We investigate by statistical means a Bose-Einstein condensate held in a one-dimensional optical lattice whose phase undergoes a fast oscillation. The averaged potential experienced by the atoms yields a periodic potential having the same spatial period but with a renormalized depth. However, the atomic dynamics also contains a micromotion whose main features are revealed by a Kolmorogov-Smirnov analysis of the experimental momentum distributions. Furthermore, we discuss the impact of the micromotion on a quench process corresponding to a proper sudden change of the driving amplitude which reverses the curvature of the averaged potential.

We present a fermionic description of nonequilibrium multilevel systems. Our approach uses the Keldysh path-integral formalism and allows us to take into account periodic drives, as well as dissipative channels. The technique is based on the Majorana fermion representation of spin-1/2 models which follows earlier applications in the context of spin and Kondo systems. We apply this formalism to problems of increasing complexity: a dissipative two-level system, a driven-dissipative multilevel atom, and a generalized Dicke model describing many multilevel atoms coupled to a single cavity. We compare our theoretical predictions with recent QED experiments and point out the features of a counterlasing transition. Our technique provides a convenient and powerful framework for analyzing driven-dissipative quantum systems, complementary to other approaches based on the solution of Lindblad master equations.

The electron affinity of the lanthanum atom was recently measured by slow-electron velocity map imaging in a photodetachment experiment [Y. Lu et al., Phys. Rev. A 99, 062507 (2019)]. Several detachment threshold energies have been measured, which correspond to different energy levels of the initial ion and/or final atom. Only one measurement, however, has been exploited to determine the electron affinity. Applying the ordinary spectroscopic method to the complete set of data presented by the authors, one obtains a slightly different, more precise and more consistent value of the electron affinity of La: $449691\left(17\right)$ instead of $449697\left(20\right){\mathrm{m}}^{-1}$, i.e., $0.557546\left(20\right)$ instead of $0.557553\left(25\right)\mathrm{eV}$.

In the context of closed quantum systems, when a system prepared in its ground state undergoes a sudden quench, the resulting Loschmidt echo can exhibit zeros, resembling the Fisher zeros in the theory of classical equilibrium phase transitions. These zeros lead to nonanalytical behavior of the corresponding rate function, which is referred to as the dynamical quantum phase transition (DQPT). In this paper, we investigate DQPTs in the context of open quantum systems that are coupled to both Markovian and non-Markovian dephasing baths via a conserved quantity. The general framework is corroborated by studying the nonequilibrium dynamics of a transverse-field Ising ring. Our paper might shed light on theoretical developments of DQPTs in the strong system-bath coupling regime.

Localized solutions of the Dirac equation for an electron moving in free space and electromagnetic field lattices with periodic dependence on space-time coordinates (electromagnetic space-time crystals) are treated using the expansions in basis wave functions. The techniques for calculating these functions with any prescribed accuracy are presented. It is shown that in the crystals created by two counterpropagating plane electromagnetic waves with the same or the opposite circular polarizations, the Dirac equation describing the basis functions reduces to matrix ordinary differential equations. These functions and the corresponding mean values of velocity, momentum, energy, and spin operators are found for both types of crystals. Localized solutions describing the families of orthonormal beams in electromagnetic space-time crystals and free space, defined by a given set of orthonormal complex scalar functions on a two-dimensional manifold, are obtained. By way of illustration the orthonormal beams in free space and various localized states with complex vortex structure of probability currents, defined by the spherical harmonics, are presented. The obtained solutions have high-probability density only in very small core regions. The evolution of wave packets with one-dimensional localization in both types of crystals created by two circularly polarized waves is described.

We determine the characteristic of dissipative quantum transport in a coupled qubit network in the presence of on-site and off-diagonal external driving. The work is a generalization of the dephasing-assisted quantum transport where noise is beneficial to the transport efficiency. Using Floquet-Magnus expansion extended to Markovian open systems, we analytically derive transport efficiency and compare it to exact numerical results. We find that periodic driving may increase the efficiency at frequencies near the coupling rate. On the other hand, at some other frequencies the transport may be suppressed. We then propose the enhancement mechanism as the ramification of interplay between driving frequency, dissipative, and trapping rates.

We present a general procedure to implement a not gate by composite pulses robust against both offset uncertainties and control field variations. We define different degrees of robustness in this two-parameter space, namely, along one, two, or all directions. We show that the phases of the composite pulse satisfy a nonlinear system and can be computed analytically or numerically.

In recent years there has been a great deal of focus on a globe-spanning quantum network, including linked satellites for applications ranging from quantum key distribution to distributed sensors and clocks. In many of these schemes, relativistic transformations may have deleterious effects on the purity of the distributed entangled pairs. In this paper, we make a comparison of several entanglement distribution schemes in the context of special relativity. We consider three types of entangled photon states: polarization, single photon, and Laguerre-Gauss mode entangled states. All three types of entangled states suffer relativistic corrections, albeit in different ways. These relativistic effects become important in the context of applications such as quantum clock synchronization, where high fidelity entanglement distribution is required.

