We solve the master equation of a two-qubit system with the induced unitary term and the dissipation term coming from the system-environment coupling. We explain under this situation how entanglement between two qubits can be generated and persist at the asymptotic time.

We demonstrate time-dependent polarization transfer from molecular rotation to nuclear spin. The ${\mathrm{H}}^{35}\mathrm{Cl}$ $(v=2,J=1,M=1)$ state is excited with a $1.7\mu \mathrm{m}$ laser pulse, and then dissociated with a delayed $235\mathrm{nm}$ laser pulse to produce ${}^{35}\mathrm{Cl}$ atoms. Time-dependent polarizations of both $\mathrm{H}{}^{35}\mathrm{Cl}$ $(v=2,J=1)$ molecules and ${}^{35}\mathrm{Cl}\left({}^2\mathrm{P}_{3∕2}\right)$ atoms, which vary due to hyperfine quantum beating, are measured. The ${}^{35}\mathrm{Cl}$ nuclear spin is highly polarized $(⟨M_{\mathrm{Cl}}⟩\approx 1.1)$ at a pump-probe delay of $145\mathrm{ns}$. Densities surpassing ${10}^{14}{\mathrm{cm}}^{-3}$ are achieved. The technique is applicable to many atoms not amenable to optical pumping.

The generalized gradient approximation based on density functional theory is used to study which effects are brought by the dichlophenyl group ${\mathrm{C}}_6{\mathrm{H}}_3{\mathrm{Cl}}_2$ on the geometric structure, electronic properties, and static linear polarizability of $\mathrm{La}@{\mathrm{C}}_{74}$. It is found that the most favorable endohedral site of a La atom in $\mathrm{La}@{\mathrm{C}}_{74}$, similar to the cases of $\mathrm{Ca}@{\mathrm{C}}_{74}$ and $\mathrm{Eu}@{\mathrm{C}}_{74}$, is off-center under a [6, 6] double bond along the $C_2$ axis on the ${\sigma }_h$ plane, yielding a structure marked as $\mathrm{La}@{\mathrm{C}}_{74}\text{−}2$. It is interesting that the $\mathrm{La}@{\mathrm{C}}_{74}$ molecule has $1{\mu }_B$ magnetic moment, while $\mathrm{La}@{\mathrm{C}}_{74}\left({\mathrm{C}}_6{\mathrm{H}}_3{\mathrm{Cl}}_2\right)$ has a closed-shell electronic structure. With respect to the static linear polarizability, $\mathrm{La}@{\mathrm{C}}_{74}\text{−}2$ has a nonzero value only in the positive $z$ direction $679.6{\mathrm{\AA }}^3$ and shows a giant anisotropic polarizability due to its low $C_{2v}$-symmetric structure in contrast to the isotropic polarizability of ${\mathrm{C}}_{60}$ with ${\mathrm{I}}_h$-symmetric structure as well as the transference of about three electrons from the La atom to the carbon cage. However, the three components along the $x$, $y$, and $z$ directions for $\mathrm{La}@{\mathrm{C}}_{74}\left({\mathrm{C}}_6{\mathrm{H}}_3{\mathrm{Cl}}_2\right)$ are, respectively, 998.7, 821.4, and $710.3{\mathrm{\AA }}^3$ with the mean value $843.5{\mathrm{\AA }}^3$, much larger than that of $\mathrm{La}@{\mathrm{C}}_{74}\text{−}2$. The static linear polarizability anisotropy of $\mathrm{La}@{\mathrm{C}}_{74}\left({\mathrm{C}}_6{\mathrm{H}}_3{\mathrm{Cl}}_2\right)$ $251.9{\mathrm{\AA }}^3$ is much smaller than that of $\mathrm{La}@{\mathrm{C}}_{74}$ $679.6{\mathrm{\AA }}^3$ because the inclined dichlorophenyl group completely destroys the $C_{2v}$ symmetry of $\mathrm{La}@{\mathrm{C}}_{74}\text{−}2$.

We demonstrate an experimental approach to read and write populations and relative phases of vibrational eigenstates within a wave packet created in the $B$ state of the iodine molecule by using a pair of phase-locked femtosecond laser pulses. Our highly stabilized optical interferometer provides attosecond stability and resolution in the interpulse delay. The stability and resolution have realized an exquisite tuning of the interference of two vibrational wave packets to manipulate the relative populations and the relative quantum phases among the vibrational eigenstates within the wave packets. These populations and phases have been retrieved by measuring fluorescence from the upper $E$ state induced by another nanosecond (ns) or femtosecond (fs) probe laser pulse. The bandwidth of the ns probe pulse is narrow enough to select only a small portion of the rotational progression of a particular vibrational band of the $E-B$ transition. By scanning the probe wavelength, we measure the population distribution of the vibrational eigenstates within the wave packet. The fs probe pulse is used to measure quantum beats arising from the temporal evolution of the wave packet. Combining these two complementary measurements, we can read both population and phase information written and stored in the wave packet.

