We study the scattering of a matter wave from an interacting system of bosons in an optical lattice, focusing on the strong-interaction regime. Analytical expressions for the many-body scattering cross section are derived from a strong-coupling expansion and a site-decoupling mean-field approximation and compared to numerically obtained exact results. In the thermodynamic limit, we find a nonvanishing inelastic cross section throughout the Mott insulating regime, which decays quadratically as a function of the boson-boson interaction.

We study the creation process of electron-positron pairs from the quantum electrodynamical vacuum under very strong electric fields by solving the quantum field theoretical Dirac equation on a space-time grid. We investigate the role of bound-bound state mixing in such a process, which can be studied if the external force can be modeled by a combination of a potential barrier and a potential well. By increasing the magnitude of the two potentials, discrete states that originate from the positive and negative energy continua can become quasidegenerate in the mass gap region (between $\text{–}{mc}^2$ and ${mc}^2$). We show that this bound-bound state mixing is quite different from the usual bound-continuum state mixing where the particles are created until the Pauli exclusion principle inhibits this process. In the case of bound-bound mixing the particle number exhibits a characteristic oscillatory behavior that in principle can last forever. These findings can be modeled by an effective two-state model.

Here, we propose a way to control the interaction between qubits with always-on Ising interaction. Unlike the standard method to change the interaction strength with unitary operations, we fully make use of nonunitary properties of projective measurements so that we can effectively turn the interaction on or off via feedforward. Our scheme is useful for generating two- or three-dimensional cluster states that are universal resources for fault-tolerant quantum computation with this scheme, and it provides an alternative way to realize a scalable quantum processor.

In this work, we calculated the fluorescence and Coster-Kronig yields for the $K$ shell and the $L$ and $M$ subshells of Ne, Ar, Kr, Xe, Rn, and Uuo $\left(Z=118\right)$, using a Dirac-Fock model which provides a better description of the electron-electron interaction than previous approaches, and is suitable to handle superheavy elements. The results are compared with available data from other authors. In what concerns Ne, Ar, Kr, Xe, and Rn $K$ shells, the obtained results are in very good agreement with the adopted values of Krause [25] and with experiment when available. For the $L$ subshells, our results are in line with existing ones. For the $M$ subshells and for all shells of Uuo there are no previous experimental and theoretical results to compare to our calculations.

We report nonsequential and sequential fragmentation dynamics of ${{\mathrm{CO}}_2}^{3+}$ investigated by electron collision at an impact energy of 500 eV. The dissociation mechanisms are clearly distinguished by combined use of the Dalitz plot together with momentum correlation spectra. The angular distributions and kinetic-energy releases (KERs) of different fragmentation processes are obtained. The dissociation channels of higher excited states of the ${{\mathrm{CO}}_2}^{3+}$ molecular ion are opened, which are quite different from the previous studies of heavy-ion collision [N. Neumann, D. Hant, L. Ph. H. Schmidt, J. Titze, T. Jahnke, A. Czasch, M. S. Schöffler, K. Kreidi, O. Jagutzki, H. Schmidt-Böcking, and R. Dörner, Phys. Rev. Lett. 104, 103201 (2010)] and intense laser field [C. Wu, C. Wu, D. Song, H. Su, Y. Yang, Z. Wu, X. Liu, H. Liu, M. Li, Y. Deng, Y. Liu, L.-Y. Peng, H. Jiang, and Q. Gong,  Phys. Rev. Lett. 110, 103601 (2013)]. By analyzing KERs together with the help of potential-energy curves exploration at the multireference configuration interaction level, we conclude that the sequential fragmentation occurs in the ${}^2\mathrm{\Pi },{}^4\mathrm{\Pi }$, and ${}^2{\mathrm{\Sigma }}^{+}$ states of the ${{\mathrm{CO}}_2}^{3+}$ ion. The bond length and bond angle are also determined based on the linear fragmentation, indicating that electron impact fragmentation is a potential method to precisely reconstruct the geometry of neutral molecules.

Femtosecond laser-induced Coulomb-explosion imaging of 3,5-dibromo-$3{}^{\prime },5{}^{\prime }$-difluoro-$4{}^{\prime }$-cyanobiphenyl molecules prealigned in space is explored using a pixel-imaging mass-spectrometry (PImMS) camera. The fast-event-triggered camera allows the concurrent detection of the correlated two-dimensional momentum images, or covariance maps, of all the ionic fragments resulting from fragmentation of multiple molecules in each acquisition cycle. Detailed simulation of the covariance maps reveals that they provide rich information about the parent molecular structure and fragmentation dynamics. Future opportunities for imaging the real-time dynamics of intramolecular processes are considered.

