In ghost imaging with thermal light (GITL), the bucket signals are analog signals playing the role of weights in the algorithms of image reconstruction. However, diverse bucket signals seemed to be completely ignored in conditional-averaging GITL, which was recently proposed by two groups [Appl. Phys. Lett. 100, 131114 (2012); Chin. Phys. Lett. 29, 074216 (2012)]. Even so, positive and negative ghost images were obtained by averaging just partial reference signals. To understand this effect, one needs to determine the statistical relation between random bucket signals and reference signals. Here, we apply probability theory to GITL by regarding thermal light intensities as stochastic variables, and then deduce the joint probability density function between the bucket and reference signals. Positive and negative images are formed in ensemble averaging of the product of the reference signal and two logic quantities of the bucket signals. The visibility and contrast-to-noise ratio of these images in conditional-averaging GITL are analyzed in detail in both theory and experiment. As for applications, we perform an experiment of remote conditional-averaging GITL and verify that the image quality is maintained when the bucket signals are extremely attenuated. Our method can be applied to any ghost imaging scenarios in which the specific statistics of the optical source is given.

Spin collective phenomena including superradiance are even today being intensively investigated with experimental tests performed based on state-of-the-art quantum technologies. Such attempts are not only for the simple experimental verification of predictions from the last century, but also as a motivation to explore new applications of spin collective phenomena and the coherent control of the coupling between spin ensembles and reservoirs. In this paper, we investigate the open quantum dynamics of two spin ensembles (double-spin domains) coupled to a common bosonic reservoir. We analyze in detail the dynamics of our collective state and its structure by focusing on both the symmetry and asymmetry of this coupled spin system. We find that when the spin size of one of the double domains is larger than that of the other domain, at the steady state this system exhibits two unusual collective behaviors: the negative-temperature-state relaxation in the smaller spin domain and the reservoir-assisted quantum entanglement between the two domains. These results are the consequence of the asymmetry of this system and the decoherence driven by the common reservoir.

We investigate the entanglement dynamics of two local two-state quantum systems coupling to a bosonic mode and a common structured bath. Here the bosonic mode is damped by a broadband reservoir and acts as a probe mode for the use of continuous quantum measurement feedback, and the bath has a structure characterized by the Ohmic spectral density. We first present the non-Markovian master equation for the two-state quantum systems and then the corresponding hierarchical equations of motion (HEOM). We illustrate the equivalence between the HEOM approach and the perturbation theory. HEOM-calculations show that the quantum measurement feedback plays a positive role in the entanglement generation, and the non-Markovian effect of the Ohmic bath could greatly enhance this action. We reveal in detail the dependence of the scheme on the spectral density parameters, the temperature of the bath, and the measurement feedback strength. The numerical results are obtained with the HEOM truncated at the third order, which are equal to those obtained with sixth-order perturbation theory.

The implementation of a high-fidelity two-qubit quantum logic gate remains an outstanding challenge for isolated solid-state qubits such as nitrogen-vacancy (NV) centers in diamond. In this work, we show that by driving pairs of NV centers to undergo photon scattering processes that flip their qubit states simultaneously, we can achieve a unitary two-qubit gate conditioned upon a single photon-detection event. Further, by exploiting quantum interference between the optical transitions of the NV centers' electronic states, we realize the existence of two special drive frequencies: a “magic” point where the spin-preserving elastic scattering rates are suppressed and a “balanced” point where the state-flipping scattering rates are equal. We analyzed four different gate operation schemes that utilize these two special drive frequencies, and various combinations of polarizations in the drive and collection paths. Our theoretical and numerical calculations show that the gate fidelity can be as high as 98%. The proposed unitary gate, combined with available single-qubit unitary operations, forms a universal gate set for quantum computing.

Blind quantum computation (BQC) is a type of quantum computation model. For a client (Alice) who does not have enough sophisticated technology and knowledge to perform universal quantum computation, BQC allows her to resort to a remote quantum computation server (Bob) to delegate universal quantum computation. During the computation, Bob cannot know Alice's inputs, algorithm, and outputs. A single-server BQC protocol requires Alice to prepare and distribute single-photon states to Bob. Unfortunately, the distributed single photons will suffer from noise. The noise not only leads to the decoherence of the single-photon state, but also leads to photon loss. In this protocol, we describe an antinoise single-server BQC protocol. This protocol has three advantages. First, Alice does not require any auxiliary resources, which reduces the client's economic cost. Second, this protocol not only can protect the state from the collective noise, but also can distill the single photon from photon loss. Third, the noise processer in Bob is based on the linear optics so that it is feasible in experiment. This protocol may show that it is possible to perform a single-server BQC protocol in a noise channel.

