The use of linearly independent signal states in realistic implementations of quantum key distribution (QKD) enables an eavesdropper to perform unambiguous state discrimination. We explore quantitatively the limits for secure QKD imposed by this fact taking into account that the receiver can monitor, to some extent the photon-number statistics of the signals even with todays standard detection schemes. We compare our attack to the beam-splitting attack and show that security against the beam-splitting attack does not necessarily imply security against the attack considered here.

We develop a first-principles molecular dynamics for slow ion-atom collisions, where a many-electron system is described over time allowing for its self-consistent coupling to evolving nuclear motions. Our treatment combines an eikonal description of nuclear motions and time-dependent Hartree-Fock states and introduces molecular orbitals written as linear combinations of traveling atomic functions to derive general matrix equations for state-to-state cross sections and to calculate properties. A “relax-and-drive” procedure is employed to propagate solutions to the coupled equations with different time scales for electronic transitions and nuclear displacements. Reduced differential cross sections have been calculated for elastic, electron transfer, and excitation processes in ${\mathrm{He}}^{+}+\mathrm{D}$ collisions, and they compare very well to experimental values and other calculations. In addition, we present results for the evolution of atomic populations during collisions.

A weak Auger transition at $870.5\pm 0.3\hspace{0.5em}\mathrm{eV}$ has been observed in neon for electron impact energies above the double K-shell ionization threshold (1863 eV). The Auger energies and transition rates identify the transition as the strongest Auger hypersatellite decay channel, $\mathrm{KK}-{\mathrm{KL}}_{23}{L}_{23}^{2}D,$ of the hollow ${1s}^o{}^{1}S$ state of ${\mathrm{Ne}}^{2+}.$ The production cross section of the ${1s}^o{}^{1}S$ state by electron impact was determined for energies up to three times the threshold ionization energy and found to agree reasonably with the binary encounter calculations.

We have studied energy transfer, fragmentation, and ionization in collisions between protons and ${\mathrm{C}}_{60}$ molecules. The collision energy is varied between 1 and 300 keV and we thus cover a velocity range from 0.2 to 3.5 atomic units. The distributions of intact and fragmented ${\mathrm{C}}_{60}$ ions have been measured by means of time-of-flight mass spectrometry using pulsed beams or emitted electrons to clock the collision events. The processes leading to multiple ionization and evaporation of neutral dimers are analyzed as functions of the projectile velocity. In contrast to recent findings for ${\mathrm{He}}^{+}{-\mathrm{C}}_{60}$ collisions by Schlathölter et al., our results show that the cross sections for ionization and evaporation first increase with velocity, have maxima at around 1–2 a.u., and decrease again for higher velocities. This velocity dependence closely resembles the one for the electronic stopping power indicating that electronic excitations are the main means of energy transfer to ${\mathrm{C}}_{60}$ and that this energy is rapidly distributed among the vibrational degrees of freedom. This conclusion is supported by Monte Carlo calculations of nuclear and electronic energy loss for protons passing through and closely outside the ${\mathrm{C}}_{60}$ cage.

A photodissociation study is reported on the simply structured molecular ion ${\mathrm{Ar}}_2^{+}$ that is, to good approximation a two-electronic state system (one bound, the other dissociative) under the conditions of this experiment. Nonlinear behavior is manifest in characteristically changed dissociation dynamics as a function of laser intensity in strong laser fields $(<~5\times {10}^{12}$ ${\mathrm{W}/\mathrm{c}\mathrm{m}}^2).\mathrm{}$ Light-induced molecular potentials (molecular dressed states) provide a convenient description under such conditions and predict surprising effects regarding molecular dynamics in a light field. Much care has been taken to assure a meaningful quantitative comparison of experimental results with numerical simulations. This leads to the identification of dissociation by tunneling through a light-induced molecular potential well. ${\mathrm{Ar}}_2^{+}$ is generated in a dc-discharge, accelerated, and formed into a mass selected molecular beam (7.5 keV). For some experiments, molecules are prepared in low vibrational states only, before they interact with a high intensity laser beam at a wavelength of 532 nm. The momentum distribution of photofragments is measured using an imaging technique for neutral fragments.

