We introduce a class of propagation invariant spatiotemporal optical wave packets with spherical harmonic symmetries in their field configurations. The evolution of these light orbitals is considered theoretically in anomalously dispersive media, and their spinning dynamics are analyzed in terms of their corresponding energy flows. Similarly, localized waves generated via spherical superposition from Archimedean and Platonic solids in $\vec{k}-\omega$ space are investigated in this work.

In this Brief Report, we investigate the validity of the adiabatic approximation in the tunneling time problem of a two-level atomic wave packet transmitting through a laser beam. We calculate the tunneling time of ground-state atoms using adiabatic approximation and compare it to the exact solution. We find that the adiabatic solution misses an important interference dip in the tunneling time which indicates that the adiabatic approximation is not always valid although the adiabatic conditions are well satisfied.

We study communication between an inertial observer and one of two causally disconnected counteraccelerating observers. We will restrict the quantum channel considering inertial-to-accelerated bipartite classical and quantum communication over different sets of Unruh modes (single-rail or dual-rail encoding). We find that the coherent information (and therefore the amount of entanglement that can be generated via state merging protocol) in this strongly restricted channel presents some interesting monogamy properties between the inertial and only one of the accelerated observers if we take a fixed choice of the Unruh mode used in the channel. The optimization of the controllable parameters is also studied and we find that they deviate from the values usually employed in the literature.

In usual security proofs of quantum protocols the adversary (Eve) is expected to have full control over any quantum communication between any communicating parties (Alice and Bob). Eve is also expected to have full access to an authenticated classical channel between Alice and Bob. Unconditional security against any attack by Eve can be proved even in the realistic setting of device and channel imperfection. In this paper we show that the security of quantum key distribution protocols is ruined if one allows Eve to possess a very limited access to the random sources used by Alice. Such knowledge should always be expected in realistic experimental conditions via different side channels.

We report on the measurement of the linear polarization degree of the $3s{}^2{S}_{1/2}$ $\to$ $3p{}^2{P}_{3/2}$ $\to$ $5s{}^2{S}_{1/2}$ transition in atomic sodium. A linearly polarized pump laser creates an alignment in the excited ${}^{23}$Na $3p{}^2{P}_{3/2}$ level. From the spectra, we extracted the inelastic collision cross section for the disalignment of the excited atoms by collisions with ground-level argon or xenon atoms. Measurements were done by monitoring the intensity of the cascade fluorescence from the $4p{}^2{P}_{1/2}$ $\to$ $3s{}^2{S}_{1/2}$ transition at 330.29 nm, in the presence of Ar and Xe for pressures ranging from 5.3 to 146.6 mbar. We demonstrated that the polarization measurement can provide important information about the alignment-dependent inelastic process in alkali-metal–rare-gas collisions. A depolarization cross-section value of 196 (22) Å${}^2$ for Na-Ar and 357 (21) Å${}^2$ for Na-Xe are obtained.

We present left-right spin asymmetries for elastic and inelastic scattering of electrons from thallium. Being a heavy target ($Z=81$), the spin asymmetries can be mediated by relativistic effects (spin-orbit interactions) in addition to exchange effects. We find overall excellent agreement with experiment.

The two-center convergent-close-coupling method is extended to the calculation of positron scattering from atoms with two valence electrons above an inert ion core. Calculations of positron scattering on magnesium have been performed over a wide energy range. Elastic scattering, Positronium formation, target excitation, and direct ionization cross sections have been calculated. Results are in reasonably good agreement with the experimental data where available. Current results confirm the existence of a low-energy shape resonance predicted by variational and single-center convergent-close-coupling methods.

For energetic atomic beams grazingly incident at a surface along low-index directions, the fast motion of the projectile in the surface plane and the slow motion in the direction perpendicular to the surface appear nearly decoupled. Fast-atom diffraction (FAD) experiments reveal two-dimensional (2D) diffraction patterns associated with exchange of the reciprocal vector perpendicular to the low-index direction of fast motion. These results are usually interpreted within the axial-channeling approximation, where the effective 2D potential experienced by the projectile is set as an average of the 3D surface potential along the atomic strings forming the channel. In this work, using the example of grazing scattering of He atoms at a LiF(001) surface, we address theoretically the range of validity of the axial-channeling approximation. Full quantum wave-packet-propagation calculations are used to study the transition from the 2D (fast atom) to the 3D diffraction pattern characteristic for low-energy atomic and molecular projectiles scattered from surfaces. Along with exact calculations, a semianalytical perturbative treatment based on the Lippmann-Schwinger equation allows an explanation of why the diffraction processes involving the exchange of reciprocal-lattice vectors along the fast-motion direction are exponentially small in typical FAD conditions.

