The experimental realizations of spin-orbit coupling (SOC) in cold atom gases put topological superfluids (TSs) under the spotlight. The topological protection of suggested superfluid models comes from the local antiunitary symmetries of ten Altland-Zirnbauer symmetry classes. In this paper we theoretically study a two-dimensional BDI model describing spin-orbit-coupled bilayer fermionic gases with $s$-wave interaction to realize mirror-symmetry- (nonlocal and unitary) protected gapped TS, beyond the paradigms suggested before. This gapped phase not only shows linearly dispersive Majorana zero modes on mirror invariant one-dimensional boundaries, but also shows Majorana corner states to realize the second-order TS (TS2) with $s$-wave pairing, which is explicitly different from the nonconventional pairings in the suggested systems to realize TS2. Generally, engineering the single-particle band structures should be easier than directly engineering the complicated pairings. Considering the tremendous progress in manipulating cold atom systems, our work provides a possible route (by engineering the single-particle physics, such as SOC) to realize mirror-symmetry-protected gapped TS and TS2 in cold fermionic systems in the future.

We show how, with the use of quantum interference, we can violate, in some sense, the rule that charges of equal sign always repel each other. By considering two electrons that propagate parallel to each other in a Mach-Zehnder interferometer, we show that the quantum superposition of the electrostatic repulsion when the electrons propagate in the same path with the absence of interaction when they propagate in opposite paths may result in an effective attraction between them, when we post-select by which port each electron leaves the interferometer. We also discuss an experimental setup that could be used to verify such an effect.

For spins chains to be useful for quantum information processing tasks, the interaction between the spin chain and its environment generally needs to be suppressed. In this paper, we propose the use of strong static and oscillating control fields in order to effectively remove the spin-chain-environment interaction. We find that our control fields can also effectively transform the spin-chain Hamiltonian. In particular, interaction terms which are absent in the original spin-chain Hamiltonian appear in the time-averaged effective Hamiltonian once the control fields are applied, implying that spin-spin interactions can be engineered via the application of static and oscillating control fields. This transformation of the spin chain can then potentially be used to improve the performance of the spin chain for quantum information processing tasks. For example, our control fields can be used to achieve almost perfect quantum state transfer across a spin chain even in the presence of noise. As another example, we show how the use of particular static and oscillating control fields not only suppresses the effect of the environment, but can also improve the generation of two-spin entanglement in the spin chain.

We show that the dynamical Casimir effect in an optomechanical system can be achieved under incoherent mechanical pumping. We adopt a fully quantum-mechanical approach for both the cavity field and the oscillating mirror. The dynamics is then evaluated using a recently developed master-equation approach in the dressed picture, including both zero- and finite-temperature photonic reservoirs. This analysis shows that the dynamical Casimir effect can be observed even when the mean value of the mechanical displacement is zero. This opens up possibilities for the experimental observation of this effect. We also calculate cavity emission spectra in both the resonant and the dispersive regimes, providing useful information on the emission process.

We demonstrate a method for measuring atom-surface interactions using transmission spectroscopy of thermal vapors confined in a wedged nanocell. The wedged shape of the cell allows complementary measurements of both the bulk atomic vapor and atoms close to surfaces experiencing strong van der Waals atom-surface interaction. These are used to tightly constrain the dipole-dipole collisional parameters of a theoretical model for transmission spectra that accounts for atom-surface interactions, cavity effects, collisions with the surface of the cell, and atomic motion. We illustrate this method on a cesium vapor in a sapphire cell, demonstrating that even the weakest of the van der Waals atom-surface interaction coefficients—for ground-state alkali atom transitions—can be determined with a very good precision. This result paves the way towards a precise quantitative characterization of atom-surface interactions in a wide range of atom-based nanodevices.

