Fully relativistic treatment of the electron-atom and positron-atom bremsstrahlung is reported. The calculation is based on the partial-wave expansion of the Dirac scattering states in an external atomic field. A comparison of the electron and positron bremsstrahlung is presented for the single and double differential cross sections and the Stokes parameters of the emitted photon. It is demonstrated that the electron-positron symmetry of the bremsstrahlung spectra, which is nearly exact in the nonrelativistic regime, is to a large extent removed by the relativistic effects.

The rotation-translation thermalization of CO${}_2$ gas is investigated 500 ps after its preheating by a nonresonant short and intense laser pulse. The temperature of thermalization is optically determined with two additional short laser pulses enabling a field-free molecular alignment process and its probing, respectively. The measurements are performed for various intensities of the preheat pulse, leading to the observation of different temperatures which are in very good agreement with classical molecular dynamics simulations. The results can be regarded as a step towards real-time tracking of ultrafast relaxation pathways in molecular motion.

We deduce an expression for the critical polarization $P_C$ below which the Fulde-Ferrell-Larkin-Ovchinnikov state emerges in one-dimensional lattices with spin-imbalanced populations. We provide and explore the phase diagram of unconfined chains as a function of polarization, interaction, and particle density. For harmonically confined systems, we supply a quantitative mapping, which also allows applying our phase diagram for confined chains. We find analytically and confirm numerically that the upper bound for the critical polarization is universal: $P_C^{\max }=1/3$ for any density, interaction, and confinement strength.

We present a space-time generalization of the known spatial (monochromatic) wave vortex beams carrying intrinsic orbital angular momentum (OAM) along the propagation direction. Generic spatiotemporal vortex beams are polychromatic and can carry intrinsic OAM at an arbitrary angle to the mean momentum. Applying either (i) a transverse wave-vector shift or (ii) a Lorentz boost to a monochromatic Bessel beam, we construct a family of either (i) time-diffracting or (ii) nondiffracting spatiotemporal Bessel beams, which are exact solutions of the Klein-Gordon wave equations. The proposed spatiotemporal OAM states are able to describe either photon or electron vortex states (both relativistic and nonrelativistic) and have potential applications in particle collisions, optics of moving media, quantum communications, and astrophysics.

We provide a full-wave electromagnetic (EM) scattering theory of discussing the electromagnetic scattering efficiency of radially anisotropic nanocylinders. In the long-wavelength limit, we derive the conditions for observing unusual EM scattering including non-Rayleigh vanishing and diverging ones. To verify our theoretical predictions, both full-wave numerical results and numerical simulations are performed, and good agreement is found. Moreover, physical origins of the anomalous phenomena are given. Therefore, the anisotropic nanowires under certain conditions can be hardly visible or exhibit superscattering. These results may find potential applications in different fields of nanotechnology.

We show that partial transposition of any $2\otimes n$ state can have at most $(n-1)$ number of negative eigenvalues. This extends a decade old result of the $2\otimes 2$ case by Sanpera et al. [Phys. Rev. A 58, 826 (1998)]. We then apply this result to critically assess an important conjecture recently made by Girolami and Adesso [Phys. Rev. A 84, 052110 (2011)], namely, the (normalized) geometric discord should always be lower bounded by squared negativity. This conjecture has strengthened the common belief that measures of generic quantum correlations should be more than those of entanglement. Our analysis shows that unfortunately this is not the case and we give several counterexamples to this conjecture. All the examples considered here are in finite dimensions. Surprisingly, there are counterexamples in $2\otimes n$ for any $n>2$. Coincidentally, it appears that the $4\otimes 4$ Werner state, when seen as a $2\otimes 8$ dimensional state, also violates the conjecture. This result contributes significantly to the negative side of the current ongoing debates on the defining notion of geometric discord as a good measure of generic correlations.

We propose an experiment which can demonstrate quantum correlations in a physical scenario as discussed in the seminal work of Einstein, Podolsky, and Rosen. Momentum-entangled massive particles are produced via the four-wave mixing process of two colliding Bose-Einstein condensates. The particles’ quantum correlations can be shown in a double–double-slit experiment or via ghost interference.

