We observe large density modulations in time-of-flight images of elongated Bose-Einstein condensates, initially confined in a harmonic trap and in the presence of weak disorder. The development of these modulations during the time of flight and their dependence with the disorder are investigated. We render an account of this effect using numerical and analytical calculations. We conclude that the observed large density modulations originate from the weak initial density modulations induced by the disorder and not from initial phase fluctuations (thermal or quantum).

We have studied the effects of a disordered optical potential on the transport and phase coherence of a Bose-Einstein condensate (BEC) of ${}^7\text{L}\text{i}$ atoms. At moderate disorder strengths $\left(V_D\right)$, we observe inhibited transport and damping of dipole excitations, while in time-of-flight images, random but reproducible interference patterns are observed. In situ images reveal that the appearance of interference is correlated with density modulation, without complete fragmentation. At higher $V_D$, the interference contrast diminishes as the BEC fragments into multiple pieces with little phase coherence.

Slow light, produced in a medium with two widely spaced absorption resonances, is analytically studied. We show that a pulse with frequency tuned in the middle of such a doublet structure slows down due to its energy storage in the excited atomic states. If the lifetime of the atomic excitation is much longer than the pulse duration, the pulse shape is nicely reproduced at the output of a thick sample for pulses with smooth envelopes. Pulses with sharp edges are fairly reproduced at the output. Each abrupt change of the pulse amplitude is followed by transients, which are delayed together with the pulse. A possibility to slow down a single photon is also considered.

We show that the relativistic hydrogen atom possesses an SO(4) symmetry by introducing a kind of pseudospin vector operator. The same SO(4) symmetry is still preserved in the relativistic quantum system in presence of a U(1) monopolar vector potential as well as a non-Abelian vector potential. Lamb shift and SO(4) symmetry breaking are also discussed.

Above $54.4\mathrm{eV}$, two-photon double ionization of helium is dominated by a sequential absorption process, producing characteristic behavior in the single and triple differential cross sections. We show that the signature of this process is visible in the nuclear recoil cross section, integrated over all energy sharings of the ejected electrons, even below the threshold for the sequential process. Since nuclear recoil momentum imaging does not require coincident photoelectron measurement, the predicted images present a viable target for future experiments with new short-pulse vacuum ultraviolet (vuv) and soft x-ray sources.

We present a rigorous study of vibrational relaxation in $\text{para}\text{−}{\mathrm{H}}_2+\text{para}\text{−}{\mathrm{H}}_2$ collisions at cold and ultracold temperatures and identify an efficient mechanism of rovibrational energy transfer. If the colliding molecules are in different rotational and vibrational levels, the internal energy may be transferred between the molecules through an extremely state-selective process involving simultaneous conservation of internal energy and total rotational angular momentum. The same transition in collisions of distinguishable molecules corresponds to the rotational energy transfer from one vibrational state of the colliding molecules to another.

Sensitivity coefficients to temporal variation of the fine-structure constant $\alpha$ for transitions between the fine-structure (FS) sublevels of the ground states of C I, Si I, S I, Ti I, Fe I, N II, Fe II, O III, S III, Ar III, Fe III, Mg V, Ca V, Na VI, Fe VI, Mg VII, Si VII, Ca VII, Fe VII, and Si IX are calculated. These transitions lie in the mid- and far-infrared regions and can be observed in spectra of high-redshift quasars and infrared bright galaxies with active galactic nuclei. Using FS transitions to study $\alpha$ variation over cosmological time scales allows one to improve the limit on $\Delta \alpha /\alpha$ by several times as compared to contemporaneous optical observations $\left(\left|\Delta \alpha /\alpha \right|<{10}^{-5}\right)$, and to suppress considerably systematic errors of the radial velocity measurements caused by the Doppler noise. Moreover, the far infrared lines can be observed at redshifts $z\gtrsim 10$, far beyond the range accessible to optical observations $\left(z\lesssim 4\right)$. We have derived a simple analytical expression which relates the FS intervals and the sensitivity of the FS transitions to the change of $\alpha$.

We present a class of noisy $N$-partite nonlocal boxes which reduces all communication complexity problems of $N$ parties to triviality. The noise level is constant for any number of parties and gives a probability of simulating the nonlocal box only slightly higher than that of quantum mechanics. Intriguingly, this class of multipartite nonlocal boxes corresponds to the Bell-Svetlichny inequality, which manifests genuine multipartite nonseparability. These results provide further support for the recent conjecture by Brassard et al., [Phys. Rev. Lett. 96, 250401 (2006)] that had nature been more nonlocal than quantum mechanics allows, communication complexity would have been trivial.

We study the process by which quantum correlations are created when an interaction Hamiltonian is repeatedly applied to a system of two harmonic oscillators for some characteristic time interval. We show that, for the case where the oscillator frequencies are equal, the initial Maxwell-Boltzmann distributions of the uncoupled parts evolve to a new Maxwell-Boltzmann distribution through a series of transient Maxwell-Boltzmann distributions, or quasistationary, nonequilibrium states. Further, we discuss why the equilibrium reached when the two oscillator frequencies are unequal is not a thermal one. All the calculations are exact and the results are obtained through an iterative process, without using perturbation theory.

