We report experimental observations of correlated-photon statistics in the single-photon detection rate. The usual quantum interference in a two-photon polarization interferometer always accompanies a dip in the single-detector counting rate, regardless of whether a dip or a peak is seen in the coincidence rate. This effect is explained by taking into account all possible photon number states that reach the detector, rather than considering just the state postselected by the coincidence measurement. We also report an interferometeric scheme in which the interference peak or dip in the coincidence corresponds directly to a peak or a dip in the single-photon detection rate.

We show that classical-quantum correspondence of center-of-mass motion in two coupled $\delta$-kicked rotors is enhanced by the entanglement of the center of-mass motion to the internal degree of freedom. The observed correspondence can be attributed to the decoherence generated from chaotic internal dynamics with a few degrees of freedom.

The quantum limits of stochastic cooling of trapped atoms are studied. The energy subtraction due to the applied feedback is shown to contain an additional noise term due to atom-number fluctuations in the feedback region. This effect is shown to dominate the cooling efficiency near the condensation point. Furthermore, we show results that indicate that Bose-Einstein condensation could be reached via stochastic cooling.

We have used trapped ${}^{85}\mathrm{Rb}$ atoms to demonstrate an atom interferometric measurement of atomic recoil in the frequency domain. The measurement uses echo techniques to generate a Ramsey fringe pattern. The pattern exhibits recoil components consistent with theoretical predictions. We find the measurement to be insensitive to magnetic field gradients and discuss the prospects for a precision measurement of the recoil frequency.

“Decoherence of quantum superpositions through coupling to engineered reservoirs” is the topic of a recent paper by Myatt et al. [Nature 403, 269 (2000)] which has attracted much interest because of its relevance to current research in fundamental quantum theory, quantum computation, teleportation, entanglement, and the quantum-classical interface. However, the preponderance of theoretical work on decoherence does not consider the effect of an external field. Here, we present an analysis of such an effect in the case of the random $\delta$-correlated force discussed by Myatt et al.

We prove a rigorous inequality that estimates the purity of a reduced density matrix of a composite quantum system in terms of cross correlation of the same state and an arbitrary product state. Various immediate applications of our result are proposed, in particular, concerning Gaussian wave-packet propagation under classically regular dynamics.

We examine the validity of the Kraus representation in the presence of initial correlations and show that it is assured only when a joint dynamics is locally unitary.

Intermediate states are known from intercept/resend eavesdropping in the Bennett-Brassard 1984 (BB84) quantum cryptographic protocol. But they also play fundamental roles in the optimal eavesdropping strategy on the BB84 protocol and in the CHSH (Clauser-Horne-Shimony-Holt) inequality. We generalize the intermediate states to an arbitrary dimension and consider intercept/resend eavesdropping, optimal eavesdropping on the generalized BB84 protocol and present a generalized Clauser-Horne-Shimony-Holt inequality for two entangled qudits based on these states.

By means of a simple example, it is demonstrated that the task of finding and identifying certain patterns in an otherwise (macroscopically) unstructured picture (dataset) can be accomplished efficiently by a quantum computer. Employing the powerful tool of the quantum Fourier transform, the proposed quantum algorithm exhibits an exponential speedup in comparison with its classical counterpart.

We examine stochastic maps in the context of quantum optics. Making use of the master equation, the damping basis, and the Bloch picture we calculate a nonunital, completely positive, trace-preserving map with unequal damping eigenvalues. This results in what we call the squeezed vacuum channel. A geometrical picture of the effect of stochastic noise on the set of pure state qubit density operators is provided. Finally, we study the capacity of the squeezed vacuum channel to transmit quantum information and to distribute Einstein-Podolsky-Rosen states.

We study the physical implementation of a qutrit quantum computer in the context of trapped ions. Qutrits are defined in terms of electronic levels of trapped ions. We concentrate our attention on a universal two-qutrit gate, which corresponds to a controlled-NOT gate between qutrits. Using this gate and a general gate of an individual qutrit, any gate can be decomposed into a sequence of these gates. In particular, we show how this works for performing the quantum Fourier transform for n qutrits.

We investigate the asymptotic rates of entanglement transformations for bipartite mixed states by local operations and classical communication (LOCC). We analyze the relations between the rates for different transitions and obtain simple lower and upper bounds for these transitions. In a transition from one mixed state to another and back, the amount of irreversibility can be different for different target states. Thus in a natural way, we get the concept of “amount” of irreversibility in asymptotic manipulations of entanglement. We investigate the behavior of these transformation rates for different target states. We show that with respect to asymptotic transition rates under LOCC, the maximally entangled states do not have a special status. In the process, we obtain that the entanglement of formation is additive for all maximally correlated states. This allows us to show irreversibility in asymptotic entanglement manipulations for maximally correlated states in $2\otimes 2.$ We also show that the possible nonequality of distillable entanglement under LOCC and that under operations preserving the positivity of partial transposition is related to the behavior of the transitions (under LOCC) to separable target states.