An evolution between a system and its environment which leads to pure dephasing of the system may either be a result of entanglement building up between the system and the environment or not (the second option is only possible for initially mixed environmental states). We find a way of distinguishing between entangling and nonentangling evolutions for systems which are larger than a single qubit. The generalization of the single-qubit separability criterion to larger systems is not sufficient to make this distinction (it constitutes a necessary condition of separability). A set of additional conditions for the operators describing the evolution of the environment depending on the state of the system is required. We find that the commutation of these environmental operators with the initial state of the environment does not guarantee separability; products of the operators need to commute among themselves for a pure-dephasing evolution not to be accompanied by system-environment entanglement generation. This is a qualitative difference with respect to the single-qubit case, since it allows for a system to entangle with an initially completely mixed environment.

Adopting the geometric description of steering assemblages and the local-hidden-state (LHS) model, we construct the optimal LHS model for some two-qubit states under continuous projective measurements and obtain a sufficient steering criterion for all two-qubit states. Using the criterion, we show more two-qubit states that are asymmetric in the steering scenario under projective measurements. Then we generalize the geometric description into higher-dimensional bipartite cases, calculate the steering bound of two-qutrit isotropic states, and discuss more general cases.

We propose an efficient method to generate multiparticle entangled states of nitrogen-vacancy (NV) centers in a spin-mechanical system, where the spins interact through a collective coupling of the Lipkin-Meshkov-Glick (LMG) type. We show that through adiabatic transitions in the ground state of the LMG Hamiltonian, the Greenberger-Horne-Zeilinger (GHZ)-type or the W-type entangled states of the NV spins can be generated with this hybrid system from an initial product state. Because of adiabaticity, this scheme is robust against practical noise and experimental imperfection, and may be useful for quantum information processing.

We address quantum spatial search on graphs and its implementation by continuous-time quantum walks in the presence of dynamical noise. In particular, we focus on search on the complete graph and on the star graph of order $N$, also proving that noiseless spatial search shows optimal quantum speedup in the latter, in the computational limit $N\gg 1$. The noise is modeled by independent sources of random telegraph noise (RTN), dynamically perturbing the links of the graph. We observe two different behaviors depending on the switching rate of RTN: fast noise only slightly degrades performance, whereas slow noise is more detrimental and, in general, lowers the success probability. In particular, we still find a quadratic speedup for the average running time of the algorithm, while for the star graph with external target node, we observe a transition to classical scaling. We also address how the effects of noise depend on the order of the graphs and discuss the role of the graph topology. Overall, our results suggest that realizations of quantum spatial search are possible with current technology and indicate the star graph as the perfect candidate for the implementation by noisy quantum walks, owing to its simple topology and nearly optimal performance for just a few nodes also.

Electron scattering by rare-gas dimers is studied for very low incident energies using the zero range potential (ZRP) method. Beyond the traditional ZRP, we consider an alternative formulation accounting for the atomic polarization, inspired in the modified effective range theory. The scattering calculations are reported in fixed nuclei, rigid rotor, and rovibrational approximations, the first two being analytical. An expression for the electron-molecule scattering length is obtained. We find that short-range interactions are the dominant mechanism for rotational transitions in electron scattering by rare-gas dimers, while the long-range interactions may be neglected. Our results show how the elastic, rotational, and rovibrational cross sections depend on the electron-atom scattering length, on the molecular parameters, and also on the inclusion of polarization effects. The principle of detailed balance is discussed in the context of the ZRP method. Finally, we show that the rovibrational coupling considerably affects the rotational cross sections when the rotational constant becomes comparable to the vibrational constant.

The longitudinal momentum of the electron at the tunneling exit is a useful quantity to make sense of the tunneling ionization process. It was usually assumed to be zero from a classical argument, but recent experiments show that it must be nonzero in order to explain the measured electron momentum distributions. In this article we show that the flow momentum of the probability fluid is a sensible quantum mechanical definition for tunneling-exit momentum, and it can be (and in general is) nonzero at the tunneling exit point where the kinetic energy is zero by definition. We show that this longitudinal momentum is nonzero even in the static or adiabatic limit, and this nonzero momentum is a purely quantum mechanical effect determined by the shape of the wave function in the vicinity of the tunneling exit point. Nonadiabaticity or finite wavelength may increase this momentum substantially, and the detailed value depends on both the atomic and the laser parameters.

Motion of electrons in plasmonic excitation of ring-shaped clusters ${\mathrm{Na}}_{4n+2}(n=1,2,...,5)$ is analyzed by inspecting trajectories of the Thouless parameters which describe photoexcitation to the plasmonic-excited state. We examine the Floquet state produced by irradiation of stationary light inducing the transition between the ground state and the plasmonic-excited state. The trajectories of the Thouless parameters demonstrate that collective excitation of $4n+2$ valence electrons occurs when the intensity of light exceeds a threshold. The occurrence of the collective excitation requires that the strength of the light field overwhelms the effect of electron correlation in the ground state. The threshold intensity is found to decrease with the cluster size.

