Within the broad research scenario of quantum secure communication, two-way quantum key distribution (TWQKD) is a relatively new proposal for sharing secret keys that is not yet fully explored. We analyze the security of TWQKD schemes that use qubits prepared in nonorthogonal states to transmit the key. Investigating protocols that employ an arbitrary number of bases for the channel preparation, we show, in particular, that the security of the LM05 protocol cannot be improved by the use of more than two preparation bases. We also provide an alternative proof of unconditional security for a deterministic TWQKD protocol recently proposed in Beaudry et al., Phys. Rev. A 88, 062302 (2013). In addition, we introduce a deterministic protocol named “TWQKD six-state” and compute an analytical lower bound (which can be tightened) for the maximum amount of information that an eavesdropper could extract in this case. An interesting advantage of our approach to the security analysis of TWQKD is the great simplicity and transparency of the derivations.

We model sympathetic cooling of ground-state CaF molecules by ultracold Li and Rb atoms. The molecules are moving in a microwave trap, while the atoms are trapped magnetically. We calculate the differential elastic cross sections for CaF-Li and CaF-Rb collisions, using model Lennard-Jones potentials adjusted to give typical values for the $s$-wave scattering length. Together with trajectory calculations, these differential cross sections are used to simulate the cooling of the molecules, the heating of the atoms, and the loss of atoms from the trap. We show that a hard-sphere collision model based on an energy-dependent momentum transport cross section accurately predicts the molecule cooling rate but underestimates the rates of atom heating and loss. Our simulations suggest that Rb is a more effective coolant than Li for ground-state molecules, and that the cooling dynamics is less sensitive to the exact value of the $s$-wave scattering length when Rb is used. Using realistic experimental parameters, we find that molecules can be sympathetically cooled to $100\mu \mathrm{K}$ in about 10 s. By applying evaporative cooling to the atoms, the cooling rate can be increased and the final temperature of the molecules can be reduced to $1 \mu \mathrm{K}$ within 30 s.

Motivated by recent experiments on two-component systems, we investigate the ground-state phase diagram of a mixture of two bosonic species by means of path-integral quantum Monte Carlo simulations by a two-worm algorithm. The mixture is trapped in a square lattice at different filling conditions. Various quantum phases are stabilized depending on the interplay between intra- and interspecies interactions and on the filling factors. We show that the ground-state phase diagram at half-filling features a demixed superfluid phase and a demixed Mott-insulator phase when the interspecies interaction becomes greater than the intraspecies repulsion, and a double-superfluid phase or a supercounterflow otherwise. We show that demixing, characterized by spatial separation of the two species, can be detected experimentally through the effects of anisotropy revealed by time-of-flight images. We also study how demixing effects depend on the filling factor of the two components. Finally, we find that the supercounterflow phase is preserved in the presence of unbalanced populations.

We present a general method for the high-temperature expansion of the self-energy of interacting particles. Although the method is valid for fermions and bosons, we illustrate it for spin-one-half fermions interacting via a zero range potential, in the Bose-Einstein-condensate–Bardeen-Cooper-Schrieffer (BEC-BCS) crossover. The small parameter of the expansion is the fugacity $z$. Our results include terms of order $z$ and $z^2$, which take into account, respectively, two- and three-body correlations. We give results for the high-temperature expansion of Tan's contact at order $z^3$ in the whole BEC-BCS crossover. We apply our method to calculate the spectral function at the unitary limit. We find structures that are different from those discussed in previous approaches, which included only two-body correlations. This shows that including three-body correlations can play an important role in the structures of the spectral function.

By using perturbation theory, we show that a hydrogen atom with magnetic moment due to the orbital angular momentum of the electron has so-called hidden momentum in the presence of an external electric field. This means that the atomic electronic cloud has a nonzero linear momentum in its center-of-mass rest frame due to a relativistic effect. This is completely analogous to the hidden momentum that a classical current loop has in the presence of an external electric field. We discuss how this effect is essential for the validity of the Lorentz-force law in quantum systems. We also connect our results to the long-standing Abraham-Minkowski debate about the momentum of light in material media.

The adiabatic Aharonov-Bohm (AB) effect is a manifestation of the Berry phase acquired when some slow variables take a planar spin around a loop. While the effect has been observed in molecular spectroscopy, direct measurement of the topological phase shift in a scattering experiment has been elusive in the past. Here, we demonstrate an adiabatic AB effect by explicit simulation of the dynamics of unpolarized very slow neutrons that scatter on a long straight current-carrying wire.