We present ab initio calculations of the complete gauge-invariant set of two-photon exchange graphs for the ${\mathrm{}\left(1s\right)\mathrm{}}^2{2p}_{3/2}$ electron configuration in Li-like ions. These calculations are an important step towards the precise theoretical determination of the ${2p}_{3/2}-2s$ transition energy in the framework of QED.

The energies and lifetimes of doubly excited sextet states of boron-like O IV, F V, and Ne VI are calculated with the multiconfiguration Hartree-Fock approach, including QED and higher-order corrections, and also with the multiconfiguration Dirac-Fock GRASP code. The wavelengths and transition rates of electric-dipole transitions from the inner-shell excited terms ${1s2s2p}^{2}3s{}^{6}P–{1s2p}^{3}3s{}^6{S}^o$ are investigated by beam-foil spectroscopy in the XUV spectral region. The predicted transition wavelengths agree with the experiment. The higher-order corrections, fine structures, and spectrum with high wavelength resolution are found to be critically important in these comparisons. Nine new lines have been identified. The ground sextet states of boronlike atoms are metastable and well above several ionization levels. These are possible candidates for XUV and soft x-ray lasers.

In terms of the generalized Sturm-Liouville theorem, the Levinson theorem for the Dirac equation with a spherically symmetric potential in $D+1\mathrm{}$ dimensions is uniformly established as a relation between the total number of bound states and the sum of the phase shifts of the scattering states at $E=\pm M$ with a given angular momentum. The critical case, where the Dirac equation has a half bound state, is analyzed in detail. A half bound state is a zero-momentum solution if its wave function is finite but does not decay fast enough at infinity to be square integrable.

The time dependence of extreme ultraviolet (EUV) fluorescence following an ionizing radiation event in liquid helium is observed and studied in the temperature range from 250 mK to 1.8 K. The fluorescence exhibits significant structure including a short $\left(\sim 10\mathrm{ns}\right)$ strong initial pulse followed by single photons whose emission rate decays exponentially with a $1.6-\mu \mathrm{s}$ time constant. At an even longer time scale, the emission rate varies as “1/time” (inversely proportional to the time after the initial pulse). The intensity of the “1/time” component from $\beta$ particles is significantly weaker than those from $\alpha$ particles or neutron capture on ${}^3\mathrm{He}.$ It is also found that for $\alpha$ particles, the intensity of this component depends on the temperature of the superfluid helium. Proposed models describing the observed fluorescence are discussed.

${\mathrm{AlH}}_3$ and ${\mathrm{Al}}_2{\mathrm{H}}_6$ are very stable molecules with high ionization potentials and low electron affinities. Yet, we report the findings of higher aluminum hydrides, ${\mathrm{Al}}_n{\mathrm{H}}_{3n}$ $(n=3\mathrm{}–7)$ with cyclic or linear polymeric structures that are even more stable. These represent the possibilities of associating higher contents of hydrogen with aluminum clusters. We use first-principles calculations with a plane-wave basis as well as a linear combination of atomic-orbitals method. The binding energies and the highest occupied–lowest unoccupied molecular-orbital gaps of these molecules are significantly higher as compared to the values for the three-dimensional structures of hydrogenated aluminum clusters. The energetic and fragmentation behavior shows that these molecules should be stable up to a size of at least $n=4\mathrm{}$ in cyclic or polymeric forms.

Fast ionization of ${\mathrm{D}}_2$ leads to the coherent population of many vibrational states of ${\mathrm{D}}_2^{+}.$ Usually, only the squared absolute values of the vibrational state amplitudes, known as Franck-Condon factors, are observed since insufficient experimental time resolution averages out all coherence effects. We propose a Coulomb explosion imaging method to visualize the coherent motion of bound wave packets using ultrashort (5 fs), intense pump-probe laser pulses. With this type of experiment decoherence times in the fs to ps range may become directly observable and provide essential information for coherent control.

The Mott scattering in the presence of a linearly polarized laser field is investigated in the first Born approximation. The theoretical results indicate that at medium and large scattering angles, a large amount of multiphoton processes take place in the course of scattering. The photoabsorption (inverse bremsstrahlung) predominates the photoemission (bremsstrahlung). The sum rule for the multiphoton cross sections is notably violated. When the laser polarization deviates from the incident direction, the laser-modified (summed) cross section shows a considerable dependence on the azimuthal angle of the scattered electron. The dependencies of the cross section on the field strength, the frequency, the polarization direction, and the electron-impact energy are studied.