We study the shear viscosity of a dilute Fermi gas as a function of the scattering length in the vicinity of the unitarity limit. The calculation is based on kinetic theory, which provides a systematic approach to transport properties in the limit in which the fugacity $z=n{\lambda }^3/2$ is small. Here, $n$ is the density of the gas and $\lambda$ is the thermal wavelength of the fermions. At leading order in the fugacity expansion, the shear viscosity is independent of density, and the minimum shear viscosity is achieved at unitarity. At the next order, medium effects modify the scattering amplitude as well as the quasiparticle energy and velocity. We show that these effects shift the minimum of the shear viscosity to the Bose-Einstein condensation side of the resonance, in agreement with the result of recent experiments.

We explore the manipulation in phase space of many-body wave functions that exhibit self-similar dynamics under the application of sudden force and/or in the presence of a constant acceleration field. For this purpose, we work out a common theoretical framework based on the Wigner function. We discuss squeezing in position space, phase-space rotation, and its implications in cooling for both noninteracting and interacting gases and time-reversal operation. We discuss various optical analogies and calculate the role of a spherical-like aberration in cooling protocols. We also present the equivalent of a spin-echo technique to improve the robustness of velocity dispersion reduction protocols.

We demonstrate squeezing of a strongly interacting optoelectromechanical system using a parametric drive. By employing real-time feedback on the phase of the pump at twice the resonance frequency the thermomechanical noise is squeezed beyond the 3 dB instability limit. Surprisingly, this method can also be used to generate highly nonlinear states. We show that using the parametric drive with feedback on, classical numberlike and catlike states can be prepared. This presents a valuable electro-optomechanical state-preparation protocol that is extendable to quantum regime.

The onset of unconventional vacuum properties in intense fields has long been an active field of research. In this paper the vacuum polarization effect is investigated via a pump probe scheme in which a probe light propagates in the vacuum excited by a standing wave composed of two counterpropagating laser beams. The modified light-cone condition of the probe light is derived analytically for the situation that it passes through the electric (magnetic) antinode plane of the pump field and thus the probe light effectively experiences a time-dependent electric (magnetic) field. The derivation does not follow the commonly adopted assumption of treating the pump field as a constant field. Differences from the conventional light-cone conditions are identified. The light-cone condition is used to calculate the ellipticity value for a conceptual vacuum birefringence measurement. The quasistatic treatment is partly justified and the implications of the unconventional correction to previous results are discussed.

Helicity amplitudes and the differential cross section of high-energy $e^{+}{e}^{-}$ photoproduction accompanied by bremsstrahlung in the electric field of a heavy atom (i.e., the amplitudes of the process ${\gamma }_{1}Z\to e^{+}{e}^{-}{\gamma }_{2}Z)$ are derived. The results are exact in the nuclear charge number and are obtained in the leading quasiclassical approximation. They correspond to the leading high-energy small-angle asymptotics of the amplitude. It is shown that, in general, accounting for the Coulomb corrections essentially modify the differential cross section, which is different from the Born result. When the initial photon is circularly polarized, the Coulomb corrections lead to the asymmetry in the distribution over the azimuth angles ${\phi }_i$ of produced particles with respect to the replacement ${\phi }_i\to -{\phi }_i$.

This paper unveils some features of a discrete-time quantum walk on the line whose coin depends on the temporal variable. After considering the most general form of the unitary coin operator, it focuses on the role played by the two phase factors that one can incorporate there, and shows how both terms influence the evolution of the system. A closer analysis reveals that the probabilistic properties of the motion of the walker remain unaltered when the update rule of these phases is chosen adequately. This invariance is based on a symmetry with consequences not yet fully explored.

We analyze the performance of a generalized Kitaev's phase-estimation algorithm where $N$ phase gates, acting on $M$ qubits prepared in a product state, may be distributed in an arbitrary way. Unlike the standard algorithm, where the mean square error scales as $1/N$, the optimal generalizations offer the Heisenberg $1/N^2$ error scaling and we show that they are in fact very close to the fundamental Bayesian estimation bound. We also demonstrate that the optimality of the algorithm breaks down when losses are taken into account, in which case the performance is inferior to the optimal entanglement-based estimation strategies. Finally, we show that when an alternative resource quantification is adopted, which describes the phase estimation in Shor's algorithm more accurately, the standard Kitaev's procedure is indeed optimal and there is no need to consider its generalized version.

We study the field-ionization threshold behavior when a Rydberg atom is ionized by a short single-cycle pulse field. Both hydrogen and sodium atoms are considered. The required threshold field amplitude is found to scale inversely with the binding energy when the pulse duration becomes shorter than the classical Rydberg period, and, thus, more weakly bound electrons require larger fields for ionization. This threshold scaling behavior is confirmed by both three-dimensional classical trajectory Monte Carlo simulations and numerically solving the time-dependent Schrödinger equation. More surprisingly, the same scaling behavior in the short pulse limit is also followed by the ionization thresholds for much lower bound states, including the hydrogen ground state. An empirical formula is obtained from a simple model, and the dominant ionization mechanism is identified as a nonzero spatial displacement of the electron. This displacement ionization should be another important mechanism beyond the tunneling ionization and the multiphoton ionization. In addition, an “ionization window” is shown to exist for the ionization of Rydberg states, which may have potential applications to selectively modify and control the Rydberg-state population of atoms and molecules.

