The evolution equation of a weakly interacting boson system is derived. The model includes the interaction between the atoms in the condensate and the surrounding gas of noncondensed particles. The Bogoliubov transformation is introduced in a full quantum context and the scattering kernel between dressed particles and the condensate phase is obtained. The final system is expressed by the Boltzmann evolution equation for noncondensed particles coupled to the Gross-Pitaevskii equation for the condensate. We consider an out-of-equilibrium situation that induces a fast production of condensed particles. We apply our model to study the condensation dynamics of positronium atoms by evaporation.

The dynamics of Bose-Einstein condensate (BEC) is studied at nonzero temperatures using our variational time-dependent-Hartree-Fock-Bogoliubov formalism. We have shown that this approach is an efficient tool to study the expansion and collective excitations of the condensate, the thermal cloud, and the anomalous correlation function at nonzero temperatures. We have found that the condensate and the anomalous density have the same breathing oscillations. We have investigated, on the other hand, the behavior of a single quantized vortex in a harmonically trapped BEC at nonzero temperatures. Generalized expressions for vortex excitations, vortex core size, and Kelvin modes have been derived. An important and somehow surprising result is that the numerical solution of our equations predicts that the vortex core is partially filled by the thermal atoms at nonzero temperatures. We have shown that the effect of thermal fluctuations is important and it may lead to enhancing the size of the vortex core. The behavior of the singly anomalous vortex has also been studied at nonzero temperatures.

Within the combined potential of an optical lattice and a harmonic magnetic trap, it is possible to form matter wave packets by intensity modulation of the lattice. An analysis of the production and motion of these wave packets provides a detailed understanding of the dynamical evolution of the system. The modulation technique also allows for a controllable transfer (deexcitation) of atoms from such wave packets to a state bound by the lattice. Thus, it acts as a beam splitter for matter waves that can selectively address different bands, enabling the preparation of atoms in localized states. The combination of wave packet creation and deexcitation closely resembles the well-known method of pump-probe spectroscopy. Here, we use the deexcitation for spectroscopy of the anharmonicity of the combined potential. Finally, we demonstrate that lattice modulation can be used to excite matter wave packets to even higher momenta, producing fast wave packets with potential applications in precision measurements.

We investigated the transient responses of coupled optical resonators, after they were injected with square modulated temporal pulses. A sharp spike, attributed to the optical precursor in coupled-resonator-induced transparency, appeared when the coupling between the resonators was weak. As the coupling strength increased, the resonance spectrum developed clearly separated double dips of Autler-Townes splitting, and the precursor spike transformed into an oscillatory structure. These temporal oscillations were attributed to the coherent energy exchange between two resonators. Theoretical calculations were in good agreement with the experimental observations.

Single-photon sources represent a fundamental building block for optical implementations of quantum information tasks ranging from basic tests of quantum physics to quantum communication and high-resolution quantum measurement. In this paper, in order to compare the effectiveness of different designs, we introduce a single-photon source performance index, based on the maximum probability of generating a single photon that still guarantees a given signal-to-noise ratio. We then investigate the performance of a multiplexed system based on asymmetric configuration of multiple heralded single-photon sources. The performance and scalability comparison with both currently existing multiple-source architectures and faint laser configurations reveals an advantage the proposed scheme offers in realistic scenarios. This analysis also provides insights on the potential of using such architectures for integrated implementation.

We propose a dissipation-engineering scheme that prepares and protects a maximally entangled state of a pair of superconducting qubits. This is done by off-resonantly coupling the two qubits to a low-$Q$ cavity mode playing the role of a dissipative reservoir. We engineer this coupling by applying six continuous-wave microwave drives with appropriate frequencies. The two qubits need not be identical. We show that our approach does not require any fine-tuning of the parameters and requires only that certain ratios between them be large. With currently achievable coherence times, simulations indicate that a Bell state can be maintained over arbitrary long times with fidelities above $94\%$. Such performance leads to a significant violation of Bell's inequality (Clauser-Horne-Shimony-Holt correlation larger than 2.6) for arbitrary long times.