The experimental realization of emergent spin-orbit coupling through laser-induced Raman transitions in ultracold atoms paves the way for exploring novel superfluid physics and simulating exotic many-body phenomena. A recent proposal with the use of Laguerre-Gaussian lasers enables another fundamental type of coupling between spin and orbital angular momentum (SOAM) in ultracold atoms. We hereby study quantum phases of a realistic Bose-Einstein condensate (BEC) with this synthetic SOAM coupling in a disk-shaped geometry, respecting radial inhomogeneity of the Raman coupling. We find that the experimental system naturally resides in a strongly interacting regime in which the phase diagram significantly deviates from the single-particle picture. The interplay between SOAM coupling and interaction leads to rich structures in spin-resolved position and momentum distributions, including a stripe phase and various types of immiscible states. Our results would provide a guide for an experimental investigation of SOAM-coupled BECs.

We analyze the Wannier-Stark spectrum of a quantum particle in generic one-dimensional double-periodic lattices. In the limit of a weak static field, the spectrum is shown to be a superposition of two Wannier-Stark ladders originating from two Bloch subbands. As the strength of the field is increased, the spectrum rearranges itself into a single Wannier-Stark ladder. We derive analytical expressions that describe the rearrangement employing the analogy between the Wannier-Stark problem and a driven two-level system in the strong-coupling regime.

We analyze the Wannier-Stark spectrum of a quantum particle in tilted two-dimensional lattices with the Bloch spectrum consisting of two subbands, which could be either separated by a finite gap or connected by the Dirac points. For rational orientations of the static field given by an arbitrary superposition of the translation vectors, the spectrum is a ladder of energy bands. We obtain asymptotic expressions for the energy bands in the limit of large and weak static fields and study them numerically for intermediate field strength. We show that the structure of energy bands determines the rate of spreading of localized wave packets, which is the quantity measured in laboratory experiments. It is shown that wave-packet dispersion becomes a fractal function of the field orientation in the long-time regime of ballistic spreading.

The high-order intensity correlation function of $N$-photon interference with thermal light observed in a recent experiment [S. Oppel, T. Büttner, P. Kok, and J. von Zanthier, Phys. Rev. Lett. 109, 233603 (2012)] is analyzed. The terms in the expansion of the $N\mathrm{th}$-order correlation function are put into three groups. One group contributes a homogeneous background. Both of the other two contribute $(N-1)$-fold super-resolving fringes. In principle they correspond to coherent and incoherent superpositions of classical optical fields, respectively. Therefore similar super-resolving fringes can be obtained without intensity-correlation measurements. We report the experimental results of the coherent and incoherent super-resolving diffraction fringes, which are observed directly in the intensity distribution. The $N-1$ sources in both the coherent and incoherent cases are set in certain definite positions. In the coherent case, moreover, the phase difference between two adjacent source fields is $\pi$. The fringe visibility is unity in the incoherent case, while it decreases as $N$ increases in the incoherent case.

We demonstrate the possibility of optical nonreciprocal response in a three-mode optomechanical system where one mechanical mode is optomechanically coupled to two linearly coupled optical modes simultaneously. The optical nonreciprocal behavior is induced by the phase difference between the two optomechanical coupling rates, which breaks the time-reversal symmetry of the three-mode optomechanical system. Moreover, the three-mode optomechanical system can also be used as a three-port circulator for two optical modes and one mechanical mode, which we refer to as an optomechanical circulator.

We examine the effect of quantum fluctuations in a tunable cold Fermi gas on phonon propagation. We show that these fluctuations can be interpreted as inducing a stochastic space-time. This effect can be displayed in the variation in the travel time of phonons, at its greatest in the crossover region between Bose-Einstein condensate and BCS regimes.

We build in this paper the counterpart of the celebrated Nielsen theorem for coherence manipulation. This offers an affirmative answer to the open question: whether, given two states $\rho$ and $\sigma$, either $\rho$ can be transformed into $\sigma$ or vice versa under incoherent operations [Baumgratz et al., Phys. Rev. Lett. 113, 140401 (2014)]. As a consequence, we find that there exist essentially different types of coherence. Moreover, incoherent operations can be enhanced in the presence of certain coherent states. These extra states are coherent catalysts: they allow uncertain incoherent operations to be realized without being consumed in any way. Our main result also sheds light on the construction of coherence measures.

