This paper explores quantum-coherence phenomena in a semiconductor quantum-dot structure. The calculations predict the occurrence of inversionless gain, electromagnetically induced transparency, and refractive-index enhancement in the transient regime for dephasing rates typical under room temperature and high excitation conditions. They also indicate deviations from atomic systems because of strong many-body effects. Specifically, Coulomb interaction involving states of the quantum dots and the continuum belonging to the surrounding quantum well leads to collision-induced population redistribution and many-body energy and field renormalizations that modify the magnitude, spectral shape, and time dependence of quantum-coherence effects.

Quantum-cloning machine for arbitrary mixed states in symmetric subspaces is proposed. This quantum-cloning machine can be used to copy part of the output state of another quantum-cloning machine and is useful in quantum computation and quantum information. The shrinking factor of this quantum cloning achieves the well-known upper bound. When the input is identical pure states, two different fidelities of this cloning machine are optimal.

We have calculated the recombination yield for swift ${\mathrm{H}}_2^{+}$ molecular ions at the exit of thin amorphous carbon foils, as a function of the dwell time and incident energy. Our results are based on a detailed simulation of the motion through the target of the ${\mathrm{H}}_2^{+}$ molecular ion (before dissociation takes place) and its constituent fragments (after dissociation), including the following effects: Coulomb repulsion, nuclear scattering, electron capture and loss, as well as self-retarding and wake forces, which provide the relative distance and velocity of the dissociated fragments at the foil exit. The recombination of an ${\mathrm{H}}_2^{+}$ ion at the exit of the foil depends on the interproton separation and internal energy of the dissociated fragments, and on their probability to capture an electron. Comparison of our results with the available experimental data shows a good agreement.

Low-momentum excitations of a dilute Bose-Einstein condensate behave as phonons and move at a finite velocity $v_s.$ Yet the atoms making up the phonon excitation each move very slowly; $v_a=p/\overrightarrow{m}0.$ A simple “cartoon picture” is suggested to understand this phenomenon intuitively. It implies a relation $v_s{/v}_a\approx N_{\mathrm{ex}},$ where $N_{\mathrm{ex}}$ is the number of excited atoms making up the phonon. This relation does indeed follow from the standard Bogoliubov theory.

In this paper, collective excitations in a homogeneous fermion-fermion mixture with different Fermi surfaces are studied. In the Fermi liquid phase, the zero-sound velocity is found to be larger than the largest Fermi velocity. With attractive interactions, the superfluid phase appears below a critical temperature, and the phase mode is the low-energy collective excitation. The velocity of the phase mode is significantly different from the zero-sound velocity. The difference between the two velocities may serve as a tool to detect the superfluid phase.

We report an experimental quantum key distribution that utilizes pulsed homodyne detection, instead of photon counting, to detect weak pulses of coherent light. Although our scheme inherently has a finite error rate, homodyne detection allows high-efficiency detection and quantum state measurement of the transmitted light using only conventional devices at room temperature. Our prototype system works at $1.55\mu \mathrm{m}$ wavelength and the quantum channel is a 1-km standard optical fiber. The probability distribution of the measured electric-field amplitude has a Gaussian shape. The effect of experimental imperfections such as optical loss and detector noise can be parametrized by the variance and the mean value of the Gaussian distribution.

Excitation and fluorescence ultraviolet spectra of ${\mathrm{Cd}}_2$ recorded at the ${}^1{0}_u^{+}\left(5\mathrm{}{}^1{P}_1\right)–X{}^1{0}_g^{+}$ transition are reported. The ${\mathrm{Cd}}_2$ molecules (seeded in Ar) produced in a continuous free-jet supersonic beam were excited in a vacuum chamber with a pulsed dye-laser beam. A well-resolved vibrational structure of the ${}^1{0}_u^{+}\leftarrow X{}^1{0}_g^{+}$ excitation spectrum as well as the isotopic structure of the vibrational components were recorded. Analysis of the spectrum yielded vibrational constants for the ${}^1{0}_u^{+}$ state: ${\omega }_e^{\prime }=100.50\mathrm{}\pm 0.25{\mathrm{cm}}^{-1},$ ${\omega }_e^{\prime }{x}_e^{\prime }=0.325\mathrm{}\pm 0.003{\mathrm{cm}}^{-1},$ $D_0^{\prime }=8638\mathrm{}\pm 15{\mathrm{cm}}^{-1},$ $D_e^{\prime }=8688\mathrm{}\pm 15{\mathrm{cm}}^{-1},$ and $\Delta R_e{=R}_e^{″}-R_e^{\prime }=1.04\mathrm{}\pm 0.01\AA$ derived for the ${}^{226}{\mathrm{Cd}}_2$ isotopomer. The ${}^1{0}_u^{+}$ state potential-energy (PE) curve was obtained numerically using an inverse perturbation approach (IPA) procedure. Condon internal diffraction (CID) patterns in the ${}^1{0}_u^{+}\to X{}^1{0}_g^{+}$ fluorescence band, emitted upon the selective excitation of the $v^{\prime }=38\mathrm{}$ and $v^{\prime }=39\mathrm{}$ vibrational components of the ${}^{226}{\mathrm{Cd}}_2$ isotopomer, were observed and improved the $v^{\prime }$ assignments derived from the analysis of the isotopic structure. Analysis of the fluorescence spectrum yielded information on the repulsive part of the ground-state interatomic potential. The result confirms a relatively soft repulsion between two Cd atoms in the short-range (2.53–4.05 Å) region and makes allowance for a covalent admixture to the ground-state van der Waals bonding. Quasirelativistic valence ab initio calculations on the PE curves for the investigated states have been performed at the complete-active-space multiconfiguration self-consistent-field (CASSCF/CAS) multireference second-order perturbation theory (CASPT2) level with the total of 40 correlated electrons. In the calculations, the Cd atom is considered as a 20-valence electron system whereas the ${\mathrm{Cd}}^{20+}$ core is replaced by an energy-consistent pseudopotential which also accounts for scalar-relativistic effects and spin-orbit interaction within the valence shell. A comparison with results obtained from other experiments and ab initio calculations is presented.

