We present molecular above-threshold ionization (ATI) by multiple-frequency circularly polarized laser pulses. Simulations are performed on aligned ${\mathrm{H}}_2{}^{+}$ by numerically solving corresponding time-dependent Schrödinger equations. Molecular ATI spectra show maximum cutoff and multiple ionization trajectories which depend on the pulse frequency combination. We describe the laser-induced electron dynamics by multiple frequency circularly polarized pulses in a rotating frame where the total ponderomotive energy of electrons is defined by the average frequency of the pulses. The influence of Coriolis effects in the photoionization is also studied. The present results illustrate the dependence of molecular photoionization on the pulse frequency combination and helicity, thus allowing for the control of circular-polarization electron dynamics in the more complex molecular systems.

We investigate the physics of a gas of ultracold atoms coupled to three single-mode optical cavities and transversely pumped with a laser. Recent work has demonstrated that, for two cavities, the ${\mathbb{Z}}_2$ symmetries of each cavity can be combined into a global U(1) symmetry [J. Léonard et al., Nature (London) 543, 87 (2017)]. Here we show that when adding an extra cavity mode, the low-energy description of this system can additionally exhibit an SO(3) rotational symmetry which can be spontaneously broken. This leads to a superradiant phase transition in all the cavities simultaneously, and the appearance of Goldstone and amplitude modes in the excitation spectrum. We determine the phase diagram of the system, which shows the emergence and breaking of the continuous symmetries and displays first- and second-order phase transitions. We also obtain the excitation spectrum for each phase and discuss the atomic self-organized structures that emerge in the different superradiant phases. We argue that coupling the atoms equally to $n$ different modes will in general generate a global $\text{SO}\left(n\right)$ symmetry if the mode frequencies can be tuned to the same value.

Quantized vortices have been observed in a variety of superfluid systems, from ${}^4\mathrm{He}$ to condensates of alkali-metal bosons and ultracold Fermi gases along the BEC-BCS crossover. In this article we study the stability of singly quantized vortex lines in dilute dipolar self-bound droplets. We first discuss the energetic stability region of dipolar vortex excitations within a variational ansatz in the generalized nonlocal Gross-Pitaevskii functional that includes quantum fluctuation corrections. We find a wide region where stationary solutions corresponding to axially symmetric vortex states exist. However, these singly charged vortex states are shown to be unstable, either by splitting the droplet in two fragments or by vortex-line instabilities developed from Kelvin-wave excitations. These observations are the results of large-scale fully three-dimensional simulations in real time. We conclude with some experimental considerations for the observation of such states and suggest possible extensions of this work.

The variational JWKB method is used to determine experimentally accessible macroscopic quantum tunneling regimes of quasibound Bose-Einstein condensates in two quasi-one-dimensional trap configurations. The potentials can be created by magnetic and optical traps: a symmetric trap from two offset Gaussian barriers and a tilt trap from a linear gradient and Gaussian barrier. Scaling laws in barrier parameters, ranging from inverse polynomial to square root times exponential, are calculated and used to elucidate different dynamical regimes, such as when classical oscillations dominate tunneling rates in the symmetric trap. The symmetric trap is found to be versatile, with tunneling times at and below 1 s, able to hold ${10}^3–{10}^4$ atoms, and realizable for atoms ranging from rubidium to lithium, with unadjusted scattering lengths. The tilt trap produces subsecond tunneling times, is able to hold a few hundred atoms of lighter elements such as lithium, and requires the use of Feshbach resonance to reduce scattering lengths. To explore a large parameter space, an extended Gaussian variational ansatz is used, which can approximate large traps with Thomas-Fermi profiles. Nonlinear interactions in the Gross-Pitaevskii equation are shown to produce additional effective mean-field barriers, affecting scaling laws.

We design a single microwave photon switch through a one-dimensional waveguide coupled with a side Jaynes-Cummings system, namely, a single-mode cavity with an embedded Rydberg atom ${}^{87}\mathrm{Rb}$. Since the energy spectra of the ${}^{87}\mathrm{Rb}$ atom depend on the electrostatic field, the ${}^{87}\mathrm{Rb}$ atom can couple with the cavity in tune and out of tune, corresponding to the switch on and off states, respectively. The influences of the single photon pulse width, cavity dissipation, and Rabi coupling on the on/off ratio are studied in detail by a non-Hermitian Hamiltonian. We find that the on/off ratio can be larger than 100 in a wide parameter space and the decay time and rise time are both about 0.75 ns. Within the master equation of the density matrix we also demonstrate that the on/off ratio can be reduced by the quantum noise but is still large enough for application. The transition energy of the two-level Rydberg atom ${}^{87}\mathrm{Rb}$ gives the switch operating frequency which is in the range of microwave regime. Accordingly, the switch is viable in practice and has potential in quantum informatics.