High-order above-threshold ionization (ATI) is characterized by the rescattering of the ionized electron off the parent ion. The ATI amplitude can be calculated applying the saddle-point (SP) method and the uniform approximation to a five-dimensional integral over the ionization time, intermediate electron momentum, and rescattering time. The so-obtained amplitude is a coherent sum of amplitudes associated to the two classes of the SP solutions. The low-energy structures in the ATI spectra are determined by the forward-scattering SP solutions, while the high-energy spectra (plateau and cutoff) are determined by the backward-scattering SP solutions. In order to properly describe the intermediate-electron-energy spectra, additional SP solutions should be taken into account. For these solutions, the imaginary part of the rescattering time can be large and, for their explanation, we introduce complex-time quantum orbits.

In the present work, we theoretically propose and numerically illustrate a mean-field analog of the Hong-Ou-Mandel experiment with bright solitons. More specifically, we scatter two solitons off of each other (in our setup, the bright solitons play the role of a classical analog to the quantum photons of the original experiment), while the role of the beam splitter is played by a repulsive Gaussian barrier. In our classical scenario, distinguishability of the particles yields, as expected, a $0.5$ split mass on either side. Nevertheless, for very slight deviations from the completely symmetric scenario, a near-perfect transmission can be constructed instead, very similarly to the quantum-mechanical output. We demonstrate this as a generic feature under slight variations of the relative soliton speed, or of the relative amplitude in a wide parametric regime. We also explore how variations of the properties of the “beam splitter” (i.e., the Gaussian barrier) affect this phenomenology.

We analyze the properties of electromagnetic waves inside materials with induced polarization and magnetization. We show that if the polarization and magnetization of the material are sufficiently large and appropriately phased, then the system supports the formation of left-handed waves. In some respects, such a system behaves similarly to materials with a negative index of refraction, yet there is one important advantage: Left-handed waves in materials with induced polarization and magnetization do not require as stringent material properties (such as the strength of resonances and the density of radiators). We numerically investigate the formation and propagation of such left-handed waves using finite-difference approximation to Maxwell's equations. We also discuss possible experimental observation of these ideas in a rare-earth-doped crystal.

We have experimentally investigated high-order-harmonic generation driven by a few-cycle truncated Bessel (TB) laser beam which propagates through optical elements of finite aperture sizes. The TB beam was first investigated by Nisoli et al. [Phys. Rev. Lett. 88, 033902 (2002)], who assumed an infinite size for the optical elements so they concluded that the phase and intensity of the laser field oscillate dramatically around the laser focus in space. However, in all real experiments, the optical elements are always finite in size and would further truncate the TB beam, and so the oscillations would dwindle substantially. In this paper we take the finite size of the optical elements into account. We find that the further truncated TB beam has two intensity peaks around the focus. In front of the second peak position the curvatures of the laser phase front and the atomic-dipole phase front have the same absolute values but opposite signs, so the generated harmonic has a flat wavefront and hence a minimized angular divergence. In addition, at this position the pump intensity is not much less than its maximal value. This result is of significant importance in practical applications due to the finite aperture size of all real optical elements.

The concept of bilocality was introduced to study the correlations which arise in an entanglement-swapping scenario, where one has two sources which can naturally be taken to be independent. This additional constraint leads to stricter requirements than simply imposing locality, in the form of bilocality inequalities. In this work we consider a natural generalization of the bilocality scenario; namely, the star network consisting of a single central party surrounded by $n$ edge parties, each of which shares an independent source with the center. We derive inequalities which are satisfied by all local correlations in this scenario, for the cases when the central party performs (i) two dichotomic measurements (ii) a single Bell-state measurement. We demonstrate quantum violations of these inequalities and study both the robustness to noise and to losses.

Finding the consequences of symmetry for open-system quantum dynamics is a problem with broad applications, including describing thermal relaxation, deriving quantum limits on the performance of amplifiers, and exploring quantum metrology in the presence of noise. The symmetry of the dynamics may reflect a symmetry of the fundamental laws of nature or a symmetry of a low-energy effective theory, or it may describe a practical restriction such as the lack of a reference frame. In this paper, we apply some tools of harmonic analysis together with ideas from quantum information theory to this problem. The central idea is to study the decomposition of quantum operations—in particular, states, measurements, and channels—into different modes, which we call modes of asymmetry. Under symmetric processing, a given mode of the input is mapped to the corresponding mode of the output, implying that one can only generate a given output if the input contains all of the necessary modes. By defining monotones that quantify the asymmetry in a particular mode, we also derive quantitative constraints on the resources of asymmetry that are required to simulate a given asymmetric operation. We present applications of our results for deriving bounds on the probability of success in nondeterministic state transitions, such as quantum amplification, and a simplified formalism for studying the degradation of quantum reference frames.