Aiming to combine density functional theory (DFT) and wave-function theory, we study a mapping from the many-body interacting system to an effectively interacting Kohn-Sham system instead of a noninteracting Kohn-Sham system. Because a ground state of effectively interacting systems requires having a solution for the correlated many-body wave functions, this provides a natural framework to many-body wave-function theories such as the configuration interaction and the coupled-cluster method in the formal theoretical framework of DFT. Employing simple one-dimensional two-electron systems—namely, the one-dimensional helium atom, the hydrogen molecule, and the heteronuclear diatomic molecule—we investigate properties of many-body wave functions and exact exchange-correlation potentials of effectively interacting Kohn-Sham systems. As a result, we find that the asymptotic behavior of the exact exchange-correlation potential can be controlled by optimizing that of the effective interaction. Furthermore, the typical features of the exact noninteracting Kohn-Sham system, namely, a spiky feature and a step feature in the exchange-correlation potential for the molecular dissociation limit, can be suppressed by a proper choice of the effective interaction. These findings open a possibility to construct numerically robust and efficient exchange-correlation potentials and functionals based on the effectively interacting Kohn-Sham scheme.

The results of simulations of cooling based on sawtooth-wave adiabatic passage (SWAP) are presented, including the possibility of population leaking to states outside of the cycling transition. The amount of population leaking can be substantially suppressed compared to Doppler cooling, which could be useful for systems that are difficult to repump back to the cycling transition. The suppression of the leaked population was more effective when simulating the slowing of a beam than in cooling a thermal distribution. As expected, calculations of the leaked population versus branching ratio of spontaneous emission show that the suppression is more effective for narrow linewidth transitions. In this limit, using SWAP to slow a beam may be worth pursuing even when the branching ratio out of the cycling transition is greater than 10%.

We analyze the control of the motion of a charged particle by means of an external electric field. The system is constrained to move along a given direction. The goal of the control is to change the speed of the particle in a fixed time with zero initial and final accelerations, while minimizing a cost functional in order to achieve a smooth transport of the system. We solve this linear control problem by using shortcut to adiabaticity (STA) methods and optimal control techniques. STA protocols are built upon local constraints. By extending the number of space variables, we explain how optimal control protocols can accommodate such constraints and provide robust solutions against slight initial and final time uncertainties. Conversely, optimal control can guide the choice of the class of functions involved in STA processes. Such mutual benefices of the two approaches remain valid beyond the specific example studied here.

The newly developed method of time-of-flight (TOF) momentum microscopy was used to analyze the cold electron emission from a Cs three-dimensional magneto-optical trap (MOT). Three-step resonant photoionization was implemented via two intermediate states ($6P_{3/2}$ pumped with an 852-nm laser and $7S_{1/2}$ with a 1470-nm laser) and a tunable femtosecond (fs) Ti:sapphire laser for the final ionization step. The magnetic field of the MOT is switched off during the photoionization step. The natural bandwidth of the fs laser is reduced to 4 meV using optical spectral filters. Precise tuning of the photon energy makes it possible to observe the transition regime between direct photoemission into the open continuum and field-induced ionization of highly excited Rydberg states. The paths can be identified by their characteristic dependency on the extraction field and on the Ti:sapphire polarization. TOF analysis allowed us to disentangle the ionization paths and the dependence of the spatiotemporal distribution of the cold electrons on the polarization of the ionizing laser.

We derive a computationally efficient expression of the photon-counting distribution for a uniformly illuminated array of single-photon detectors. The expression takes the number of single detectors, their quantum efficiency, and their dark count rate into account. Using this distribution we compute the error of the array detector by comparing the output to that of an ideal detector. We conclude from the error analysis that the quantum efficiency must be very high in order for the detector to resolve a handful of photons with high probability. Furthermore, we conclude that in the worst-case scenario the required array size scales quadratically with the number of photons that should be resolved. We also simulate a temporal array and investigate how large the error is for different parameters, and we compute the optimal size of the array that yields the smallest error.

We introduce the first-principle model describing frequency comb generation in a whispering-gallery-mode microresonator with the backscattering-induced coupling between the counterpropagating waves. The elaborated model provides deep insights and an accurate description of the complex dynamics of nonlinear processes in such systems. We analyze the backscattering impact on the splitting and reshaping of the nonlinear resonances and demonstrate backscattering-induced modulational instability in the normal dispersion regime and subsequent frequency comb generation. We present and discuss features of the soliton comb dynamics induced by the backward wave.