The bright soliton solutions of the mixed coupled nonlinear Schrödinger equations with two components (2-CNLS) with linear self- and cross-coupling terms have been obtained by identifying a transformation that transforms the corresponding equation to the integrable mixed 2-CNLS equations. The study on the collision dynamics of bright solitons shows that there exists periodic energy switching, due to the coupling terms. This periodic energy switching can be controlled by the new type of shape changing collisions of bright solitons arising in a mixed 2-CNLS system, characterized by intensity redistribution, amplitude dependent phase shift, and relative separation distance. We also point out that this system exhibits large periodic intensity switching even with very small linear self-coupling strengths.

We show that an absolute coherent phase of a laser can be used to manipulate the entanglement of photon pairs in two-photon laser. We present simple physics behind a general master equation for two-photon laser. Our focus is on the generation of a continuous source of macroscopically entangled photon pairs in the double $\Lambda$ (or Raman) scheme. We show how the steady-state photon numbers and entanglement depend on the laser parameters, especially the phase. We obtain a relationship between entanglement and two-photon correlation. We derive conditions that give steady-state entanglement for the spontaneous Raman-electromagnetic-induced transparency scheme and use it to identify the region with macroscopic entanglement. No entanglement is found for the double resonant Raman scheme.

We discuss the correlation properties of current-carrying states of one-dimensional insulators, which could be realized by applying an impulse to atoms loaded onto an optical lattice. While the equilibrium noise has a gapped spectrum, the quantum uncertainty encoded in the amplitudes for the Zener process gives a zero-frequency contribution out of equilibrium. We derive a general expression for the generating function of the full counting statistics and find that the particle transport obeys binomial statistics with doubled charge, resulting in super-Poissonian noise that originates from the coherent creation of particle-hole pairs.

We perform a variational quantum Monte Carlo simulation of an interacting Fermi gas confined in a three dimensional harmonic potential. This gas is considered as the precursor system from which a molecular bosonic gas is formed. Based on the results of two-body calculations for trapped atoms, we propose a family of variational many-body wave functions that takes into account the qualitative different nature of the Bardeen-Cooper-Schieffer and Bose-Einstein condensate regimes as a function of the scattering length. Energies, densities, and correlation functions are calculated and compared with previous results for homogeneous gases. Universality tests at the unitarity limit are performed including the verification of the virial relation and the evaluation of the $\beta$ parameter.

A general relation between the relativistic dwell time and Larmor time is derived by a time-dependent method. It is explicitly shown that the relativistic dwell time is equal to the Larmor time in an infinitesimal magnetic field.

We strengthen the set of Bell-type inequalities presented by Sun and Fei [Phys. Rev. A 74, 032335 (2006)] that give a classification for biseparable correlations and entanglement in tripartite quantum systems. We will furthermore consider the restriction to local orthogonal spin observables and show that this strengthens all previously known such tripartite inequalities. The quadratic inequalities we find indicate a type of monogamy of maximal biseparable tripartite quantum correlations, although the nonmaximal ones can be shared. This is contrasted to recently found monogamy inequalities for bipartite Bell correlations in tripartite systems.

Electron-positron pair creation due to combined nuclear Coulomb and strong laser fields is investigated for the tunneling regime. The energy spectra and angular distributions of the pair are found analytically. The energy spectrum for each lepton exhibits a sharp maximum located well above the threshold for any polarization of the laser field. The angular distributions of leptons depend on the polarization: for linear polarization, both leptons move predominantly along the laser beam direction; for circular polarization, leptons are emitted in a thin-walled cone centered on the laser beam. The spectral and angular distributions found are governed by the intensity and frequency of the field, and the frequency-independent total pair creation rates comply with the previously known results. A method of calculation—the vicinal approximation—that uses the fact that the pair production takes place in the close vicinity of the nucleus is suggested.

The odd-parity $J=11∕2$ autoionizing states of europium atom below the $5d$ ${}^{9}D_4$ threshold are investigated by a two-step laser photoionization scheme via the $4f^{7}5d6p$ ${}^{10}F_{9∕2}$ intermediate state. The observed resonances are classified into two autoionizing Rydberg series (with $n=15$ to 55) converging to the $5d$ ${}^{9}D_4$ limit and eight members ($n=13$ to 20) of two autoionizing Rydberg series converging to the higher $5d$ ${}^{9}D_5$ limit. The single series converging to the $5d$ ${}^{9}D_3$ limit is not efficiently excited. The observed four Rydberg series are assigned in $jj$ coupling as $5dnd$ $(4,j)$ and $5dnd$ $(5,j)$ with $j=3∕2$ and $5∕2$. Apart from the $5dnd$ $(5,5∕2)$ series, the majority of autoionizing resonances exhibits narrow linewidths and quasisymmetric line shapes, reflecting this way the weak coupling between the discrete levels and the adjacent continua. The experimental data are analyzed through the semiempirical phase-shifted multichannel quantum defect theory. The theoretically calculated energy level positions and excitation profiles are in good agreement with the experimental ones, thus confirming the identification of the observed structures. The constructed Lu-Fano plot and computed admixture coefficients reveal weak and localized interactions among the Rydberg series members.