Mechanical oscillators can be cooled by coupling them to an optical or microwave cavity. Going beyond the standard quantum noise approach, we find an analytic expression for the steady-state phonon number in systems where the position of the mechanical oscillator modulates the cavity frequency as well as the cavity linewidth. We trace the origin for the quantum limit of cooling to fluctuations in the optical force both at and away from the mechanical frequency. Finally, we calculate the minimal phonon number for the different types of couplings. Our study elucidates how to beneficially combine dispersive and dissipative optomechanical couplings.

We investigate the resonant quantum dynamics of a laser-pumped real or artificial two-level single-atom system embedded in a leaking microcavity. We found that for stronger laser-atom-cavity couplings the generated microcavity photons exhibit larger steady-state correlations. In particular, the second- and third-order photon correlation functions are greater than the corresponding ones obtained for an incoherent light source, respectively. Furthermore, the emitted microcavity photon flux is enhanced in comparison to weaker coupling cases.

We analyze the dependence of the transmission profiles of an atom in a tripod configuration on the polarizations of the coupling and the probe beams and use room-temperature metastable helium (${}^4$He*) as a model system. We show that, by rotating the orthogonally polarized coupling-probe beams with respect to an applied small magnetic field, one can manipulate the detuned peaks due to electromagnetically induced transparency [Kumar, Lauprêtre, Ghosh, Bretenaker, and Goldfarb, Phys. Rev. A 84, 023811 (2011)] and the central peak arising because of ground-state coherent population oscillations [Lauprêtre, Kumar, Berger, Faoro, Ghosh, Bretenaker, and Goldfarb, Phys. Rev. A 85, 051805(R) (2012)] observed earlier separately. Our experimental results match well with our numerical simulation using the Floquet method.

Boykin and Roychowdhury [Phys. Rev. A 67, 042317 (2003)] discussed the optimal encryption of quantum bits, gave a necessary and sufficient condition (NASC) for the security of quantum encryption, and suggested an optimal quantum one-time pad encryption scheme. In this Comment, we point out that the proof of the NASC for the security of quantum encryption is not correct. Moreover, we reprove the NASC for the security of quantum encryption.

We show that time induces a dynamical renormalization of the system-environment coupling in open-quantum-system dynamics. The renormalizability condition, of the interactions being either local, or, alternatively, defined on a finite continuum support, is generally fulfilled for both discrete and continuous environments. As a consequence, we find a generalized Lieb-Robinson bound to hold for local and, surprisingly, also for nonlocal interactions. This unified picture allows us to devise a controllable approximation for arbitrary non-Markovian dynamics with an a priori estimate of the worst-case computational cost.

The possibility to explain quantum correlations via (possibly) unknown causal influences propagating gradually and continuously at a finite speed $v>c$ has attracted some attention recently. In particular, it could be shown that this assumption leads to correlations that can be exploited for superluminal communication. This was achieved studying the set of possible correlations that are allowed within such a model and comparing them to correlations produced by local measurements on a four-party entangled quantum state. Here, we report on a quantum state that allows for the same conclusion involving only three parties.

We investigate the structure of bipartite entanglement in one-dimensional spin systems. The standard approach is based on a reduced two-qubit density matrix and uses concurrence as the measure of entanglement. We show that for a spin system consisting of four qubits the structure of bipartite entanglement can be classified by rigged string configurations in a strictly combinatorial manner.

Using the Deutsch model of closed timelike curves, we show that a universal cloner can be constructed that when acting on a completely arbitrary qubit state, exceeds the no-cloning bound for fidelity. Since the no-cloning theorem has played a central role in the development of quantum information science, it is clear that the existence of closed timelike curves that behave according to Deutsch's model would radically change the rules for quantum information technology. Nevertheless, we show that this type of cloning does not violate no-signaling criteria.

The Runge-Gross [E. Runge and E. K. U. Gross, Phys. Rev. Lett. 52, 997 (1984)] action functional of time-dependent density-functional theory leads to a well-known causality paradox; that is, a perturbation of the electronic density in the future affects the response of the system in the present. This paradox is known to be caused by an inconsistent application of the Dirac-Frenkel variational principle. In view of the recent solutions to this problem, the action functional employed by Runge and Gross in their formulation of time-dependent density-functional theory is analyzed in the context of the Keldysh contour technique. The time-dependent electronic density and the concept of causality are extended to the contour. We derive a variational equation that obeys causality and relates the exchange-correlation potential with its kernel and the functional derivative of the exchange-correlation action functional with respect to the density. It is shown that the adiabatic local-density approximation is a consistent solution of this equation and that the time-dependent optimized potential method can also be derived from it. The formalism presented here can be used to find new approximation methods for the exchange-correlation potential and to avoid the causality dilemma.