Manipulating at will the propagation dynamics of high power laser pulses is a long-standing dream whose accomplishment would lead to the control of fascinating physical phenomena emerging from laser-matter interaction. The present work represents a significant step towards such a control by manipulating the nonlinear optical response of the gas medium. This is accomplished by shaping an intense laser pulse experiencing filamentation at the subcycle level with a relatively weak $\left(\simeq 1\%\right)$ third-harmonic radiation. The control results from quantum interference between a single- and a two-color (mixing the fundamental frequency with its third-harmonic) ionization channel. This mechanism, which depends on the relative phase between the two electric fields, is responsible for wide refractive index modifications in relation with significant enhancement or suppression of the ionization rate. As a first application, we demonstrate the production and control of an axially modulated plasma channel.

We report a study of the effect of relativity on the tunneling time for the process of the tunneling ionization. Our results indicate that the relativistic effects make the tunneling time nonzero. A time interval of a few attoseconds is required for the electron to traverse the barrier. This finding agrees with the basic postulate of the special relativity that two events which are in causal relationship cannot occur simultaneously.

Dipolar Bose-Einstein condensates are systems well suited for the investigation of effects caused by the nonlocal and anisotropic dipole-dipole interaction. In this paper we are interested in properties which are directly connected to the realization of a condensate with ${}^{164}\mathrm{Dy}$, such as stability and phase diagrams. Additionally, we study the expansion of dipolar condensates and find signatures of the dipole-dipole interaction in terms of structured states and a deviation of the well-known inversion of the aspect ratio of the cloud during a time of flight. Our analysis is based on the extended Gross-Pitaevskii equation, which we solve numerically exactly on a grid by means of an imaginary- and real-time evolution.

Molecular high-order harmonic generation from the ${{\mathrm{H}}_2}^{+}$ ion driven by spatial inhomogeneous fields consisting of the chirped pulse and a terahertz pulse has been theoretically investigated by numerically solving the non–Born-Oppenheimer time-dependent Schrödinger equation. It shows that with the introduction of the chirp as well as the spatial inhomogeneity of the pulse, not only the harmonic cutoff is remarkably extended, but also the single short quantum path is selected to contribute to the harmonic spectra. Moreover, through investigation the effects of the laser and the molecular parameters on the inhomogeneous harmonic generation, we found 1.92- and 3.3-dB enhanced fields for the chirp-free and chirped inhomogeneous pulses, respectively. Isotopic effect shows that intense harmonics can be generated from the lighter molecule. Furthermore, with the enhancement of the initial vibrational state and by properly adding a terahertz controlling pulse, the harmonic yield is enhanced by almost five orders of magnitude compared with the initial single chirped case. As a result, a 362-eV supercontinuum (which corresponds to a 4.0-dB laser field enhancement) with five orders of magnitude improvement is obtained. Finally, by properly superposing the harmonics, a series of intense extreme ultraviolet pulses with durations from 22 to 52 as can be produced.

We present a simple and efficient Bayesian recursive algorithm for the data-pattern scheme for quantum state reconstruction, which is applicable to situations where measurement settings can be controllably varied efficiently. The algorithm predicts the best measurements required to accurately reconstruct the unknown signal state in terms of a fixed set of probe states. In each iterative step, this algorithm seeks the measurement setting that minimizes the variance of the data-pattern estimator, which essentially measures the reconstruction accuracy, with the help of a data-pattern bank that was acquired prior to the signal reconstruction. We show that, with this algorithm, it is possible to minimize the number of measurement settings required to obtain a reasonably accurate state estimator by using just the optimal settings and, at the same time, increasing the numerical efficiency of the data-pattern reconstruction.

We study how to generate in minimum time special unitary transformations for a two-level quantum system under the assumptions that (i) the system is subject to a constant drift, (ii) its dynamics can be affected by three independent, bounded controls, and (iii) the bounds on the controls are asymmetric, that is, the constraint on the control in the direction of the drift is independent of that on the controls in the orthogonal plane. Using techniques recently developed for the analysis of $\text{SU}\left(2\right)$ transformations, we fully characterize the reachable sets of the system and the optimal control strategies for any possible target transformation.

We show that an unknown quantum state can be transferred with neither quantum nor classical particle traveling in the transmission channel. Our protocol does not require prearranged entangled photon pairs and Bell measurements. By utilizing quantum Zeno effect and counterfactuality, we can entangle and disentangle a photon and an atom by nonlocal interaction. It is shown that quantum information is completely transferred from an atom to photon due to controllable disentanglement processes. There is no need to cross-check the result via classical channels.

A general procedure to construct criteria for identifying genuine multipartite continuous-variable entanglement is presented. It relies on the definition of adequate global operators describing the multipartite system, the positive partial transpose criterion of separability, and quantum-mechanical uncertainty relations. As a consequence, each criterion encountered consists of a single inequality that is nicely computable and experimentally feasible. Violation of the inequality is a sufficient condition for genuine multipartite entanglement. Additionally, we show that the previous work of van Loock and Furusawa [P. van Loock and A. Furusawa, Phys. Rev. A 67, 052315 (2003)] is a special case of our result.