We report experiments on propagation of scalar and vector optical beams through random phase screens mimicking turbulence and show that the intensity profile of the beam containing a C-point polarization singularity shows maximally robust behavior. This observation is explained in terms of the polarization and orbital angular momentum (OAM) diversity in the beam. The $l=0$ and $l=1$ OAM states whose vector superposition leads to the C-point singularity are seen to produce complementary speckle intensity patterns with significant negative correlation on propagation through a random phase screen. This unique property of C-point singularity makes it superior to other inhomogeneous polarization states as demonstrated in our simulations and experiments. The results provide an important generic guideline for designing beams that can maintain superior beam intensity profile on passing through random phase fluctuations and are expected to have a number of applications.

The capture and confinement of a large and pure sample of anions into an ion trap is challenging due to the fragile nature of these weakly bound atomic systems. We present the results of an experimental study using ${\mathrm{Au}}^{-}$ in which anions from a keV-energy continuous beam were loaded into a cylindrical Penning trap. Experimental parameters such as stopping potential and axial trap depth were found to be crucial. The transfer of axial to radial momentum due to ion–ion collisions was identified as a main contributing factor to successful capture. A total number of up to ${10}^7{\mathrm{Au}}^{-}$ ions were loaded. The cooling of anions by simultaneously confined electrons was observed and seen to result in centrifugal separation of the two species. After storage times of several 10 s, electron cooling of the anions to the temperature of the cryogenic trap was achieved.

The Jaynes-Cummings system is one of the most fundamental models of how light and matter interact. When driving the system with a coherent state (e.g., laser light), it is often assumed that whether the light couples through the cavity or atom plays an important role in determining the dynamics of the system and its emitted field. Here, we prove that the dynamics are identical in either case except for the offset of a trivial coherent state. In particular, our formalism allows for both steady state and the treatment of any arbitrary multimode coherent state driving the system. Finally, the offset coherent state can be interferometrically canceled by appropriately homodyning the emitted light, which is especially important for nanocavity quantum electrodynamics where driving the atom is much more difficult than driving the cavity.

We discuss quantum optimal control of Bose-Einstein condensates trapped in magnetic microtraps. The objective is to transfer a condensate from the ground state to the first-excited state. This type of control problem is typically solved using derivative-based methods in a high-dimensional control space such as gradient-ascent pulse engineering (GRAPE) and Krotov's method or derivative-free methods in a reduced control space such as Nelder-Mead with a chopped random basis (CRAB). We discuss how these methods can be combined in gradient optimization using parametrization (GROUP) including the finite bandwidth of the control electronics. We compare these methods and find that GROUP converges much faster than Nelder-Mead with CRAB and achieves better results than GRAPE and Krotov's method on the control problem presented here.

Local and intrinsic quantum coherence, as two contributions of total quantum coherence, have been introduced by Radhakrishnan et al. [C. Radhakrishnan, M. Parthasarathy, S. Jambulingam, and T. Byrnes, Phys. Rev. Lett. 116, 150504 (2016)]. In the present work we study the property of local, intrinsic, and total quantum coherence in critical systems such as the $XY$ model, extended $XY$ model, and Ashkin-Teller model to analyze their capability in identifying the second-order, infinite-order quantum phase transitions, as well as the topological quantum phase transitions. It is shown that not only the total coherence, but also the local and intrinsic coherence can spotlight the critical points efficiently. Moreover, the long-range properties of total and local coherence, and the short-range properties of intrinsic coherence are analyzed. The total and local coherence for spin-pairs farther than nearest neighbors can highlight the critical points while the intrinsic coherence is absent for long-distance spin-pairs.