The problem of sharing entanglement over large distances is crucial for implementations of quantum cryptography. A possible scheme for long-distance entanglement sharing and quantum communication exploits networks whose nodes share Einstein-Podolsky-Rosen (EPR) pairs. In Perseguers et al. [Phys. Rev. A 78, 062324 (2008)] the authors put forward an important isomorphism between storing quantum information in a dimension $D$ and transmission of quantum information in a $D+1$-dimensional network. We show that it is possible to obtain long-distance entanglement in a noisy two-dimensional (2D) network, even when taking into account that encoding and decoding of a state is exposed to an error. For 3D networks we propose a simple encoding and decoding scheme based solely on syndrome measurements on 2D Kitaev topological quantum memory. Our procedure constitutes an alternative scheme of state injection that can be used for universal quantum computation on 2D Kitaev code. It is shown that the encoding scheme is equivalent to teleporting the state, from a specific node into a whole two-dimensional network, through some virtual EPR pair existing within the rest of network qubits. We present an analytic lower bound on fidelity of the encoding and decoding procedure, using as our main tool a modified metric on space-time lattice, deviating from a taxicab metric at the first and the last time slices.

The dispersed laser induced fluorescence resulting from excitation of the $U\left(\right|\Omega |=1)-X{}^1{\Sigma }^{+}(0,0)$, $L\left(\right|\Omega |=1)-X{}^1{\Sigma }^{+}(0,0)$, and $I\left(\right|\Omega |=1)-X{}^1{\Sigma }^{+}(0,0)$ bands near 398, 402, and 512 nm, respectively, of thorium oxide, ThO, has been recorded and analyzed. The branching ratios for the fluorescence decay from the $U\left(\right|\Omega |=1)(v=0)$, $L\left(\right|\Omega |=1)(v=0)$, and $I\left(\right|\Omega |=1)(v=0)$ states are determined. Radiative lifetimes of 57 ± 2, 17 ± 1, 115 ± 4, and 468 ± 30 ns for the $U\left(\right|\Omega |=1)(v=0)$, $L\left(\right|\Omega |=1)(v=0)$, $I\left(\right|\Omega |=1)(v=0)$, and $C{}^1{\Pi }_1(v=0)$ states, respectively, were determined from the analysis of the fluorescence decay curves. Transition dipole moments for numerous bands involving the $U\left(\right|\Omega |=1)(v=0)$, $L\left(\right|\Omega |=1)(v=0)$, $I\left(\right|\Omega |=1)(v=0)$, and $C{}^1{\Pi }_1(v=0)$ states are determined. Possible optical pumping and detection schemes are proposed for experiments using ThO to search for the electric dipole moment of the electron.

High-order-harmonic generation (HHG) spectra have been calculated for ${{\mathrm{H}}_2}^{+}$ molecules aligned parallel to the polarization of the laser field. We make use of the Jacobi coordinates and neglect the rotation of the nuclei. The remaining time-dependent Schrödinger equation is three dimensional in spatial coordinates, one of them being the internuclear separation and the other two describing the electronic motion. The problem is solved using the accurate and efficient time-dependent generalized pseudospectral method in prolate spheroidal coordinates for the electronic coordinates and Fourier grid method for the internuclear separation. Laser pulses with a carrier wavelength of 800 nm, a duration of ten optical cycles, and a peak intensity of $2\times {10}^{14}$ ${\text{W/cm}}^2$ have been used in the calculations. Our HHG spectra, which incorporate the effect of nuclear vibration, generally exhibit a significant deviation from those calculated for the fixed internuclear separations. The low-energy regions of the spectra, however, resemble those for the nuclei fixed at larger separations, while the high-energy regions are closer to those for the nuclei fixed at smaller internuclear distances. The dynamics of the nuclear vibrational wave packet is also obtained and analyzed.

We introduce a one-dimensional spatially inhomogeneous Bose-Hubbard model (BHM) with the strength of the onsite repulsive interactions growing, with the discrete coordinate $z_j$, as $|z_j{|}^{\alpha }$ with $\alpha >0$. Recently, the analysis of the mean-field (MF) counterpart of this system has demonstrated self-trapping of robust unstaggered discrete solitons, under the condition $\alpha >1$. By using the numerically implemented method of the density matrix renormalization group, we demonstrate that, in a certain range of the interaction, the BHM also features self-trapping of the ground state into a soliton-like configuration, at $\alpha >1$, and remains weakly localized at $\alpha <1$. An essential quantum feature found in the BHM is a residual quasi-constant density of the background surrounding the soliton-like peak in the ground state, while in the MF limit the finite-density background is absent. Very strong onsite repulsion eventually destroys soliton-like states, driving the system, at integer densities, into the Mott phase with a spatially uniform density.

We show that the relativistic signatures on the transition probability of atoms moving through optical cavities are very sensitive to their spatial trajectory. This allows for the use of internal atomic degrees of freedom to measure small time-dependent perturbations in the proper acceleration of an atomic probe, or in the relative alignment of a beam of atoms and a cavity.