Security of coherent-state quantum key distribution with homodyne or heterodyne detection through phase-insensitive amplifying channel is analyzed. Contrary to a lossy channel, the security against collective attacks is guaranteed for any ideal amplification in the channel if the direct reconciliation is applied. If there is excess noise during amplification, the threshold on the security against the collective attacks is determined. Opposite to the lossy channel, the most robust setup against the excess noise is a coherent-state key distribution with homodyne detection and direct reconciliation.

We discuss cryptographic applications of single-qubit rotations from the perspective of trapdoor one-way functions and public-key encryption. In particular, we present an asymmetric cryptosystem whose security relies on fundamental principles of quantum physics. A quantum public key is used for the encryption of messages while decryption is possible by means of a classical private key only. The trapdoor one-way function underlying the proposed cryptosystem maps integer numbers to quantum states of a qubit and its inversion can be infeasible by virtue of the Holevo’s theorem.

Spectra of $L$ x rays emitted from thick solid targets containing elements with atomic numbers ranging from 49 (In) to 67 (Ho) were measured in high resolution using a curved crystal spectrometer. The projectiles included heavy ions (Ne, Ar, Kr, and Xe) ranging in energy from 6 to $15\mathrm{MeV}∕\mathrm{amu}$. A scaling rule was established for the apparent average fraction of $M$ vacancies at the time of $L$ x-ray emission and the result was compared to one obtained previously for the apparent average fraction of $L$ vacancies at the time of $K$ x-ray emission. The apparent degree of $L$-shell ionization at the time of $L$ x-ray emission was examined as a function of target atomic number (for $6\mathrm{MeV}∕\mathrm{amu}$ Kr projectiles), projectile atomic number (for an ${}_{51}\mathrm{Sb}$ target), and projectile energy.

We report results of second-order Born-approximation (SBA) calculations for electron-impact double ionization of He at large momentum transfer. Comparisons are made with symmetric noncoplanar $\left(e,3-1e\right)$ experiments at impact energies of 2080 and 4260 eV [N. Watanabe et al., Phys. Rev. A 72, 032705 (2005); Phys. Rev. A 75, 052701 (2007)]. The SBA calculations clearly show that second-order two-step mechanisms play crucial roles in double ionization of He under the kinematical conditions studied. Furthermore, contributions of the two-step mechanisms are found to be more prominent with the increase in the slow ejected electron energy.

We analyze the process of above-threshold ionization (ATI) in light diatomic molecules exposed to an intense linearly polarized femtosecond laser pulse. We concentrate on the problem of extracting high-accuracy dynamic information about the molecular structure from the electron spectra. The results of the frozen-nuclei and moving-nuclei simulations are presented. For a molecule with fixed nuclei, we obtained the corrections to the formula for the interference minima in the electron angular distributions, which improve substantially the determination of the internuclear distance, as confirmed by the comparison with the results of numerical solution of the time-dependent Schrödinger equation (TDSE). To study ATI in molecules with moving nuclei, we used a semiclassical approach in which one solves numerically TDSE for the electron wave function while the nuclear motion is described classically. The influence of a nuclear motion on the electron diffraction images is discussed for the cases of long and few-cycle laser pulses.

Quantitative investigations are made into the quantum enhancements achieved in laser-assisted photoionizations for attosecond extreme-ultraviolet (xuv) pulses with different chip properties. An improved pulse reconstruction scheme is presented based on the analysis of two photoelectron energy spectra (PESs) measured with different laser intensities. A direct procedure is developed to completely reconstruct the intensity and chirp of an isolated pulse (within one-half the laser period) with high precision. Results show root-mean-square frequency (time) differences less than 0.8 eV (2 as, except for a few data points near the start and/or end of the pulse). A stereo measurement (recording PESs from two opposite directions along the direction of laser polarization) is needed to reconstruct a broad bandwidth xuv pulse with a nonmonotonous instantaneous frequency.

We discuss a general physical mechanism for arbitrary control of the quantum states of multiple qubits in the symmetric Dicke subspace. The qubit-qubit coupling leads to unequal energy spacing in the symmetric Dicke subspace. This allows one to manipulate a prechosen transition with an external driving source, with other transitions remaining off-resonant. Any entangled state in the symmetric Dicke subspace can be created from the initial ground state by tuning the driving source. We illustrate the idea in cavity QED, but it should be applicable to other related systems.

Quantum light depolarization is handled through a master equation obtained by coupling dispersively the field to a randomly distributed atomic reservoir. This master equation is solved by transforming it into a quasiprobability distribution in phase space and the quasiclassical limit is investigated.

An optical implementation of the recently proposed unambiguous identification of coherent states is presented. Our system works as a programmable discriminator between two, in general nonorthogonal, weak coherent states. The principle of operation lies in the interference of three light beams–two program states and one unknown coherent state which can be equal to either one of the two program states. The experiment is based on fiber optics. Its results confirm theoretical predictions, and the experimental setup can be straightforwardly extended for higher numbers of program states.

We have investigated the supercontinuum spectra generated in an irregularly microstructured elliptic core fiber, a fiber with an elliptic core surrounded by irregularly arranged air holes. The supercontinuum spectra are strongly dependent on the polarization direction of the linearly polarized input pulse because of the large ellipticity of the core caused by the irregularity of air holes. From the simulation results obtained by solving numerically the generalized nonlinear Schrödinger equation with the adaptive split-step Fourier method, we show that the supercontinuum spectra can be analyzed as the sum of two supercontinuum spectra for the respective fundamental modes of the elliptic core. The polarization-dependent supercontinuum spectra could be useful in the implementation of a tunable broadband source.