A laser-driven Bose-Einstein condensate of ultracold atoms loaded into a lossy high-finesse optical resonator exhibits critical behavior and, in the thermodynamic limit, a phase transition between stationary states of different symmetries. The system realizes an open-system variant of the celebrated Dicke model. We study the transition for a finite number of atoms by means of a Hartree-Fock-Bogoliubov method adapted to a damped-driven open system. The finite-size scaling exponents are determined and a clear distinction between the nonequilibrium and equilibrium quantum criticality is found.

When a light pulse is reflected from a mirror, energy and momentum are exchanged between the electromagnetic field and the material medium. The resulting change in the energy of the reflected photons is directly related to their Doppler shifts arising from the change in the state of motion of the mirror. Similarly, the Doppler shift of photons that enter an absorber is intimately tied to the kinetic energy and momentum acquired by the absorber in its interaction with the incident light. The argument from the Doppler shift yields expressions for the exchanged energy and momentum that are identical with those obtained from Maxwell's equations and the Lorentz law of force, despite the fact that the physical bases of the two methods are fundamentally different. Here, we apply the Doppler-shift argument to a submerged partial reflector (one that absorbs a fraction of the incident light), deducing, in the process, the magnitude of the photon momentum within the submerging medium. We also discuss the case of the submerging medium having a negative refractive index and show the absence of the so-called “reversed” Doppler shift when the reflector is detached from the negative-index medium.

The nonlinear dynamics of intense truncated Airy beams in Kerr ionizing media are investigated from numerical simulations and experiments. We show numerically that a competition between the linear and nonlinear effects takes place and may be modified by tuning the width of the main lobe of the Airy beam and the size of the truncating diaphragm. Our analysis shows that the acceleration of the Airy peak, an inherent feature of linear Airy beam propagation, is preserved only for powers in the main Airy lobe below a certain threshold. Nonlinear propagation of intense Airy beams with low power in the main lobe is sustained by a continuous energy flux from its neighbors, similarly to the mechanism sustaining nonlinear Bessel beam propagation. Airy beams with higher powers in the main lobe are reshaped into a multifilamentary pattern induced by Kerr and multiphoton nonlinearities. The nucleation of new filaments and their interaction affect the acceleration of the main Airy lobes. We finally show that the size of the truncation constitutes a control parameter for the energy flux that features the Airy beam acceleration. Experiments performed in water corroborate the existence of these two distinct nonlinear propagation regimes.

High-order-harmonic-generation yield is remarkably sensitive to driving laser ellipticity, which is interesting from a fundamental point of view as well as for applications. The most well-known example is the generation of isolated attosecond pulses via polarization gating. We develop an intuitive semiclassical model that makes use of the recently measured initial transverse momentum of tunneling ionization. The model is able to predict the dependence of the high-order-harmonic yield on driving laser ellipticity and is in good agreement with experimental results and predictions from a numerically solved time-dependent Schrödinger equation.

In this paper we define the quantum version of the Tsallis entropy in terms of quantum phase space distribution functions. The quantum Tsallis entropy is compared with Kenfack's nonclassicality indicator, for different systems, such as the Schrödinger cat state, the thermal state, a superposition of the ground and the first excited number states, and the harmonic oscillator state. These comparisons indicate that the Wigner representation gives us complete information about the state with the nonextensivity parameter $q=1$, while the Husimi representation hides some information with the nonextensivity parameter $q<1$.

Knowledge of optimal quantum measurements is important for a wide range of situations, including quantum communication and quantum metrology. Quantum measurements are usually optimized with an ideal experimental realization in mind. Real devices and detectors are, however, imperfect. This has to be taken into account when optimizing quantum measurements. In this paper, we derive the optimal minimum-cost and minimum-error measurements for a general model of imperfect detection.

The study and control of coherence in quantum systems is one of the most exciting recent developments in physics. Quantum coherence plays a crucial role in emerging quantum technologies as well as fundamental experiments. A major obstacle to the utilization of quantum effects is decoherence, primarily in the form of dephasing that destroys quantum coherence, and leads to effective classical behavior. We show that there are universal relationships governing dephasing that constrain the relative rates at which quantum correlations can disappear. These lead effectively to speed limits which become especially important in multipartite systems.

