoretical study of bound-free electron-positron pair production in the interaction of $\gamma$ rays with bare ions. Special attention is paid to the longitudinal polarization of both the emitted positrons and the produced hydrogenlike ions. To evaluate this polarization we employed exact solutions of the relativistic Dirac equation and treat the electron-photon coupling within the framework of first-order perturbation theory. Detailed calculations have been performed for both low- and high-$Z$ ions and for a wide range of photon energies. The results of these calculations suggest that bound-free pair production can be a source of strongly polarized positrons and ions.

In this paper we present a protocol to evaluate partial and total Auger decay rates combining the restricted active space self-consistent field electronic structure method for the bound part of the spectrum and numerically obtained continuum orbitals in the single-channel scattering theory framework. Additionally, the two-step picture is employed to evaluate the partial rates. The performance of the method is exemplified for the prototypical Auger decay of the neon $1s{}^{-1}3p$ resonance. Different approximations to obtain the continuum orbitals, the partial rate matrix elements, and the electronic structure of the bound part are tested against theoretical and experimental reference data. It is demonstrated that the partial and total rates are most sensitive to the accuracy of the continuum orbitals. For instance, it is necessary to account for the direct Coulomb potential of the ion for the determination of the continuum wave functions. The Auger energies can be reproduced quite well already with a rather small active space. Finally, perspectives of the application of the proposed protocol to molecular systems are discussed.

The feasibility of producing the molecular antihydrogen anion ${{\overline{\mathrm{H}}}_2}^{-}$ in the laboratory is investigated. Utilizing reaction rates calculated here involving the interaction of laser excited-state antihydrogen atoms held in magnetic minimum traps, key processes are identified that could lead to anion production, as well as competing effects leading to anti-atom loss. These are discussed in the context of present-day and near-future experimental capabilities.

In trapped-ion quantum information processing, interactions between spins (qubits) are mediated by collective modes of motion of an ion crystal. While there are many different experimental strategies to design such interactions, they all face both technical and fundamental limitations to the achievable coherent interaction strength. In general, obtaining strong interactions and fast gates is an ongoing challenge. Here, we extend previous work [W. Ge, B. C. Sawyer, J. W. Britton, K. Jacobs, J. J. Bollinger, and M. Foss-Feig, Phys. Rev. Lett. 122, 030501 (2019)] and present a general strategy for enhancing the interaction strengths in trapped-ion systems via parametric amplification of the ions' motion. Specifically, we propose a stroboscopic protocol using alternating applications of parametric amplification and spin-motion coupling. In comparison with the previous work, we show that the current protocol can lead to larger enhancements in the coherent interaction that increase exponentially with the gate time.

It is shown that the dynamic differential scalar polarizability of the $S_{1/2}-D_{5/2}$ transition in ${}^{138}\mathrm{Ba}^{+}$ can be determined to an inaccuracy below $0.5\%$ across a wide wavelength range ($\lambda >700\mathrm{nm}$). This can be achieved using measurements for which accurate determination of laser intensity is not required, and most of the required measurements are already in the literature. Measurement of a laser-induced ac-Stark shift of the clock transition would then provide an in situ measurement of the laser's intensity to the same $0.5\%$ level of inaccuracy, which is not easily achieved by other means. This would allow accurate polarizability measurements for clock transitions in other ions, through comparison with ${}^{138}\mathrm{Ba}^{+}$. The approach would be equally applicable to ${\mathrm{Sr}}^{+}$ and ${\mathrm{Ca}}^{+}$, with the latter being immediately applicable to ${\mathrm{Al}}^{+}/{\mathrm{Ca}}^{+}$ quantum logic clocks.

We analytically study the expansion of a Bose-Einstein condensate in a ring-shaped trap with an increasing central radius. The evolution of the ground state is described using a scaling transform. Additionally, the dynamics of excited azimuthal modes over the varying ground state is analyzed through a generalization of the Bogoliubov–de Gennes approach. Our results explain some of the features observed in recent experiments focused on testing the applicability of the system as a parallel of cosmological inflationary models. The radial dynamics, which corresponds to the inflaton field of the cosmological counterpart, is analytically characterized: The expansion is found to induce the oscillatory displacement of the condensate as well as the coupled variation of the radial and vertical widths. Our findings account also for the observed redshift and emergence of the Hubble friction in the evolution of initially prepared azimuthal modes. Our description, which traces the role of the different components of the setup in the expansion, enhances the controllability and therefore the potential of the system as a ground for emulating the inflationary dynamics of cosmological models.

Nonstandard Bose-Hubbard models can exhibit rich ground-state phase diagrams, even when considering the one-dimensional limit. Using a self-consistent Gutzwiller diagonalization approach, we study the mean-field ground-state properties of a long-range interacting atomic gas in a one-dimensional optical lattice. We first confirm that the inclusion of long-range two-body interactions to the standard Bose-Hubbard model introduces density-wave and supersolid phases. However, the introduction of pair and density-dependent tunneling can result in new phases with two-site periodic density, single-particle transport, and two-body transport order parameters. These staggered phases are potentially a mean-field signature of the known novel twisted superfluids found via a density-matrix renormalization group approach [Phys. Rev. A 94, 011603(R) (2016)]. We also observe other unconventional phases which are characterized by sign staggered order parameters between adjacent lattice sites.

We propose a three-mode optomechanical system to realize optical nonreciprocal transmission with unidirectional amplification, where the system consists of two coupled cavities and one mechanical resonator which interacts with only one of the cavities. Additionally, the optical gain is introduced into the optomechanical cavity. It is found that for a strong optical input, the optical transmission coefficient can be greatly amplified in a particular direction and suppressed in the opposite direction. The expressions of the optimal transmission coefficient and the corresponding isolation ratio are given analytically. Our results pave a way to design high-quality nonreciprocal devices based on optomechanical systems.

We discuss how, in appropriately designed configurations, solenoids carrying a semifluxon can be used as topological energy barriers for charged quantum systems. We interpret this phenomenon as a consequence of the fact that such solenoids induce nodal lines in the wave function describing the charge, which on itself is a consequence of the Aharonov-Bohm effect. Moreover, we present a thought experiment with a cavity where two solenoids are sufficient to create bound states.