We report a joint theoretical-experimental investigation of electron collision with dimethyl ether (DME) in the low- and intermediate-energy ranges. Experimental absolute differential, integral, and momentum-transfer cross sections for elastic $e^{-}$-DME scattering are reported in the 100–1000 eV energy range. Our measurements were performed using a crossed electron-beam–molecular-beam geometry. The angular distribution of the scattered electrons was converted to absolute cross section using the relative flow technique. Theoretically, elastic differential, integral, and momentum-transfer cross sections, as well as the grand-total and total absorption cross sections for electron collision with DME are calculated in the 1–1000 eV energy range. A single-center-expansion technique combined with the Padé approximant method is used in our calculations. A comparison between the present experimental and theoretical data shows very good agreement. Moreover, comparison with theoretical and experimental data for $e^{-}$-ethanol (an isomer of DME) scattering shows interesting isomeric effects.

The time-dependent Schrödinger equation for the hydrogen atom and its interaction with coherent intense high-frequency short laser pulses is solved numerically exactly by propagating single-electron wave packets. Thereby, the wave function is followed in space and time for times longer than the pulse duration. Results are explicitly shown for 3 and 10 fs pulses. Particular attention is paid to identifying the effect of dynamic interference of photoelectrons emitted with the same kinetic energy at different times in the rising and falling sides of the pulse as predicted by Demekhin and Cederbaum [Phys. Rev. Lett. 108, 253001 (2012)]. In order to be able to see the dynamic interference pattern in the computed electron spectra, the photoelectron wave packet has to be propagated over long distances. Clearly, the complex absorption potentials often employed to compute the spectra of emitted particles cannot be used to detect dynamic interference. For the considered high-frequency pulses of 3 and 10 fs duration, this requires enormously large spatial grids. The photoionization and above-threshold ionization spectra presently computed are found to exhibit pronounced dynamic interference patterns. The patterns are in very good agreement with previously published results on the photoionization spectra, where available, which were computed using a completely different method, thus supporting the previously made assumption that the above-threshold ionization processes are very weak for the considered pulse intensities and high carrier frequency. The quiver motion in space and time of a free electron in strong laser pulses is also investigated numerically. Finally, a discussion is presented of how fast the atom is ionized by an intense pulse.

A pump-probe laser scheme is employed to investigate orbital alignment and its dynamics in the ground state of laser-generated neutral atoms. The alignment is initiated by electron photodetachment of an atomic negative ion in a strong laser pulse. The electron density distribution in the ground state of the residual atom is probed by means of strong-field ionization in a second laser pulse at a delayed time. The principle of the probe method relies on the fact that the portion of the electron density distribution oriented along the laser polarization axis constitutes the ionization yield in the high-energy jets of emitted electrons. A systematic study is carried out on C, Si, and Ge atoms, which possess two electrons in an open $p$ shell. A pronounced temporal modulation in the yield of high-energy electrons is observed for C and Si, revealing a periodic spatial rearrangement of the electron density distribution in these atoms. Its period is defined by the beat between the $J=1$ and $J=2$ spin-orbit components of the ground state.

We study the effects of induced interactions on the pairing gap, transition temperature, and chemical potential of a quasi-two-dimensional Fermi gas of atoms with spin-orbit coupling. We find that these mean-field parameters are significantly modified when induced interactions are taken into account. We also investigate the implications of induced interaction corrections for the BEC-BCS crossover driven by spin-orbit coupling, which happens even for small (compared to the Fermi energy) values of the binding energy.

We investigate the storage and the nonlinear coupling of two Zeeman coherence gratings in an ensemble of cold cesium atoms. This nonlinear coupling was revealed through the analysis of the total energy that can be extracted from the atomic medium as a function of the relative intensities of the grating writing beams, which presented a minimum when their intensities were made equal. A theoretical model using the density matrix formalism accounts quantitatively well for the observed results and allowed us to identify several channels in which energy can be delivered in the reading process.