We derive quantitative relations among several naturally defined measures of classical and nonclassical correlations in a bipartite quantum state. We also obtain an upper bound of entanglement irreversibility and a sufficient condition for reversible entanglement. The additivity of entanglement of formation is directly related to the additivity of quantum discord as well as a certain measure of classical correlation.

We propose a hybrid quantum repeater protocol combining the advantages of continuous and discrete variables. The repeater is based on the previous work of Brask et al. [Phys. Rev. Lett. 105, 160501 (2010)] but we present two ways of improving this protocol. In the previous protocol entangled single-photon states are produced and grown into superpositions of coherent states, known as two-mode cat states. The entanglement is then distributed using homodyne detection. To improve the protocol, we replace the time-consuming nonlocal growth of cat states with local growth of single-mode cat states, eliminating the need for classical communication during growth. Entanglement is generated in subsequent connection processes. Furthermore the growth procedure is optimized. We review the main elements of the original protocol and present the two modifications. Finally the two protocols are compared and the modified protocol is shown to perform significantly better than the original protocol.

The density-based action-integral functional introduced by Runge and Gross [Phys. Rev. Lett. 52, 997 (1984)] in their foundation of time-dependent density-functional theory is reexamined. Based on a simple expansion of the original definition, it becomes apparent that the action-integral functional is a trivial construct and, moreover, nonstationary. It cannot be used to establish equations of motion for the time evolution of quantum systems at the density-function level.

The thermal correction to the energy of the Casimir-Polder interaction of atoms with a suspended graphene membrane described by the Dirac model is investigated. We show that a major impact on the thermal correction is made by the size of the gap in the energy spectrum of graphene quasiparticles. Specifically, if the temperature is much lower than the gap parameter (alternatively, higher or on the order of the gap parameter), the thermal correction is shown to be relatively small (alternatively, large). We have calculated the free energy of the thermal Casimir-Polder interaction of atoms of ${\mathrm{He}}^{*},\mathrm{Na},\mathrm{Rb}$, and Cs with graphene described by both the hydrodynamic and the Dirac models. It is shown that, in exact computations using the Dirac model, one should use the polarization operator at nonzero temperatures. The computational results for the Casimir-Polder free energy obtained in the framework of the hydrodynamic model of graphene are several times larger than in the Dirac model within the separation region below $2\mu \mathrm{m}$. We conclude that the theoretical predictions following from the two models can be reliably discriminated in experiments on quantum reflection of different atoms on graphene.

Whether the Efimov effect is possible, in principle, for $p$-wave or higher partial-wave interactions is a fundamental question. Recently, there has been a claim that three nonrelativistic particles with resonant $p$-wave interactions exhibit the Efimov effect. We point out that the assumed $p$-wave scattering amplitude inevitably causes a negative probability. This indicates that the Efimov states found there cannot be realized in physical situations. We also restate our previous argument that the Efimov effect, defined as an infinite tower of universal bound states characterized by discrete scale invariance, is impossible for $p$-wave or higher partial-wave interactions.

Resonant $p$-wave interactions can be described by a minimal zero-range model defined by a truncated effective range expansion so that the only two-body interaction parameters are the inverse scattering volume $1/a_p$ and the $p$-wave effective range $r_p$. This minimal model can be formulated as a local quantum field theory with a $p$-wave interaction between atom fields and a molecular field. In the two-atom sector, the model is renormalizable, but it has unphysical behavior at high energies because there are negative-probability states with momentum scale $r_p$. In the sector with three atoms, two of which are identical, renormalization in some parity and angular momentum channels involves an ultraviolet limit cycle, indicating asymptotic discrete scale invariance. The Efimov effect occurs in the unitary limit $a_p^{-1/3},r_p\to 0$, but this limit is unphysical because there are low-energy states with negative probability. The minimal model can be of physical relevance only at energies small compared to the energy scale set by $r_p$ where the effects of negative-probability states are suppressed.

We investigate the electron-positron pair creation process in a supercritical static electric field in the presence of a static magnetic field that is perpendicular. If both fields vary spatially in one direction the dynamics can be reduced to a set of one-dimensional systems. Using a generalized computational quantum field theoretical procedure, we calculate the time dependence of the spatial density for the created electrons. In the presence of the magnetic field, a significant amount of suppression of pair creation is observed in the simulations and confirmed by an analytical analysis for the limits of short-range fields and long interaction times. This suppression might be interpreted in terms of Pauli blocking by the electron during its return to the creation region as it performs a cyclotronlike motion in the magnetic field.