Electron-ion radiative recombination assisted by a bicircular laser field that consists of two circularly polarized fields counterrotating in the $xy$ plane and having the frequencies $r\omega$ and $s\omega$, which are integer multiples of the fundamental frequency $\omega$, is considered using the $S\text{-matrix}$ theory. The energy and polarization of soft x rays generated in this process are analyzed as functions of the incident electron energy and incident electron angle with respect to the $x$ axis. Numerical results for the process of direct recombination of electrons with ${\text{He}}^{+}$ ionic targets are presented. Abrupt cutoffs of the plateau structures in the emitted x-ray energy spectra are explained by classical analysis. Simpler or more complex oscillatory structures in the spectrum may appear as a result of the interference of a different number of classical orbits. Symmetry analysis and the numerical results show that the x-ray power spectrum and ellipticity are invariant with respect to a rotation of the incident electron momentum by the angle $2\pi /(r+s)$. We have visualized this by presenting the logarithm of the differential power spectrum and polarization of the emitted x rays in false colors as functions of the incident electron angle and the x-ray energy. We have also shown that the change of the relative phase of the bicircular field is equivalent to the change of the incident electron angle. By controlling this relative phase it is possible to control the polarization of the emitted soft x rays.

We study theoretically optical tomograms of the time-evolved states generated by the evolution of different kinds of initial wave packets in a Kerr medium. An exact analytical expression for the optical tomogram of the quantum state at any instant during the evolution of a generic initial wave packet is derived in terms of Hermite polynomials. The time evolution of the optical tomogram is discussed for three kinds of initial states: a coherent state, an $m$-photon-added coherent state, and even and odd coherent states. We show the manifestation of revival and fractional revivals in the optical tomograms of the time-evolved states. We find that the optical tomogram of the time-evolved state at the instants of fractional revivals shows structures with sinusoidal strands. The number of sinusoidal strands in the optical tomogram of the time-evolved state at $l$-subpacket fractional revivals is $l$ times the number of sinusoidal strands present in the optical tomogram of the initial state. We also investigate the effect of decoherence on the optical tomograms of the states at the instants of fractional revivals for the initial states considered above. We consider amplitude decay and phase damping models of decoherence and show the direct manifestations of decoherence in the optical tomogram.

Heralded spontaneous parametric down-conversion (SPDC) has become the mainstay for single-photon-state preparation in present-day photonics experiments. Because they are heralded, in principle one knows when a single photon has been prepared. However, the heralding efficiencies in experimentally realistic SPDC sources are typically very low. To overcome this, multiplexing techniques have been proposed which employ a bank of SPDC sources in parallel and route successfully heralded photons to the output, thereby effectively boosting the heralding efficiency. However, running a large bank of independent SPDC sources is costly and requires complex switching. We analyze a multiplexing technique based on time-bin encoding that allows the heralding efficiency of just a single SPDC source to be increased. The scheme is simple and experimentally viable using present-day technology. We analyze the operation of the scheme in terms of experimentally realistic considerations, such as losses, detector inefficiency, and pump power.

We report on experimental results of four-wave mixing processes in rubidium vapor where coherence is induced on the three-photon resonant transition from $5s$ to $6p$ states via intermediate Rydberg levels. It is shown that the use of two beams in a noncollinear configuration, i.e., $\theta \ne 0$, and high atomic density unveil new features. First, the $\theta =0$ (collinear configuration) odd-photon destructive interference between the incident and generated fields is strongly inhibited for $\theta \ne 0$. Second, most importantly, the observed cooperative frequency shift of the three-photon transition is strongly enhanced for small, but nonzero, values of $\theta$ due to the factor ${(1-cos\theta )}^{-1}$, which is not present if the generated radiation field is not considered self-consistently in the Maxwell-Bloch equations.

Temporal cavity solitons (CSs) enable the generation of coherent Kerr frequency combs with a broad bandwidth. Here we derive analytically and numerically the relationship between the carrier-envelope-phase (CEP) slip of CSs and the pump phase detuning. To preserve the stability of CSs, CEP slips always equal the pump phase detuning. Since the CEP slip for solitons has a contribution from the Kerr effect, the locking between the CEP slip and pump phase detuning results in a universal constraint on the peak power for CSs. The CEP dynamics is also found to affect the stability of CSs. When the CEP slip fails to follow the pump phase detuning, CSs become unstable, exhibiting periodic or chaotic breathing. Our results can be important for the self-referencing of Kerr frequency combs.