We analyze an autonomous thermoelectric engine composed of two superconducting qubits coupled to separate heat baths and connected by a Josephson junction. Work and heat are process quantities and not observables of the engine quantum system, but their rates can be derived from the steady-state expectation value of appropriate system observables, and their fluctuations are given by correlation functions determined by the master equation and quantum regression theorem of open quantum systems. Underneath the constant steady state of the system, the temporal correlation functions reveal a cyclical, dynamical transfer of energy—the strokes of the engine.

The most common source of entangled photons is spontaneous parametric down-conversion (SPDC). The degree of energy and momentum entanglement in SPDC is determined by the nonlinear interaction volume. By reducing the length of a highly nonlinear material, we relax the longitudinal phase-matching condition and reach record levels of transverse momentum entanglement. The degree of entanglement is estimated using both correlation measurements and stimulated emission tomography in wave-vector space. The high entanglement of the state in wave-vector space can be used to massively increase the quantum information capacity of photons, but more interestingly the equivalent state measured in position space is correlated over distances far less than the photon wavelength. This property promises to improve the resolution of many quantum imaging techniques beyond the current state of the art.

Lipkin-Meshkov-Glick (LMG) models describe a set of $N$ qubits which are located at the corners of an $N$-dimensional simplex and embedded in an external magnetic field. Because of its high coordinate number, quantum correlations in the models are quite nontrivial. For instance, in the large-$N$ limit, two-site entanglement concurrence vanishes for any magnetic field. Multisite global entanglement and generalized global entanglement also tend to vanish in some quantum regions. We characterize the quantum correlations in the LMG model with multipartite nonlocality. In LMG models with anisotropy in the $x-y$ plane, for any fixed magnetic field, the ground-state nonlocality (denoted by $\mathcal{S}$) is found to scale as ${log}_{10}\mathcal{S}\sim aN+b$, with $a$ and $b$ the fitting parameters. For LMG models which are isotropic in the $x-y$ plane, nevertheless, the nonlocality for each Dicke state scales as ${log}_{10}\mathcal{S}\sim a\frac{1}{N}+b$, with $b>0$. Signals for the quantum phase transitions of the models will also be discussed. These results indicate that multipartite nonlocality captures some key ingredient of the ground-state quantum correlations in the models. In addition to the ground states, we also study multipartite nonlocality in the LMG models at finite temperatures. In the anisotropic models, the thermal stability of the nonlocality at low temperatures is determined by the energy gap. For the isotropic models, nevertheless, as the temperature increases, the nonlocality presents a sharp valley which is followed by a round peak. The behavior is far beyond the above-mentioned “thermal stability vs energy gap” picture. The underlying mechanics is the “contribution inversion” of low-lying energy states in the thermal-state nonlocality $\mathcal{S}\left({\widehat{\rho }}_T\right)$; that is, in a low-temperature region, the first-excited state “defeats” the ground state and plays a dominant role in thermal-state nonlocality.

The quantum alternating operator ansatz (QAOA) is a promising gate-model metaheuristic for combinatorial optimization. Applying the algorithm to problems with constraints presents an implementation challenge for near-term quantum resources. This paper explores strategies for enforcing hard constraints by using $XY$ Hamiltonians as mixing operators (mixers). Despite the complexity of simulating the $XY$ model, we demonstrate that, for an integer variable admitting $\kappa$ discrete values represented through one-hot encoding, certain classes of the mixer Hamiltonian can be implemented without Trotter error in depth $O\left(\kappa \right)$. We also specify general strategies for implementing QAOA circuits on all-to-all connected hardware graphs and linearly connected hardware graphs inspired by fermionic simulation techniques. Performance is validated on graph-coloring problems that are known to be challenging for a given classical algorithm. The general strategy of using $XY$ mixers is borne out numerically, demonstrating a significant improvement over the general $X$ mixer, and moreover the generalized $W$ state yields better performance than easier-to-generate classical initial states when $XY$ mixers are used.

We report a precise determination of the lifetime of the $\left(4p\right){}^2{P}_{3/2}$ state of ${}^{40}\mathrm{Ca}^{+}$, ${\tau }_{P_{3/2}}=6.639\left(42\right)\mathrm{ns}$, using a combination of measurements of the induced light shift and scattering rate on a single trapped ion. Good agreement with the result of a recent high-level theoretical calculation, 6.69(6) ns [M. S. Safronova et al., Phys. Rev. A 83, 012503 (2011)], but a 6-$\sigma$ discrepancy with the most precise previous experimental value, 6.924(19) ns [J. Jin et al., Phys. Rev. Lett. 70, 3213 (1993)], is found. To corroborate the consistency and accuracy of the new measurements, relativistically corrected ratios of reduced-dipole-matrix elements are used to directly compare our result with a recent result for the $P_{1/2}$ state, yielding a good agreement. The application of the present method to precise determinations of radiative quantities of molecular systems is discussed.