We study the dynamics of three-level $\Lambda$-type atoms driven by a coherent train of short, nonoverlapping laser pulses. We derive analytical nonperturbative expressions for the density matrix by approximating the pulses by a $\delta$ function. We demonstrate that, depending on the train parameters, several scenarios of system dynamics can be realized. We show the possibility of driving Raman transitions between the two ground states of the $\Lambda$ system, avoiding populating excited states by using pulses with effective area equal to $2\pi$. The number of $2\pi$ pulses needed to transfer the entire population from one ground state to another depends on the ratio between the Rabi frequencies of two allowed transitions. In the case of equal Rabi frequencies, the system can be transferred from one ground state to another with a single $2\pi$ pulse. When the total pulse area differs from $2\pi$ and the two-photon resonance condition is fulfilled, the system evolves into a “dark” state and becomes transparent to subsequent pulses. The third possible scenario is the quasi-steady-state regime when neither the total single-pulse area is equal to $2\pi$ nor is the two-photon resonance condition fulfilled. In this regime the radiative-decay-induced drop in the population following a given pulse is fully restored by the subsequent pulse. We derive an analytical expression for the density matrix in the quasi-steady-state regime. We analyze the dependence of the postpulse excited-state population in the quasi-steady-state regime on the train parameters. We find the optimal values for the train parameters corresponding to the maximum of the excited-state population. The maximum of the excited-state population in the steady-state regime is reached at an effective single-pulse area equal to $\pi$ and is equal to $2/3$ in the limiting case when its radiative lifetime is much shorter than the pulse repetition period.

We investigate the feasible limits for realizing a continuously evaporated atom laser with high-temperature sources. A plausible scheme for realizing a truly continuous atom laser is to outcouple atoms from a partially condensed Bose gas while continuously reloading the system with noncondensed thermal atoms and performing evaporative cooling. Here we use quantum kinetic theory to model this system and estimate feasible limits for the operation of such a scheme. For sufficiently high temperatures, the figure of merit for the source is shown to be the phase-space flux. The dominant process limiting the usage of sources with low phase-space flux is the three-body loss of the condensed gas. We conclude that certain double-magneto-optical trap sources may produce substantial mean condensate numbers through continuous evaporation and provide an atom laser source with a narrow linewidth and reasonable flux.

We report experimental generation of an azimuthally polarized (AP) beam with variable spatial coherence. The effect of spatial coherence on the propagation properties of an AP beam is studied both numerically and experimentally, and our experimental results agree well with the theoretical predictions. The dependence of the intensity distribution of an AP beam focused by a high numerical aperture objective lens on its initial spatial coherence is illustrated numerically, and it is found that we can shape the beam profile of a tightly focused AP beam by varying its initial spatial coherence. Furthermore, the radiation forces on Rayleigh particles induced by a tightly focused AP beam are studied, and we find that the tightly focused AP beam can be used to trap a Rayleigh particle whose refractive index is larger or smaller than that of the ambient by varying its initial spatial coherence. Our results will be useful for particle trapping and material thermal processing.

We have studied experimentally pressure broadening and shift of the Ag $D_1$ line caused by He, Ar, and N${}_2$ buffer gases. The measurements were done in a heat-pipe-type absorption cell at a temperature of $\approx$1000 K and gas pressures up to 1000 torr. The measured values for pressure broadening and shift (in MHz/torr) are as follows: Ag-He 5.8(1), +1.16(2); Ag-Ar 5.2(2), $-2.28\left(4\right)$; Ag-N${}_2$ 5.2(1), $-2.52\left(7\right)$. The plus and minus signs indicate the direction for the shifts towards higher and lower frequencies, respectively.

A “bucket brigade” architecture for a quantum random memory of $N=2^n$ memory cells needs $n(n+5)/2$ times of quantum manipulation on control circuit nodes per memory call. Here we propose a scheme in which only on average $n/2$ times manipulations are required to accomplish a memory call. This scheme may significantly decrease the time spent on a memory call and the average overall error rate per memory call. A physical implementation scheme is discussed for storing an arbitrary state in a selected memory cell followed by reading it out.