We present experimental studies of atom-ion collisions using buffer-gas cooled, trapped ytterbium (${\mathrm{Yb}}^{+}$) ions immersed in potassium (K) vapor. The range of the collisional temperature is on the order of several hundred kelvin (thermal regime). We have determined various collisional rate coefficients of the ${\mathrm{Yb}}^{+}$ ion per K-atom number density. We find the upper bounds of charge-exchange rate coefficients ${\kappa }_{\mathrm{ce}}$ to be $(12.7\pm 1.6)\times {10}^{-14}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$ for K-${}^{171}\mathrm{Yb}^{+}$ and $(5.3\pm 0.7)\times {10}^{-14}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$ for K-${}^{172}\mathrm{Yb}^{+}$. For both isotopes, the spin-destruction rate coefficient ${\kappa }_{\mathrm{sd}}$ has an upper bound at $(1.46\pm 0.77)\times {10}^{-9}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$. The spin-exchange rate coefficient ${\kappa }_{\mathrm{se}}$ is measured to be $(1.64\pm 0.51)\times {10}^{-9}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$. The relatively low charge-exchange rate reported here demonstrates the advantage of using K atoms to sympathetically cool ${\mathrm{Yb}}^{+}$ ions and the relatively high spin-exchange rate may benefit research work in quantum metrology and quantum information processing on an atom-ion platform using K atoms and ${\mathrm{Yb}}^{+}$ ions.

We investigate $\mathrm{Cs}+\mathrm{Cs}$ scattering in excited Zeeman and hyperfine states. We calculate the real and imaginary parts of the $s$-wave scattering length; the imaginary part directly provides the rate coefficient for two-body inelastic loss, while the real part allows us to identify regions of magnetic field where three-body recombination will be slow. We identify field regions where Cs in its $(f,m_f)=(3,+2)$ and $(3,+1)$ states may be stable enough to allow Bose-Einstein condensation, and additional regions for these and the (3, 0) and $(3,-3)$ states where high-density clouds should be long-lived.

Leading tests of the Standard Model, like measurements of the electron electric dipole moment or of matter-antimatter asymmetry, are built upon our ability to laser-cool atoms and molecules to ultracold temperatures. Unfortunately, laser-cooling remains limited to a minute collection of species with very specific electronic structures. To include more species, such as polyatomic molecules or exotic atoms like antihydrogen, new cooling methods are needed. Here we demonstrate a method based on Sisyphus cooling that was proposed for laser-cooling antihydrogen. In our implementation, atoms are selectively excited to an electronic state whose energy is spatially modulated by an optical lattice, and the ensuing spontaneous decay completes one Sisyphus cycle. We show that this method eliminates many constraints of traditional radiation-pressure-based approaches, while providing similar atom numbers with lower temperatures. This laser-cooling method can be instrumental in bringing new exotic species and molecules to the ultracold regime.

Variational quantum eigensolvers offer a small-scale testbed to demonstrate the performance of error mitigation techniques with low experimental overhead. We present successful error mitigation by applying the recently proposed symmetry verification technique to the experimental estimation of the ground-state energy and ground state of the hydrogen molecule. A finely adjustable exchange interaction between two qubits in a circuit QED processor efficiently prepares variational ansatz states in the single-excitation subspace respecting the parity symmetry of the qubit-mapped Hamiltonian. Symmetry verification improves the energy and state estimates by mitigating the effects of qubit relaxation and residual qubit excitation, which violate the symmetry. A full-density-matrix simulation matching the experiment dissects the contribution of these mechanisms from other calibrated error sources. Enforcing positivity of the measured density matrix via scalable convex optimization correlates the energy and state estimate improvements when using symmetry verification, with interesting implications for determining system properties beyond the ground-state energy.