We analyze a configuration consisting of a ${}^3\mathrm{He}$ droplet inmersed in a Bose-Einstein condensate at $T\approx 0.$ We investigate the stationary density and oscillations of the condensate, as well as the dynamics of the droplet in the frame of a simple model. Our paper takes into account the actual interaction potential between the ${}^3\mathrm{He}$ and condensate atoms and reveals the stability of the dynamics of a small ${}^3\mathrm{He}$ droplet inside the condensate.

Using the full nonlinear equations of motion, we calculate the quantum-nondemolition (QND) correlations for the traveling-wave second-harmonic generation. We find that, after a short interaction length, these are qualitatively different from results calculated previously using a linearized fluctuation analysis. We demonstrate that, although individual QND criteria can be very good in certain regions, there is no region where all three of the standard criteria are perfect, as has previously been claimed. We also show that only the amplitude quadrature of the output field can be considered as a QND quantity, with the phase quadrature not satisfying all the criteria.

We present an elementary derivation of the maximum fidelity attainable in teleportation using a single copy of two d-level systems in an arbitrary pure state. This derivation provides a complete set of necessary and sufficient conditions for optimal teleportation protocols. We also discuss the information on the teleported particle that is revealed in course of the protocol using a nonmaximally entangled state.

We point out that the discrete Sturmian set for the one-dimensional harmonic oscillator, constructed recently by Antonsen [Phys. Rev. A 60, 812 (1999)], is incomplete in $L_{x^2}^2\left(R\right)$ and thus does not form a basis in that Hilbert space. We show that for $E>\mathrm{}0,$ the spectrum of the Sturm-Liouville problem defining the Sturmian functions is mixed and consists of an infinite number of discrete positive eigenvalues, coinciding with those found by Antonsen, and the continuum of eigenvalues covering the negative real semiaxis. The discrete eigenvalues are simple while continuous eigenvalues are doubly degenerate. A basis in $L_{x^2}^2\left(R\right)$ comprises all Sturmian functions generated by that problem, i.e., those associated with discrete eigenvalues as well as those associated with continuum eigenvalues. We consider also the defining Sturm-Liouville problem in the case $E<~0$ and find that then the discrete part of its spectrum is absent: the spectrum is purely continuous, doubly degenerate, and covers the negative real semiaxis; the associated Sturmian functions form a noncountable basis in $L_{x^2}^2\left(R\right).\mathrm{}$

We analyze various scenarios for entangling two initially unentangled qubits. In particular, we propose an optimal universal entangler that entangles a qubit in unknown state $|\Psi 〉$ with a qubit in a reference (known) state $|0〉.$ That is, our entangler generates the output state that is as close as possible to the pure (symmetrized) state $(|\Psi 〉|0〉+|0〉|\Psi 〉).\mathrm{}$ The most attractive feature of this entangling machine, is that the fidelity of its performance (i.e., the distance between the output and the ideally entangled—symmetrized state) does not depend on the input and takes the constant value $\mathcal{F}=(9+3\mathrm{}\sqrt{2})/14\mathrm{}\simeq \mathrm{0.946.}$ We also analyze how to optimally generate from a single qubit initially prepared in an unknown state $|\Psi 〉$ a two qubit entangled system, which is as close as possible to a Bell state $(|\Psi 〉|{\Psi }^{\perp }〉+|{\Psi }^{\perp }〉|\Psi 〉),$ where $〈\Psi |{\Psi }^{\perp }〉=0.\mathrm{}$

An NMR realization of a two-qubit quantum gate which processes quantum information indirectly via couplings to a spectator qubit is presented in the context of the Deutsch-Jozsa algorithm. This enables a successful comprehensive NMR implementation of the Deutsch-Jozsa algorithm for functions with three argument bits and demonstrates a technique essential for multi-qubit quantum computation.

It is shown that the analysis by Marinescu and You [Phys. Rev. A 59, 1936 (1999)] of the Casimir-Polder long-range interaction potential between a pair of ground-state alkali-metal atoms is incomplete. Missing is the contribution to the energy shift arising from the diamagnetic-diamagnetic interaction term. The leading term of this interaction is evaluated and found to have an inverse seventh-power dependence on interatomic separation distance.