We propose a feasible scheme to achieve quantum computation based on geometric manipulation of ensembles of atoms and analyze it for neutral rubidium atoms magnetically trapped in planoconcave microcavities on an atom chip. The geometric operations are accomplished by optical excitation of a single atom into a Rydberg state in a constant electric field. Strong dipole-dipole interactions and incident lasers drive the dark state of the atom ensembles to undergo cyclic evolutions that realize a universal set of quantum gates. Such geometric manipulation turns out naturally to protect the qubits from the errors induced by nonuniform laser illumination as well as cavity loss. The gate performance and decoherence processes are assessed by numerical simulation.

This article provides an introduction to surface code quantum computing. We first estimate the size and speed of a surface code quantum computer. We then introduce the concept of the stabilizer, using two qubits, and extend this concept to stabilizers acting on a two-dimensional array of physical qubits, on which we implement the surface code. We next describe how logical qubits are formed in the surface code array and give numerical estimates of their fault tolerance. We outline how logical qubits are physically moved on the array, how qubit braid transformations are constructed, and how a braid between two logical qubits is equivalent to a controlled-not. We then describe the single-qubit Hadamard, $\widehat{S}$ and $\widehat{T}$ operators, completing the set of required gates for a universal quantum computer. We conclude by briefly discussing physical implementations of the surface code. We include a number of Appendices in which we provide supplementary information to the main text.

In this paper we set forth a connection between the Bennett 1992 protocol and a Bell inequality. This allows us to extend the usual prepare-and-measure protocol to its entanglement-based formulation. We exploit a recent result in the framework of device-independent quantum key distribution to provide a simple, model-independent, security proof for the protocol. The minimum efficiency required for a practical implementation of the scheme can be as low as 75% in the completely untrusted case and 50% in case the sender is assumed to be trusted.

Relativistic linearized coupled-cluster single-double approximation with third-order corrections is used to calculate positron-atom bound states. The method is tested on closed-shell atoms such as Be, Mg, Ca, Zn, Cd, and Hg, where a number of accurate calculations are available. It is then used to calculate positron binding energies for a range of open-shell transition metal atoms from Sc to Cu, from Y to Pd, and from Lu to Pt. These systems possess Feshbach resonances, which can be used to search for positron-atom binding experimentally through resonant annihilation or scattering.

Classical mechanics and classical ensembles have provided numerous insights into the dynamics of strong-field double ionization. In this paper, we show that in classical multidimensional modeling, the laser intensity at which sequential ionization begins to dominate depends on the softening of the interaction between the electron and nucleus. We show that an unsoftened interaction in two or three dimensions can lead to classical orbits in which an electron can start deep in the nuclear potential-energy well, gain energy from the oscillating laser field, and ionize over the barrier without any recollision. We discuss how this energy gain occurs, with the electron orbit favoring one side of the nucleus or the other, depending on which side corresponds with the rising potential-energy curve.

We study the pinning quantum phase transition in a Tonks-Girardeau gas, both in equilibrium and out of equilibrium, using the ground-state fidelity and the Loschmidt echo as diagnostic tools. The ground-state fidelity will have a dramatic decrease when the atomic density approaches the commensurate density of one particle per lattice well. This decrease is a signature of the pinning transition from the Tonks to the Mott insulating phase. We study the applicability of the fidelity for diagnosing the pinning transition in experimentally realistic scenarios. We find that fidelity can predict the particle number(s) at which the pinning occurs. In addition, we explore the out-of-equilibrium dynamics of the gas following a sudden quench with a lattice potential. We find that all properties of the ground-state fidelity are reflected in the Loschmidt echo dynamics, i.e., in the nonequilibrium dynamics of the Tonks-Girardeau gas initiated by a sudden quench of the lattice potential.