A theory of coherent resonant energy transfer with initial system-bath correlations [Qin et al., Phys. Rev. A 99, 032111 (2019)] is extended for a coherent initial system condition, which is a linear superposition of donor and acceptor excitations, and is more general and actual in many respects. The nonequilibrium bath state, containing initial system-bath correlations, is expanded in powers of coupling strength within the polaron formalism of second-order time-convolutionless quantum master equation. Detailed expressions for both homogeneous and inhomogeneous terms are derived and calculations are performed under the assumption of super-Ohmic spectral densities where the case of common bath modes is also included. Numerical results indicate that, for certain initial system conditions, the nonequilibrium due to the initial system-bath correlations brings about larger amplitude of population oscillation. And the initial system condition is of crucial importance in determining whether the initial system-bath correlations would play a significant role in the transfer dynamics. We identify a sensitive window in which the transfer dynamics is especially vulnerable to the bath such that there exists a greatly obvious difference between the dynamics with and without initial correlations. Besides, the system-bath coupling strength, the evolution time, and the nature of common bath modes are also shown to contribute to these effects. Rate equations based on Förster-Dexter energy transfer theory are derived for comparison.

Coherence is one of the most basic concepts and resources in quantum information. To clear the role coherence plays on the essential operator level in Grover's search algorithm, here we discuss the coherence dynamics of the state after each basic operator is applyied. As it is known, Grover's search algorithm repeats the application of Grover operator $G$, which can be decomposed into $G=H^{\otimes n}P{H}^{\otimes n}O$, where $H$ is Hadamard operator, $P$ is the condition phase-shift operator, and $O$ is the oracle operator. First, we show that $O$ and $P$ are incoherent operators while $H^{\otimes n}$ is coherent. Second, we prove that the amount of the operator coherence of the first $H^{\otimes n}$ and the operator coherence produced or depleted by $H^{\otimes n}$ depends not only on the size of the database and the success probability, but also on target states. Moreover, the amount of operator coherence is larger when the superposition state of targets is entangled rather than product. Third, we show that the two $H^{\otimes n}$ have different effects on coherence that one produces coherence and the other depletes coherence, and the depletion plays a major role. Therefore, the coherence is vibrating during the search process and the overall effect is that coherence is in depletion.

Quantum process tomography, a primitive in many quantum information processing tasks, can be cast within the framework of the theory of design of experiments (DOE), a branch of classical statistics that deals with the relationship between inputs and outputs of an experimental setup. Such a link potentially gives access to the many ideas of the rich subject of classical DOE for use in quantum problems. The classical techniques from DOE, however, cannot be directly applied to the quantum process tomography due to the basic structural differences between the classical and quantum estimation problems. Here we properly formulate quantum process tomography as a DOE problem and examine several examples to illustrate the link and the methods. In particular, we discuss the common issue of nuisance parameters and point out interesting features in the quantum problem absent in the usual classical setting.

We report an alternative scheme for implementing generalized quantum measurements that does not require the usage of an auxiliary system. Our method utilizes solely (a) classical randomness and postprocessing, (b) projective measurements on a relevant quantum system, and (c) postselection on nonobserving certain outcomes. The scheme implements arbitrary quantum measurement in dimension $d$ with the optimal success probability $1/d$. We apply our results to bound the relative power of projective and generalized measurements for unambiguous state discrimination. Finally, we test our scheme experimentally on an IBM quantum processor. Interestingly, due to noise involved in the implementation of entangling gates, the quality with which our scheme implements generalized qubit measurements outperforms the standard construction using an auxiliary system.