Entangled multiqubit states are an essential resource for quantum information and computation. Solid-state emitters can mediate interactions between subsequently emitted photons via their spin, thus offering a route towards generating entangled multiphoton states. However, existing schemes typically rely on the excitation-relaxation of the emitter, resulting in single photons limited by the emitter's radiative lifetime, suffering from considerable practical limitations, for self-assembled quantum dots most notably the limited spin coherence time due to Overhauser magnetic field fluctuations. We here propose an alternative approach based on a spin-$\mathrm{\Lambda }$ system that overcomes the limitations of previous proposals. Studying the example of spin-flip Raman scattering of self-assembled quantum dots in Voigt geometry, we argue that weakly driven hole spins constitute a promising platform for the practical generation of frequency-entangled photonic cluster states.

Singly ionized lutetium has a number of fortuitous properties well suited for a design of an optical clock and corresponding applications. In this work, we study ${\mathrm{Lu}}^{+}$ properties relevant to a development of the clock using the relativistic high-precision method combining configuration interaction and the linearized coupled-cluster approaches. The systematic effects due to interaction of an external electric-field gradient with the quadrupole moment and the dynamic correction to the blackbody radiation shift are studied and uncertainties are estimated. The value of the $5d6s{}^1{D}_2$ polarizability is predicted. We also demonstrate that ${\mathrm{Lu}}^{+}$ is a good candidate to search for variation of the fine-structure constant.

We propose to correct the ionization factor in the quantitative rescattering (QRS) theory for a higher precision calculation of macroscopic high harmonic generation (HHG) in a gaseous medium. Compared to the first-order strong-field approximation (SFA-1) and the Ammosov-Delone-Krainov (ADK) ionization models, the Perelomov-Popov-Terent'ev (PPT) model achieves the best agreement with the macroscopic HHG spectra of Ar based on the solution of the time-dependent Schrödinger equation (TDSE). The investigation of the ionization probability dependence on laser intensity indicates that the PPT model is consistent with the TDSE, both in the tunneling and multiphoton ionization regimes, and the SFA-1 or the ADK mostly works in the tunneling regime. This explains the difference in the total macroscopic harmonic spectra, mainly due to off-axis harmonic emissions generated by small laser intensities. The examples for Ne and Xe by using three ionization models in the QRS theory also show the significance of ionization correction by the PPT model to precisely simulate the macroscopic HHG spectra. This study will help further increase the accuracy of simulating measured HHG spectra quantitatively under a variety of macroscopic conditions.

Nondipole effects in the atomic dynamic interference are investigated by numerically solving the time-dependent Schrödinger equation (TDSE) of hydrogen. It is found that the inclusion of nondipole corrections in the TDSE can induce momentum shifts of photoelectrons in the opposite direction of the laser propagation. The magnitude of the momentum shift is roughly proportional to the laser peak intensity and to the momentum component of the photoelectron along the laser propagation. By including the nondipole corrections of the Volkov phase into a semianalytical model previously developed under the dipole approximation, all the main features of the momentum shifts can be nicely reproduced. Through an analytic expression, the origin of such momentum shifts is attributed to the nondipole phase difference between the two electron wave packets ejected in the rising edge and the falling edge of the laser pulse, which will interfere with each other and result in the final fringe pattern. One important consequence of such momentum shifts is that they can smooth out the peak splitting induced by the dynamic interference in the photoelectron energy spectrum. Nevertheless, it should be emphasized that the dynamic interference persists in the photoelectron momentum distributions and is not suppressed at all for the laser parameters considered in this work.

A first-order type phase transition between Mott lobes has been reported in Phys. Rev. Lett. 109, 135302 (2012) for a two-dimensional Bose-Hubbard model in the presence of an attractive three-body interaction. We revisit the scenario in systems of ultracold bosons both in one- and two-dimensional lattices using the density matrix renormalization group method and the self-consistent cluster mean-field theory approach, respectively. We show that an unconventional pairing of particles occurs due to the competing repulsive two-body and attractive three-body interactions. This leads to a pair superfluid phase sandwiched between the Mott insulator lobes corresponding to densities density one and three in the strongly interacting regime. This is in contrast to the direct first-order jump as predicted before. Interestingly, the Mott to pair superfluid phase transitions are found to be continuous in nature. We also show that the pair superfluid phase is robust with respect to the ratio between the two- and three-body interaction strengths. In the end, we establish a connection between the Bose-Hubbard model presented in Phys. Rev. Lett. 109, 135302 (2012) with a more general Bose-Hubbard model and analyze the fate of the pair superfluid phase in the presence of an external trapping potential.