The structural features and energetics of ${\mathrm{Zn}}_{n-m}{\mathrm{Cd}}_m$ $\mathrm{}(n=7,8\mathrm{})\mathrm{}$ microclusters and ${\mathrm{Zn}}_{50},$ ${\mathrm{Cd}}_{50},$ and ${\mathrm{Zn}}_{25}{\mathrm{Cd}}_{25}$ nanoparticles have been investigated by performing molecular-dynamics computer simulations using a recently developed empirical many-body potential-energy function. The most stable structures were found to be compact and three-dimensional for all elemental and mixed clusters. An interesting structural feature of the mixed clusters is that Zn and Cd atoms do not mix in mixed clusters; they come together almost without mixing.

The multielectron tunneling ionization probabilities have been calculated for Ar and Kr atoms in circularly polarized laser fields with the pulse durations of 50 fs and 5 fs. The channels of sequential, nonsequential ionization, and ionization with ionic core excitation (inelastic tunneling) have been taken into account. The calculated results demonstrate that many-body effects in tunneling, connected mostly with ionic core excitation, are very important for creation of ${\mathrm{Ar}}^{4+},$ ${\mathrm{Ar}}^{5+},$ ${\mathrm{Kr}}^{4+},$ and ${\mathrm{Kr}}^{5+}$ ions.

We compare the experimental stability diagram of a Fermi-Bose mixture of ${}^{40}\mathrm{K}$ and ${}^{87}\mathrm{Rb}$ atoms with attractive interaction to the predictions of a mean-field theoretical model. We discuss how this comparison can be used to give a better estimate of the interspecies scattering length, which is currently known from collisional measurements with larger uncertainty.

The mutual dipole-dipole interaction of atoms in a trap can affect their fluorescence. Extremely large effects were reported for double jumps between different intensity periods in experiments with two and three ${\mathrm{Ba}}^{+}$ ions for distances in the range of about ten wave lengths of the strong transition while no effects were observed for ${\mathrm{Hg}}^{+}$ at 15 wavelengths. In this theoretical paper we study this question for configurations with three and four levels, which model those of ${\mathrm{Hg}}^{+}$ and ${\mathrm{Ba}}^{+},$ respectively. For two systems in the ${\mathrm{Hg}}^{+}$ configuration we find cooperative effects of up to 30% for distances around one or two wavelengths, about 5% around ten wavelengths, and, for larger distances in agreement with experiments, practically none. This is similar for two V systems. However, for two four-level configurations, which model two ${\mathrm{Ba}}^{+}$ ions, cooperative effects are practically absent, and this latter result is at odds with the experimental findings for ${\mathrm{Ba}}^{+}.$

The calculation of Bethe logarithm for the ground state of the lithium atom is presented. The Bethe logarithm is the main QED effect coming from the electron self-interaction, which has not been obtained yet. Both results for the infinite nuclear mass, $\ln k_0=5.178\mathrm{}17\left(3\right),$ and the mass polarization correction, $\Delta \ln k_0=0.114\mathrm{}\left(3\right),$ significantly improve the hitherto theoretical values for the lithium ground-state energy. They allow from one side to test the theory against precise measurements of transition frequencies and from the other side, to improve the accuracy of determination of the difference in the square of nuclear charge radii from the isotope-shift measurements. The applied calculational method is based on the well adapted explicitly correlated Gaussian basis set and can be extended to other few-electron atoms and molecules.

We investigate the interaction of a two-active electron system $\left({\mathrm{Li}}^{-}\right)$ with intense single-cycle and double half-cycle pulses. The “intensity” and “frequency” considered correspond to the “multiphoton above-barrier regime.” For the single-cycle pulse (SCP), the electric field changes sign once, allowing electron wave packets created during the first half cycle to recollide with the parent ion when driven back by the field. For the double half-cycle pulse (DHP), however, the electric field does not change sign, and electron wave packets created during the first half cycle are not driven back to the parent ion. We find that both single and double ionization are significantly larger for the SCP than for the DHP, thereby elucidating the role of the rescattering mechanism. On the other hand, doubly ionized electrons produced by a half-cycle pulse and a DHP are found to have angular distributions in which one electron is ejected in the direction of the pulse field, and the other in the opposite direction. This clear signature of electron correlations suggests that “shake-off,” “knockout,” and, possibly, “multiphoton-sharing” processes are alternative contributing mechanisms for double ionization in this regime.