We have previously used the molecular three-body distorted-wave model to examine electron-impact single ionization of molecules. One of the possible weaknesses of this approach lies in the fact that the continuum electron wave functions do not depend on the orientation of the molecule. Here we introduce a model called the multicenter molecular three-body distorted-wave (MCM3DW) approach, for which the continuum electron wave functions depend on the orientation of the molecule at the time of ionization. The MCM3DW results are compared with experimental data taken from work by Dorn and colleagues [Ren et al., Phys. Rev. A 91, 032707 (2015); Phys. Rev. A 93, 062704 (2016); Phys. Rev. A 95, 022701 (2017); Phys. Rev. Lett. 109, 123202 (2012); Gong et al., Phys. Rev. A 98, 042710 (2018)] in which they measured triple differential cross sections for single ionization of molecular hydrogen while simultaneously determining the orientation of the ${{\mathrm{H}}_2}^{+}$ ion at the time of ionization. Comparisons are also made with previous theoretical calculations. It is found that orientation effects are important for low incident energy electrons. Very nice agreement with experiment and the time-dependent close coupling results is found for an incident electron energy of 26 eV. Orientation effects become relatively unimportant by the time the incident electron energy is 54 eV.

We investigate phase matching for high-order harmonic generation with linearly polarized Laguerre-Gaussian (LG) beams with nonzero orbital angular momentum (OAM). We compare the conditions for efficient phase matching for LG beams with those of Gaussian beams. In particular, we show how the OAM of the incident beams affects the phase-matching conditions for the short and long trajectories that arise from the saddle-point approximation of the dipole moment. Thereby we illustrate that the coherence length for the short trajectories decreases for LG beams near the focus compared to Gaussian beams, whereas efficient phase matching can be achieved before and behind the focus. Furthermore, we demonstrate that the coherence length for the long trajectory behind the focus plane can be controlled by the OAM. This paper provides a route for the experiment in order to have good coherence control to enhance the conversion efficiency for high-order harmonic generation with beams carrying OAM.

We study transverse instability and disintegration dynamics of a domain wall of a relative phase in two-component Bose-Einstein condensates with a coherent Rabi coupling. We obtain analytically the stability phase diagram of the stationary solution of the domain wall for the one-dimensional coupled Gross-Pitaevskii equations in the plane of the Rabi frequency and the intercomponent coupling constant. Outside the stable region, the domain wall is dynamically unstable for the transverse modulation along the direction perpendicular to the phase kink. The nonlinear evolution associated with the instability is demonstrated through numerical simulations for both the domain wall without edges and that with edges formed by the quantized vortices.

The optical forces generated by a right-handed plasmonic Archimedean spiral (PAS) have been mapped and analyzed. By changing the handedness of the circularly polarized excitation, the structure can switch from a trapping force profile to a rotating force profile. The Helmholtz-Hodge decomposition method has been used to separate the solenoidal component and the conservative component of the force and quantify their relative magnitude. It is shown that the for right-hand circularly polarized excitation, the PAS creates a significant amount of solenoidal forces. Using the decomposed force components, an intuitive explanation of the motion of micro- and nanoparticles in the force field is presented. Vector field topology is used to visualize the force components. The analysis is found to be consistent with numerical and experimental results. Due to the intuitive nature of the analysis, it can be used in the initial design process of complex laboratory-on-a-chip systems where a rigorous analysis is computationally expensive.

We use time- and frequency-resolved measurements of self-focusing and self-defocusing of 4.3-μm radiation in ${\mathrm{CO}}_2$ gas to study the resonant nonlinear optical response in the vicinity of individual rovibrational lines. Measurements over a range of lines on the $4P$ and $4R$ branch of the 000-001 transition indicate that the nonlinear response of ${\mathrm{CO}}_2$ is strongly affected by power broadening, resulting in a sign reversal of the nonlinearity as compared to that of a saturable two-level system.

We reveal the emergence of environment-induced spontaneous synchronization between two spin-$\frac{1}{2}$ quantum objects in a collision model setting. In particular, we determine the conditions for the dynamical establishment of synchronous evolution between local spin observables of a pair of spins undergoing open-system dynamics in the absence of an external drive. Exploiting the versatility of the collision model framework, we show that the formation of quantum or classical correlations between the principal spin pair is of no significant relevance to the manifestation of spontaneous quantum synchronization between them. Furthermore, we discuss the consequences of thermal effects on the environmental spins for the emergence of quantum synchronization.