The two-component Fermi gas with contact attractive interactions between different spin components can be described by the Yang-Gaudin model. Applying the Bethe ansatz approach, one finds analytical formulas for the system eigenstates that are uniquely parametrized by the solutions of the corresponding Bethe equations. Recent numerical studies of the so-called yrast eigenstates, i.e., lowest energy eigenstates at a given nonzero total momentum, in the Yang-Gaudin model show that their spectrum resembles an yrast dispersion relation of the Lieb-Liniger model which in turn matches the dark soliton dispersion relation obtained within the nonlinear Schrödinger equation. It was shown that such a conjecture in the case of the Lieb-Liniger model was not accidental and that dark soliton features emerged in the course of measurement of positions of particles when the system was initially prepared in an yrast eigenstate. Here, we demonstrate that, starting with yrast eigenstates in the Yang-Gaudin model, the key soliton signatures are revealed by the measurement of pairs of fermions. We study soliton signatures in a wide range of interaction strengths.

We experimentally and theoretically study propagation of 80-ns Gaussian-like probe pulses in hot potassium vapor under conditions of four-wave mixing (FWM). The atomic scheme for FWM is off-resonant, double-$\mathrm{\Lambda }$ atomic scheme, with pump and probe photons, mediated in the K vapor, generating new probe and conjugate photons. We define the subset of FWM parameters, one-photon pump detuning, two-photon pump-probe Raman detuning, vapor density, the pump Rabi frequencies, when slowed pulses exit the vapor are also Gaussian-like. When Gaussian-like pulses exit the cell we are able to compare theoretical and experimental results for fractional delays and broadening for the probe and conjugate. We have obtained fractional delays above 1. Results of the model are compared with the experiment, with and without the model of Doppler averaging, when the atom velocity distribution is divided into different number of groups. We analyze possible causes for pulse broadening and distortion of slowed probe pulses and show that they are the result of quite different behavior of the probe pulse in the FWM vapor. Besides presenting the first results of slowing 80-ns probe pulses, this work is a useful test of the numerical model and values of parameters taken in the model that are not known in experiments.

We show that dynamical quantum phase transitions (DQPTs) in the quench dynamics of two-dimensional topological systems can be characterized by a dynamical topological invariant defined along an appropriately chosen closed contour in momentum space. Such a dynamical topological invariant reflects the vorticity of dynamical vortices responsible for the DQPTs, and thus serves as a dynamical topological order parameter in two dimensions. We demonstrate that when the contour crosses topologically protected fixed points in the quench dynamics, an intimate connection can be established between the dynamical topological order parameter in two dimensions and those in one dimension. We further define a reduced rate function of the Loschmidt echo on the contour, which features nonanalyticities at critical times and is sufficient to characterize DQPTs in two dimensions. We illustrate our results using the Haldane honeycomb model and the quantum anomalous Hall model as concrete examples, both of which have been experimentally realized using cold atoms.

The most general class of nonlocality criteria for $N$-partite $d$-chotomic systems with $k$ number of measurement settings is derived under the constraint of measurement symmetries. It is the complete characterization of the multipartite nonlocality when the correlation is assumed to be symmetric under the choice of measurement settings. The generalized nonlocality condition is obtained using the correlation functions, which are derived from Fourier analysis of probability spectrums. It is found that the condition for the local hidden variable model is violated by multipartite quantum states and general constraints for the quantum violation of the maximally entangled state has been obtained.

We investigate the scattering properties of a parity-time-antisymmetric non-Hermitian system. The parity-time-antisymmetric scattering center possesses imaginary nearest-neighbor hoppings and real on-site potentials; it has been experimentally realized through dissipative coupling and frequency detuning between atomic spin waves. We find that such parity-time-antisymmetric system displays three salient features: First, the reflection and transmission are both reciprocal. Second, the reflection and transmission probabilities satisfy $R\pm T=1$, which depends on the parity of the scattering center size. Third, the scattering matrix satisfies $\left(S{\sigma }_{\mathrm{z}}\right){\left(S{\sigma }_{\mathrm{z}}\right)}^{*}=I$ for a scattering center with an even site; for a scattering center with an odd site, the dynamics exhibits Hermitian scattering behavior, possessing unitary scattering matrix $S{S}^{†}=I$.