We investigate the effect of the Casimir force on the entanglement in an optomechanical system formed by a Fabry-Pérot cavity with a dielectric nanosphere trapped near the cavity mirror by external driving laser beams. We find that steady-state optomechanical entanglement can be generated via the Casimir interaction between the nanosphere and the mirror, even with only one driving laser. With two driving lasers that are simultaneously responsible for trapping and cooling, the Casimir effect may enhance or reduce the degree of the steady-state optomechanical entanglement, depending on the cavity detunings, the strength of the Casimir interaction, the driving powers, and the cavity decay rates. The Casimir effect leads to an oscillation frequency shift of the nanosphere in the optomechanical system, which could be used for estimating the magnitude of the Casimir force between a sphere and a plane.

We study the evolution of an inverted spin ensemble coupled to a cavity. While the inversion itself presents an inherent instability in the system, the inhomogeneous broadening of spin-resonance frequencies provides a stabilizing mechanism. The detailed behavior of mean values and variances of the spin components and the cavity field is accounted for under both stable and unstable conditions.

The entanglement entropy and quantum fidelity in a hard-core-boson model with nearest- and next-nearest-neighbor interactions are studied numerically. By using exact diagonalization and the density-matrix renormalization group, the effects of interactions on the entanglement entropy and fidelity susceptibility in the model are investigated. We focus our attention on looking for three quantum phase transitions. It is found that the first derivative of the entanglement entropy can indicate all three phase transitions, while the fidelity susceptibility cannot predict the transition from superfluid to bond order in a finite-size system.

We discuss the desired criteria for a two-qubit phase gate and present a method for realizing such a gate for quantum computation that is measurement free and low error. The gate is implemented between qubits via an intermediate bus mode. We take a coherent state as the bus and use cross-Kerr-type interactions between the bus and the qubits. This new method is robust against parameter variations and is thus low error. It fundamentally improves on previous methods due to its deterministic nature and the lack of approximations used in the geometry of the phase rotations. This interaction is applicable both to solid-state and photonic qubit systems.

We derive the measurement strategy that optimizes the Bayes criterion, not necessarily the strategy minimizing the error probability, for distinguishing between collections of quantum states. We present a sufficient condition concerning the rank of each detection operator of an optimal measurement for the Bayes criterion. We also derive an analytical expression for an optimal measurement for the Bayes criterion for an Abelian geometrically uniform state set or for a cyclic state set under a certain constraint.

The trade-off capacity region of a quantum channel characterizes the optimal net rates at which a sender can communicate classical, quantum, and entangled bits to a receiver by exploiting many independent uses of the channel, along with the help of the same resources. Similarly, one can consider a trade-off capacity region when the noiseless resources are public, private, and secret-key bits. We identified [see Wilde, Hayden, and Guha, Phys. Rev. Lett. 108, 140501 (2012)] these trade-off rate regions for the pure-loss bosonic channel and proved that they are optimal provided that a long-standing minimum-output entropy conjecture is true. Additionally, we showed that the performance gains of a trade-off coding strategy when compared to a time-sharing strategy can be quite significant. In this paper, we provide detailed derivations of the results announced there, and we extend the application of these ideas to thermal-noise and amplifying bosonic channels. We also derive a “rule of thumb” for trade-off coding, which determines how to allocate photons in a coding strategy if a large mean photon number is available at the channel input. Our results on the amplifying bosonic channel also apply to the “Unruh channel” considered in the context of relativistic quantum information theory.

We study the lateral dependence of the Casimir energy for different corrugated gratings of arbitrary periodic profile. To this end we model the profiles as stacks of horizontal rectangular slices following the profiles' shape and evaluate numerically the Casimir energy between them for different relative lateral displacements of the two corrugated plates. We compare our results with predictions obtained within the proximity force approximation (PFA). At comparable separation of the corrugated plates and geometric parameters, we find a strong dependence of the Casimir energy on the shape of the corrugation profiles.