We have measured the loading and loss rates in a magneto-optical trap only with a few atoms by directly counting atom-number changing events. Unambiguous formulas are presented for the calculation of those rates from a step-wise time sequence of the fluorescence of the trapped atoms. With a recently developed atom-number feedback technique we could efficiently measure the loading rate as a function of the magnetic field gradient for the initial number of trapped atoms of zero. We could also measure the one- and two-atom loss rates as functions of the trap laser intensity for a precisely prepared initial number of trapped atoms. Each of these rates has been measured independently by directly counting the corresponding atom-number-changing events, without any influence from or inference to the other rates.

We apply quantum filtering and control to a particle in a harmonic trap under continuous position measurement, and show that a simple static feedback law can be used to cool the system. The final steady state is Gaussian and dependent on the feedback strength and coupling between the system and probe. In the limit of weak coupling, this final state becomes the ground state. An earlier model by Haine et al. [Phys. Rev. A 69, 13605 (2004)] without measurement backaction showed dark states: states that did not display error signals, thus remaining unaffected by the control. This paper shows that for a realistic measurement process this is not true, which indicates that a Bose-Einstein condensate may be driven toward the ground state from any arbitrary initial state.

The validity of the few-level approximation in dipole-dipole interacting collective systems is discussed. As an example system, we study the archetype case of two dipole-dipole interacting atoms, each modeled by two complete sets of angular momentum multiplets. We establish the breakdown of the few-level approximation by first proving the intuitive result that the dipole-dipole induced energy shifts between collective two-atom states depend on the length of the vector connecting the atoms, but not on its orientation, if complete and degenerate multiplets are considered. A careful analysis of our findings reveals that the simplification of the atomic level scheme by artificially omitting Zeeman sublevels in a few-level approximation generally leads to incorrect predictions. We find that this breakdown can be traced back to the dipole-dipole coupling of transitions with orthogonal dipole moments. Our interpretation enables us to identify special geometries in which partial few-level approximations to two- or three-level systems are valid.

We propose a general concept to use the spatial information encoded in the time-dependent polarization of high-order harmonic radiation generated by orthogonally polarized two-color laser fields. Based on polarization selective detection, we present two examples of applications: (i) a method for isolating a single attosecond pulse from an attosecond pulse train which is more efficient than the cutoff selection method and (ii) a technique for orbital tomography of molecules with attosecond resolution.

We present a numerical study of high-order harmonic generation in a gas medium with spatial modulation of the density. We show that, by adjusting the modulation period, the contributions of the short and long quantum paths to harmonic emission can be selectively quasi-phase-matched on axis. In particular, quasi-phase-matching of the short path contribution dramatically improves the spatial and spectral properties of the harmonic beam, increasing peak brightness by a factor close to 40 for the 45th harmonic of a Ti:sapphire laser at $17.8\mathrm{nm}$. Finally, simultaneous or selective quasi-phase-matching of the short and long paths makes it possible to control their relative weight.

We explore a dynamic signature of quantum phase transition (QPT) in an isotropic Lipkin-Meshkov-Glick (LMG) model by studying the time evolution of a central qubit coupled to it. We evaluate exactly the time-dependent purity, which can be used to measure quantum coherence, of the central qubit. It is found that distinctly different behaviors of the purity as a function of the parameter reveal clearly the QPT point in the system. It is also clarified that the present model is equivalent to an anti–Jaynes-Cummings model under certain conditions.

We have suppressed multiphoton probability of a heralded-photon source, which is vital for quantum key distribution with a higher secure key generation rate. It is accomplished by utilizing a practical photon-number-resolving detector for triggering resulting in an important step for improved practical quantum key distribution. Heralded-photon source and a practical photon-number-resolving detector capable of real-time processed multiphoton rejection are stably operable at room temperature and enable us to generate a secure key at a distance as long as an ideal single photon source is used.

We present a method for measuring the scalar polarizability of molecules by means of near-field matter-wave interferometry. This technique is applicable to a wide range of complex neutral molecules. A key feature of our experiment is the combination of good transmission and high spatial resolution, gained by imprinting a submicrometer spatial modulation onto a wide molecular beam. Our method significantly improves the measurement sensitivity of the polarizability-dependent shifts. We have successfully performed measurements on fullerenes and report the polarizability to be $\alpha =88.9\pm 0.9\pm 5.1{\mathrm{\AA }}^3$ for ${\mathrm{C}}_{60}$ and $\alpha =108.5\pm 2.0\pm 6.2{\mathrm{\AA }}^3$ for ${\mathrm{C}}_{70}$.