Estimating the parameters that dictate the dynamics of a quantum system is an important task for quantum information processing and quantum metrology, as well as fundamental physics. In this paper we develop a method for parameter estimation for Markovian open quantum systems using a temporal record of measurements on the system. The method is based on system realization theory and is a generalization of our previous work on identification of Hamiltonian parameters [Phys. Rev. Lett. 113, 080401 (2014)].

Controlling the decoherence induced by the interaction of quantum system with its environment is a fundamental challenge in quantum technology. Utilizing Floquet theory, we explore the constructive role of temporal periodic driving in suppressing decoherence of a spin-1/2 particle coupled to a spin bath. It is revealed that, accompanying the formation of a Floquet bound state in the quasienergy spectrum of the whole system including the system and its environment, the dissipation of the spin system can be inhibited and the system tends to coherently synchronize with the driving. It can be seen as an analog to the decoherence suppression induced by the structured environment in spatially periodic photonic crystal setting. Comparing with other decoherence control schemes, our protocol is robust against the fluctuation of control parameters and easy to realize in practice. It suggests a promising perspective of periodic driving in decoherence control.

In this paper, we show how to use low-fidelity operations to control the dynamics of quantum systems. Noisy operations usually drive a system to evolve into a mixed state and damage the coherence. Sometimes frequent noisy operations result in the coherent evolution of a subsystem, and the dynamics of the subsystem is controlled by tuning noisy operations. Based on this, we find that universal quantum computation can be carried out by low-fidelity (fidelity $<90\%$) operations.

A classical model based on the resolution of Hamilton equations of motion is used to determine the angular distribution of H projectiles following single-electron capture in ${\mathrm{H}}^{+}+\mathrm{H}$ collisions at an incident projectile energy of 250 eV. At such low energies, the experimental charge-exchange probability and angular differential cross sections exhibit oscillatory structures that are classically related to the number of swaps the electron experiences between the target and the projectile during the collision. These oscillations are well reproduced by models based on quantum mechanics. In the present paper, the angular distribution of H projectiles is determined classically, at angles varying from 0.1° up to 7°. The variation in intensity due to interferences caused by the indiscernibility between different trajectories is calculated, and the role of these interferences is discussed.

A kinetic theory model is developed for positron-impact ionization (PII) with neutral rarefied gases. Particular attention is given to the sharing of available energy between the postionization constituents. A simple model for the energy-partition function that qualitatively captures the physics of high-energy and near-threshold ionization is developed for PII, with free parameters that can be used to fit the model to experimental data. By applying the model to the measurements of Kover and Laricchia [Phys. Rev. Lett. 80, 5309 (1998)] for positrons in ${\mathrm{H}}_2$, the role of energy partitioning in PII for positron thermalization is studied. Although the overall thermalization time is found to be relatively insensitive to the energy partitioning, the mean energy profiles at certain times can differ by more than an order of magnitude for the various treatments of energy partitioning. This can significantly impact the number and energy distribution of secondary electrons.

In many experiments involving magneto-optical traps (MOTs), it is imperative to know the fraction of atoms left in an excited state by the cooling and trapping lasers. In most cases, researchers have used formulas that were derived for simple two-level systems interacting with a single beam of light having a well-defined polarization, and in the absence of magnetic or electric fields. However, a MOT environment is much more complex than this. Here we directly measure the excited fraction in a MOT of ${}^{85}\mathrm{Rb}$ atoms in a model-independent manner for a wide range of trapping conditions. We then fit our measured fractions to an ansatz based on a simple model. Knowing only the trapping laser's total intensity and detuning from resonance, one can then use this ansatz to accurately predict the excited fraction. The work is a companion piece to similar measurements on a MOT of ${}^{87}\mathrm{Rb}$.

We investigate a harmonically trapped two-component Bose-Einstein condensate within the miscible regime, close to its boundaries, for different ratios of effective intra- and interspecies interactions. We derive analytically a universal equation for the density around the different boundaries in one, two, and three dimensions, for both the coexisting and spatially separated regimes. We also present a general procedure to solve the Thomas-Fermi approximation in all three spatial dimensionalities, reducing the complexity of the Thomas-Fermi problem for the spatially separated case in one and three dimensions to a single numerical inversion. Finally, we analytically determine the frontier between the two different regimes of the system.