When an atom is placed into an oscillating electric field with frequency far from atomic resonances, the atomic electrons partly shield this field at the nucleus. It is conjectured that when the frequency of an electric field reaches an atomic resonance, the electric field at the nucleus may be significantly enhanced. In this paper we systematically study the mechanisms of this enhancement and show that it may reach five orders in magnitude in particular cases. As an application, we consider laser-assisted neutron capture in 139-lanthanum nucleus and screening and resonance enhancement of nuclear electromagnetic transitions by electrons.

We present our results on the emissions from ${{\text{N}}_2}^{+}$ pumped with ultrafast filaments using high-resolution pump probe. Our result shows the phase relation between transition lines at earlier delays. We show that the gain on transition lines extends beyond the time when coherence is lost.

The transient dielectric responses of cubic boron nitride (cBN) modulated by an ultrashort laser pulse are introduced based on time-dependent density functional theory calculations. We theoretically investigate whether an ultrashort laser pulse is able to reversibly drive the electric and optical properties of cBN to change profoundly without damage. With easily tunable laser parameters, the cBN can even be switched by reversal of both elliptic and hyperbolic behaviors for a particular working frequency window. We describe the evolution of the transient dielectric function throughout the duration of the pulse, which can exhibit a reversible behavior driven by the instantaneous optical field for a purposely designed laser pulse, providing a fundamental possibility for applications of all-optical signal processing and optical-field-effect devices.

We study Majorana modes in the vortex core of a two-dimensional $p+ip$ Fermi superfluid interacting with a Bose-Einstein condensate. Under a repulsive $s$-wave contact interaction between fermions and bosons, fermions are depleted from the vortex core when the bosonic density becomes sufficiently high. This gives rise to a dynamically driven local interface that emerges between fermions and bosons, along which chiral Majorana edge states should appear. We examine in detail the variation of vortex-core structures as well as the formation of chiral Majorana edge states with increasing bosonic density, where the circulation of the vortex plays an important role. Whereas both the Majorana modes and the vortex-core structures can be controlled and manipulated by tuning the bosonic density and the Bose-Fermi interaction strength, our study presents an illuminating example of how topological defects can be dynamically controlled in the context of cold-atomic gases.

We study out-of-time-order correlation (OTOC) for one-dimensional periodically driven hardcore bosons in the presence of Aubry-André (AA) potential and show that both the spectral properties and the saturation values of OTOC in the steady state of these driven systems provide a clear distinction between the localized and delocalized phases of these models. Our results, obtained via exact numerical diagonalization of these boson chains, thus indicate that OTOC can provide a signature of drive-induced delocalization, even for systems which do not have a well-defined semiclassical (and/or large $N$) limit. We demonstrate the presence of such a signature by analyzing two different drive protocols for hardcore boson chains leading to distinct physical phenomena and discuss experiments which can test our theory.

We present two diffusion Monte Carlo (DMC) algorithms for systems of ultracold quantum gases featuring synthetic spin-orbit interactions. The first one is a spin-integrated DMC method which provides fixed-phase energy estimates. The second one is a discrete spin generalisation of the T-moves spin-orbit DMC [Melton et al., J. Chem. Phys. 144, 244113 (2016)], which provides an upper bound to the fixed-phase energy. The former is a more accurate method but it is restricted to spin-independent two-body interactions. We report a comparison between both algorithms for different systems. As a check of the efficiency of both methods, we compare the DMC energies with results obtained with other numerical methods, finding agreement between both estimations.

We reveal an interesting phenomenon of the acceleration and deceleration of the phase rotation for an orbital-angular-momentum-carrying Laguerre-Gaussian beam in a chiral medium. Under this circumstance, the differences between the optical force and optical torque exerted on a dielectric spherical particle for chiral and achiral cases are demonstrated. The optical force and torque are exerted on the particle simultaneously, which makes the particle revolve and spin simultaneously, as a planet in the solar system. We hope the results may point to a unique routine of optical manipulation.

We propose theoretically a four-mode coupled optomechanical system to explore the optical-response properties of an optical trimer system consisting of a passive cavity, a no-loss-gain cavity, and an active cavity coupled with a mechanical oscillator. In the study, the passive cavity is driven by an external laser so that the stability of the coupled system depends strongly on the gain-to-loss ratio and the photon tunneling between the adjacent cavities. We find that in the regime near the stable-unstable critical point, the resonance absorption of the optical trimer system changes quickly with the increasing gain of the active cavity. In contrast, when the mechanical oscillator is coupled to the passive cavity, the center absorption peak is split and a phenomenon of optomechanically induced transparency appears. Consequently, two additional resonance peaks in the absorption profile are induced by the optomechanical coupling, which hinges on the tunneling between the optical cavities and also on the gain of the active cavity. The dependence of the width of the transparency window on the optomechanical coupling and the driving strength is also discussed in detail. The results obtained here indicate that the optical properties of the trimer system can be manipulated by coupling a mechanical oscillator to the system and therefore have a potential application in quantum optics and quantum information processing.