In a recent paper [Phys. Rev. A 61, 022117 (2000)], A. Peres argued that quantum mechanics is consistent with special relativity by proposing that the operators that describe time evolution do not need to transform covariantly, although the measurable quantities need to transform covariantly. We discuss the weaknesses of this proposal.

Contrary to the claim in the preceding Comment, the operators that describe the complete time evolution of a quantum system have a covariant transformation law, even though they are built from expressions that belong to separate subsystems and do not obey Lorentz transformations. The reason is that these are open subsystems, and different parts of the universe are excluded in their description. However, the complete system still obeys Lorentz covariance.

We study a general teleportation scheme with an arbitrary state of the pair of particles $(2\mathrm{}$ and $3)$ shared by Alice and Bob, and arbitrary measurements on the input particle 1 and one of the members (2) of the pair on Alice’s side. We find an efficient iterative algorithm for identifying optimum operations on Bob’s side. In particular, we find that simple unitary transformations on his side are not always optimal even if particles 2 and 3 are perfectly entangled. We describe the most interesting protocols in the language of extremal completely positive maps.

An analysis based on intrinsic anisotropic states exposed to rotation explains why approximate rotorlike energy spectra appear in multiply excited atoms. For intrashell excited atoms, unlike molecules and nuclei, the rotor structure is not associated with rotational kinetic energy. Close examination of a general argument from nuclear physics shows that the rotorlike series are due to the behavior of wave function overlaps and matrix elements of the electron-electron repulsion between rotated anisotropic states.

The effects of quantum and thermal fluctuations upon the fringe structure predicted to be observable in the momentum distribution of coupled Bose-Einstein condensates are studied by the effective-potential method. For a double-well trap, the coherence factor recently introduced by Pitaevskii and Stringari (e-print cond-mat/0104458) is calculated using the effective potential approach and is found in good agreement with their result. The calculations are extended to the case of a one-dimensional array of condensates, showing that quantum effects are essentially described through a simple renormalization of the energy scale in the classical analytical expression for the fringe structure. The consequences for the experimental observability are discussed.

We propose a quantum measurement model where the environment is taken to be a squeezed reservoir. This kind of meter-environment coupling has a dramatic influence on the decoherence rate. Depending on the phase and the squeezing parameter the decoherence time can be enhanced or reduced arbitrarily. We derive the Wigner function for the system-meter state, and discuss its nonclassical features. Further, we test quantum nonlocality in phase space on the basis of coincidence measurements of the entangled system-meter scheme, and investigate the effect of a highly reduced decoherence rate. Surprisingly, in sharp contrast to popular notion, violations of the Bell inequality can be observed even when the meter has reached a macroscopic state.

One strategy to the discrimination problem is to identify the state with certainty, leaving a possibility of undecidability. This paper gives an upper bound for the maximal success probability of unambiguous discrimination among n states. This bound coincides with the known IDP limit when two states are considered.

We study the properties of quantum stabilizer codes that embed a finite-dimensional protected code space in an infinite-dimensional Hilbert space. The stabilizer group of such a code is associated with a symplectically integral lattice in the phase space of $2N$ canonical variables. From the existence of symplectically integral lattices with suitable properties, we infer a lower bound on the quantum capacity of the Gaussian quantum channel that matches the one-shot coherent information optimized over Gaussian input states.

Xenon $5p$ subshell photoionization has been studied in the vicinity of the Xe $4d$ shape resonance by angle and spin resolved photoelectron spectroscopy. Dipole matrix elements and relative phase shifts derived from these experimental data are compared with calculations with regard to the strength of interchannel and relativistic interactions. The comparison shows strong influence of interchannel interactions on the transition-matrix elements, particularly on the phase shift in the vicinity of the $4d$ shape resonance, but gives little evidence for relativistic interactions of comparable strength.

The cross section for ion-pair formation produced by dissociative recombination of low-energy $(<~10\mathrm{eV})$ electrons with ${\mathrm{HD}}^{+}$ is calculated using a time-dependent wave-packet method. The source of the experimentally observed oscillations as a function of energy in this cross section is confirmed to be an interference effect arising from the existence of multiple pathways to the final product. The total cross section for dissociative recombination as well as the branching ratios into the various product channels has also been calculated. These cross sections, including the ion-pair channel, are in good agreement with the available experimental data.

We study the interaction of strong femtosecond laser pulses with the ${\mathrm{C}}_{60}$ molecule employing time-dependent density-functional theory with the ionic background treated in a jellium approximation. The laser intensities considered are below the threshold of strong fragmentation but too high for perturbative treatments such as linear response. The nonlinear response of the model to excitations by short pulses of frequencies up to 45 eV is presented and analyzed with the help of Kohn-Sham orbital resolved dipole spectra. In femtosecond laser pulses of 800 nm wavelength ionization is found to occur multiphotonlike rather than via excitation of a “giant” resonance.

We propose a method for generating and controlling a spatially separated vortex-antivortex pair in a Bose-Einstein condensate trapped in a toroidal potential. Our simulations of the time-dependent Gross-Pitaevskii equation show that in toroidal condensates vortex dynamics are different from the dynamics in the homogeneous case. Our numerical results agree well with analytical calculations using the image method. Our proposal offers an effective example of coherent generation and control of vortex dynamics in atomic condensates.