We analyze the existence of activable bound entangled states in multiparticle systems. We first give a series of examples that illustrate some different ways in which bound entangled states can be activated by letting some of the parties share maximally entangled states. Then, we derive necessary conditions for a state to be distillable as well as to be activable. These conditions turn out to be also sufficient for a certain family of multiqubit states. We use these results to explicitely construct states displaying properties related to bound entanglement and its activation.

The ${}^2{S}_{1/2}{\mathrm{}(F=0\mathrm{})-}^2{F}_{7/2}\mathrm{}(F=3\mathrm{})\mathrm{}$ electric octupole transition in ${}^{171}{\mathrm{Yb}}^{+}$ has been detected by observing quantum jumps in the fluorescence of a single ion held in an electrodynamic trap. The transition frequency has been measured as 642 121 498.1 (0.8) MHz. The extreme weakness of this transition makes it ideally suited as an optical frequency reference of subhertz linewidth. The absolute frequencies of a series of Doppler-free absorptions in tellurium vapor are also reported. One of these absorptions was used as a local frequency reference while locating the electric octupole transition.

We discuss how quantum information distribution can improve the performance of some quantum computation tasks. This distribution can be naturally implemented with different types of quantum cloning procedures. We give two examples of tasks for which cloning provides some enhancement in performance, and briefly discuss possible extensions of the idea.

A 13-boson system has been qualitatively studied based on symmetry consideration. The rotational excitation of the ground state is found to be greatly hindered. The geometric feature and some specific modes of internal oscillation of low-lying states have been predicted. An idea for the classification of states has been proposed. A number of rotation bands have been suggested.

We present an analytical investigation of revival phenomena in the finite square-well potential. The classical motion, revival, and super-revival time scales are derived exactly for wave packets excited in the finite well. These time scales exhibit a richer dependence on wave-packet energy and on potential-well depth than has been found in other quantum systems: They explain, for example, the difficulties in exciting wave packets with strong classical features at the bottom of a finite well, or with clearly resolved super-revivals in a shallow well. In the proper regions of validity, the time scales predict the instances of wave-packet reformation extremely accurately. Revivals at the bottom of the well are explored as a “universal” limit of the general theory, which offers the clearest connection with the series of fractional and full revivals seen in the dynamics of the infinite square-well potential.

Even though measurement results obtained in the real world are generally both noisy and continuous, quantum measurement theory tends to emphasize the ideal limit of perfect precision and quantized measurement results. In this article, a more general concept of noisy measurements is applied to investigate the role of quantum noise in the measurement process. In particular, it is shown that the effects of quantum noise can be separated from the effects of information obtained in the measurement. However, quantum noise is required to “cover up” negative probabilities arising as the quantum limit is approached. These negative probabilities represent fundamental quantum-mechanical correlations between the measured variable and the variables affected by quantum noise.

Generation and propagation of high-order harmonics in high-pressure (0.5–4 atm) gases by intense, ultrashort laser pulses is numerically investigated. The calculations are performed utilizing a self-consistent solution of the time-dependent Schrödinger and one-dimensional wave equations. The results obtained from the second-order and from the reduced first-order wave equations are compared. It is shown that at high pressure and for the high-order harmonics the use of the reduced first-order wave equation is not adequate. At very high pressure (∼4 atm) a new regime of phase matching was observed. The applicability of the reduced first-order wave equation is also discussed.

We demonstrate a way to circumvent the interruption of evaporative cooling observed at high bias field for ${}^{87}\mathrm{Rb}$ atoms trapped in the $\mathrm{}(F=2,m=+2\mathrm{})\mathrm{}$ ground state. Our scheme uses a three-frequency rf knife achieved by mixing two rf frequencies. This compensates part of the nonlinearity of the Zeeman effect, allowing us to achieve Bose-Einstein condensation where the standard one-frequency rf knife evaporation method did not work. We are able to get efficient evaporative cooling, provided that the residual detuning between the transition and the rf frequencies is smaller than the power broadening of the rf transitions at the end of the evaporation ramp.