We present experimental investigations of rephasing optically driven atomic coherences, prepared by electromagnetically induced transparency in a Pr${}^{3+}$:Y${}_2$SiO${}_5$ crystal. In particular, we systematically study rephasing based on rapid adiabatic passage and compare the performance of the latter with standard rephasing based on Hahn spin echoes. The data clearly demonstrate the superior performance of rapid adiabatic passage for any application of rephasing, which suffers from large inhomogeneous broadenings in the medium or inevitable fluctuations in the experimental parameters.

The exactly solvable quantum many-particle model with harmonic one- and two-particle interaction terms is extended to include time dependency. We show that when the external trap potential and interparticle interaction have a time dependency, the numerically exact solutions of the corresponding time-dependent many-boson Schrödinger equation are still available. We use these exact solutions to benchmark the recently developed multiconfigurational time-dependent Hartree method for bosons (MCTDHB) [Phys. Rev. Lett. 99, 030402 (2007); Phys. Rev. A 77, 033613 (2008)]. In particular, we benchmark the MCTDHB method for (i) the ground state; (ii) the breathing many-body dynamics activated by a quench scenario where the interparticle interaction strength is suddenly turned on to a finite value; (iii) the nonequilibrium dynamic for driven scenarios where both the trap- and interparticle-interaction potentials are time-dependent. Excellent convergence of the ground state and dynamics is demonstrated. The great relevance of the self-consistency and time adaptivity, which are the intrinsic features of the MCTDHB method, is demonstrated by contrasting the MCTDHB predictions and those obtained within the standard full configuration interaction method spanning a Fock space of the same size, but utilizing as one-particle basis set the fixed-shape eigenstates of the one-particle potential. Connections of the model's results to ultracold Bose-Einstein condensed systems are addressed.

Linear response of simple (i.e., condensed) Bose-Einstein condensates is known to lead to the Bogoliubov–de Gennes equations. Here, we derive linear response for fragmented Bose-Einstein condensates, i.e., for the case where the many-body wave function is not a product of one, but of several single-particle states (orbitals). This gives one access to excitation spectra and response amplitudes of systems beyond the Gross-Pitaevskii description. Our approach is based on the number-conserving variational time-dependent mean-field theory [Alon, Streltsov, and Cederbaum, Phys. Lett. A 362, 453 (2007)], which describes the time evolution of best-mean-field states. Correspondingly, we call our linear-response theory for fragmented states LR-BMF. In the derivation it follows naturally that excitations are orthogonal to the ground-state orbitals. As applications excitation spectra of Bose-Einstein condensates in double-well potentials are calculated. Both symmetric and asymmetric double wells are studied for several interaction strengths and barrier heights. The cases of condensed and twofold fragmented ground states are compared. Interestingly, even in such situations where the response frequencies of the two cases are computed to be close to each other, which is the situation for the excitations well below the barrier, striking differences in the density response in momentum space are found. For excitations with an energy of the order of the barrier height, both the energies and the density response of condensed and fragmented systems are very different. In fragmented systems there is a class of “swapped” excitations where an atom is transferred to the neighboring well. The mechanism of its origin is discussed. In asymmetric wells, the response of a fragmented system is purely local (i.e., finite in either one or the other well) with different frequencies for the left and right fragments. This finding is in stark contrast to that for condensed systems.

We propose that the temperature dependence of a single quantity $R={\kappa }_i/\delta n_i^2$, the ratio of the local compressibility to the local number fluctuations, can be used to map out the finite temperature phase diagram, diagnose the critical region around a quantum phase transition, and identify critical points belonging to different universality classes. We test our proposal using state-of-the-art large-scale quantum Monte Carlo simulations of the two-dimensional Bose Hubbard model. Our results have implications for recently developed single-site imaging experiments.

In the context of quantum-state engineering we analyze the effect of observation on nonlinear optical $n$-photon Fock-state generation. We show that it is possible to truncate the vacuum component from an arbitrary photon number superposition without modifying its remaining parts. In the course of the full dynamical analysis of the effect of observation, it is also found that the Zeno and the anti-Zeno effects repeat periodically. We discuss the close relationship between vacuum-state truncation and so-called “interaction-free” measurement. The proposed scheme may also be considered a way to generate extremely nonclassical states.