We analyze the phenomena of condensate collapse, as described by Donley et al. [Nature 412, 295 (2001)] and N. Claussen [Ph. D thesis, University of Colorado, 2003 (unpublished)] by focusing on the behavior of excitations or fluctuations above the condensate, as driven by the dynamics of the condensate, rather than the dynamics of the condensate alone or the kinetics of the atoms. The dynamics of the condensate squeezes and amplifies the quantum excitations, mixing the positive and negative frequency components of their wave functions thereby creating particles that appear as bursts and jets. By analyzing the changing amplitude and particle content of these excitations, our simple physical picture explains well the overall features of the collapse phenomena and provides excellent quantitative fits with experimental data on several aspects, such as the scaling behavior of the collapse time and the number of particles in the jet. The prediction of the bursts at this level of approximation is less than satisfactory but may be improved by including the backreaction of the excitations on the condensate. The mechanism behind the dominant effect—parametric amplification of vacuum fluctuations and freezing of modes outside of horizon—is similar to that of cosmological particle creation and structure formation in a rapid quench (which is fundamentally different from Hawking radiation in black holes). This shows that Bose-Einstein condensate dynamics is a promising venue for doing “laboratory cosmology.”

We propose a technique of ultra-short $\gamma$-ray pulses production with a pulse duration much less than the natural lifetime of a nucleus excited state. The technique is based on compression of frequency modulated radiation of a Mossbauer source while propagating in a far-detuned resonant absorber. Numerical estimates for ${}^{57}\mathrm{Fe}$ 14.413 keV nuclear resonance energy show feasibility of experimental realization of the technique

The field of linear optical quantum computation (LOQC) will soon need a repertoire of experimental milestones. We make progress in this direction by describing several experiments based on Grover’s algorithm. These experiments range from a relatively simple implementation using only a single nonscalable controlled-NOT (CNOT) gate to the most complex, requiring two concatenated scalable CNOT gates, and thus form a useful set of early milestones for LOQC. We also give a complete description of basic LOQC using polarization-encoded qubits, making use of many simplifications to the original scheme of Knill, Laflamme, and Milburn [E. Knill, R. Laflamme, and G. J. Milburn, Nature (London) 409, 46 (2001)].

We introduce the entangled coherent-state representation, which provides a powerful technique for efficiently and elegantly describing and analyzing quantum optics sources and detectors while respecting the photon-number superselection rule that is satisfied by all known quantum optics experiments. We apply the entangled coherent-state representation to elucidate and resolve the long-standing puzzles of the coherence of a laser output field, interference between two number states, and dichotomous interpretations of quantum teleportation of coherent states.

Motivated by recent development in quantum entanglement, we study relations among concurrence C, ${\mathrm{SU}}_q\mathrm{}\left(2\right)\mathrm{}$ algebra, quantum phase transition and correlation length at the zero temperature for the $\mathrm{XXZ}$ chain. We find that at the SU(2) point, the ground state possesses the maximum concurrence. When the anisotropic parameter $\Delta$ is deformed, however, its value decreases. Its dependence on $\Delta$ scales as ${C=C}_0-C_1(\Delta -{1)}^2$ in the $\mathrm{XY}$ metallic phase and near the critical point (i.e., $1<\mathrm{}\Delta <1.3)$ of the Ising-like insulating phase. We also study the dependence of C on the correlation length $\xi ,$ and show that it satisfies ${C=C}_0-1/2\xi$ near the critical point. For different sizes of the system, we show that there exists a universal scaling function of C with respect to the correlation length $\xi .$

We studied the recollision dynamics between the electrons and ${\mathrm{D}}_2^{+}$ ions following the tunneling ionization of ${\mathrm{D}}_2$ molecules in an intense short pulse laser field. The returning electron collisionally excites the ${\mathrm{D}}_2^{+}$ ion to excited electronic states; from there ${\mathrm{D}}_2^{+}$ can dissociate or be further ionized by the laser field, resulting in ${\mathrm{D}}^{+}+\mathrm{D}$ or ${\mathrm{D}}^{+}+{\mathrm{D}}^{+},$ respectively. We modeled the fragmentation dynamics and calculated the resulting kinetic-energy spectrum of ${\mathrm{D}}^{+}$ to compare with recent experiments. Since the recollision time is locked to the tunneling ionization time which occurs only within a fraction of an optical cycle, the peaks in the ${\mathrm{D}}^{+}$ kinetic-energy spectra provide a measure of the time when the recollision occurs. This collision dynamics forms the basis of the molecular clock where the clock can be read with attosecond precision, as first proposed by Corkum and co-workers. By analyzing each of the elementary processes leading to the fragmentation quantitatively, we identified how the molecular clock is to be read from the measured kinetic-energy spectra of ${\mathrm{D}}^{+}$ and what laser parameters are to be used to measure the clock more accurately.