We study cluster-cluster collisions in one-dimensional Fermi systems with particular emphasis on the nontrivial quantum effects of the collision dynamics. We adopt the Fermi-Hubbard model and the time-dependent density-matrix renormalization-group method to simulate collision dynamics between two fermion clusters of different spin states with contact interaction. It is elucidated that the quantum effects become extremely strong with the interaction strength, leading to the transmittance being much more enhanced than expected from the semiclassical approximation. We propose a concise model as an application of the Bethe ansatz, which unveils the origin of the quantum effects and also explains the overall properties of the simulation results clearly. This model provides an intuitive perspective of the collision dynamics with contact interaction. Some potential applications, such as repeated collisions, are addressed.

We consider a broad area vertical-cavity surface-emitting laser (VCSEL) operating below the lasing threshold and subject to optical injection and time-delayed feedback. We derive a generalized delayed Swift-Hohenberg equation for the VCSEL system, which is valid close to the nascent optical bistability. We first characterize the stationary-cavity solitons by constructing their snaking bifurcation diagram and by showing clustering behavior within the pinning region of parameters. Then, we show that the delayed feedback induces a spontaneous motion of two-dimensional (2D) cavity solitons in an arbitrary direction in the transverse plane. We characterize moving cavity solitons by estimating their threshold and calculating their velocity. Numerical 2D solutions of the governing semiconductor laser equations are in close agreement with those obtained from the delayed generalized Swift-Hohenberg equation.

Recently, Gerry et al. [Phys. Rev. A 79, 022111 (2009)] studied the violation of the Bell-Clauser-Horne-Shimony-Holt inequality for two-spin systems, prepared in an entanglement of spin-coherent states, the so-called entangled spin-coherent states (ESCSs), and found maximal violations for a large class of states. In this paper, using the Holstein-Primakoff realization (HPR) of angular momentum algebra, we present an improved phase estimation scheme employing ESCSs and demonstrate that increasing the spin number gives the smallest variance in the phase parameter in comparison to NOON states under perfect and lossy conditions. The phase sensitivity of this interferometric scheme with parity detection on one of the output states is discussed.

In a dipolar Fermi gas, the anisotropic interaction between electric dipoles can be turned into an effectively attractive interaction in the presence of a rotating electric field. We show that the topological $p_x+i{p}_y$ superfluid phase can be realized in a single-component dipolar Fermi gas trapped in a two-dimensional square optical lattice with this attractive interaction at low temperatures. The $p_x+i{p}_y$ superfluid state has potential applications for topological quantum computing. We obtain the phase diagram of this system at zero temperature. When the interaction is weak, the $p$-wave superfluid state is stable for most filling factors except near half filling, where phase separation takes place. In the weak-coupling limit, the phase-separation region vanishes. When the interaction strength is above a threshold, the system is phase separated for any filling factor $0<n<1$. The transition temperature of the $p_x+i{p}_y$ superfluid state is estimated and the implication for experiments is discussed.

We investigate strongly correlated phases of two-component (or pseudospin-$1/2$) Bose gases under rapid rotation through exact diagonalization on a torus geometry. In the case of pseudospin-independent contact interactions, we find the formation of gapped spin-singlet states at the filling factors $\nu =k/3+k/3$ ($k/3$ filling for each component) with integer $k$. We present numerical evidences that the gapped state with $k=2$ is well described as a non-Abelian spin-singlet (NASS) state, in which excitations feature non-Abelian statistics. Furthermore, we find the phase transition from the product of composite fermion states to the NASS state by changing the ratio of the intercomponent-to-intracomponent interactions.

We study planar Dirac scattering for an electrostatic stratified barrier potential. The general expressions for transmitted and reflected waves are derived. Of particular interest is the information upon relative helicity phases. We also briefly discuss some possible applications.

An isolated classical chaotic system, when driven by the slow change in several parameters, responds with two reaction forces: geometric friction and geometric magnetism. By using the theory of quantum fluctuation relations, we show that this holds true also for open quantum systems and provide explicit expressions for those forces in this case. This extends the concept of Berry curvature to the realm of open quantum systems. We illustrate our findings by calculating the geometric magnetism of a damped charged quantum harmonic oscillator transported along a path in physical space in the presence of a magnetic field and a thermal environment. We find that, in this case, the geometric magnetism is unaffected by the presence of the heat bath.