For a Bose-Einstein condensate placed in a rotating trap and strongly confined along the z axis, we set a framework of study for the Gross-Pitaevskii energy in the Thomas-Fermi regime for an effective two-dimensional (2D) situation in the $x-y$ plane. We investigate an asymptotic expansion of the energy, the critical angular velocities of nucleation of vortices with respect to a small parameter $\varepsilon ,$ and the location of vortices. The limit $\varepsilon$ going to zero corresponds to the Thomas-Fermi regime. The nondimensionalized energy is similar to the Ginzburg-Landau energy for superconductors in the high-κ high-field limit and our estimates rely on techniques developed for this latter problem. We also take advantage of this similarity to develop a numerical algorithm for computing the Bose-Einstein vortices. Numerical results and energy diagrams are presented.

We demonstrate the existence of nonclassical correlations in the radiation of two atoms, that are coherently driven by a continuous laser source. The photon-photon correlations of the fluorescence light show a spatial interference pattern not present in a classical treatment. A feature of this phenomenon is that bunched and antibunched light is emitted in different spatial directions. The calculations are performed analytically. It is pointed out that the correlations are induced by state reduction due to the measurement process when the detection of the photons does not distinguish between the atoms. It is interesting to note that the phenomena show up even without any interatomic interaction.

We consider the interaction of an arbitrary quantum system with a single field cavity mode without the Rotating-Wave Approximation (RWA). The cavity mode is damped by coupling to a reservoir at zero temperature. We show that the system evolution is governed by an effective Linblad-type master equation with nonzero temperature terms leading to a nonzero stationary state, even without an external driven field. The system evolution under the RWA is also considered. Two particular cases of atom-field and field-field interaction are discussed.

Optimal N to $M\mathrm{}(M>N)\mathrm{}$ quantum cloning machines for a d-level system are presented. Unitary cloning transformations achieve the bound of the fidelity.

We provide a self-consistent theoretical model describing the vicinage effect on charge states of partially stripped ions in a large cluster traversing a thin solid foil at a high speed. Starting from Brandt-Kitagawa variational theory for the effective charge on a fast heavy ion, supplemented by the dynamically screened interionic interactions within the cluster, we obtain a nonlinear integral equation for a function describing the spatial distribution of ion charges throughout the cluster. Numerical solutions of such an equation show that the ions in the cluster interior become increasingly neutralized as the cluster size grows.

In a previous paper we have studied dressed excited states in the Friedrichs model, which describes a two-level atom interacting with radiation. In our approach, excited states are distributions (or generalized functions) in the Liouville space. These states decay in a strictly exponential way. In contrast, the states one may construct in the Hilbert space of wave functions always present deviations from exponential decay. We have considered the momentum representation, which is applicable to global quantities (trace, energy transfer). Here we study the space-time description of local quantities associated with dressed unstable states, such as, the intensity of the photon field. In this situation the excited states become factorized in Gamow states. To go from local quantities to global quantities, we have to proceed to an integration over space, which is far from trivial. There are various elements that appear in the space-time evolution of the system: the unstable cloud that surrounds the bare atom, the emitted real photons and the “Zeno photons,” which are associated with deviations from exponential decay. We consider a Hilbert space approximation to our dressed excited state. This approximation leads already to decay close to exponential in the field surrounding the atom, and to a line shape different from the Lorentzian line shape. Our results are compared with numerical simulations. We show that the time evolution of an unstable state satisfies a Boltzmann-like $\mathcal{H}$ theorem. This is applied to emission and absorption as well as scattering. The existence of a microscopic $\mathcal{H}$ theorem is not astonishing. The excited states are “nonequilibrium” states and their time evolution leads to the emission of photons, which distributes the energy of the unstable state among the field modes.

The ionization and Coulomb explosion of homonuclear ${\mathrm{D}}_n$ and ${\mathrm{T}}_n$ $(n=959\mathrm{}–8007)$ and heteronuclear $({\mathrm{D}}_2\mathrm{O}{)}_n$ and $({\mathrm{T}}_2\mathrm{O}{)}_n$ $(n=459\mathrm{}–2171)$ clusters in very intense $(I=5\mathrm{}\times {10}^{14}–5\times {10}^{18}{\mathrm{W}\mathrm{}\mathrm{cm}}^{\mathrm{-}2})$ laser fields is studied using classical dynamics simulations. The efficiency of the $d+d$ and $d+t$ nuclear fusion driven by the Coulomb explosion (NFDCE) is explored. The $d+d$ NFDCE of $({\mathrm{D}}_2\mathrm{O}{)}_n$ heteronuclear clusters is enhanced by energetic and kinematic effects for ${\mathrm{D}}^{+},$ while for $({\mathrm{T}}_2\mathrm{O}{)}_n$ heteronuclear clusters the kinetic energy of ${\mathrm{T}}^{+}$ is dominated by energetic effects. The cluster size dependence of the fusion reaction yield has been established. The heteronuclear clusters provide considerably higher $d+d$ and $d+t$ fusion reaction yields than the homonuclear clusters of the same size. The clusters consisting of both D and T atoms can provide highly efficient $d+t$ fusion reactions.

The static electric dipole polarizabilities of sodium clusters ${\mathrm{Na}}_n$ for a series of sizes in the range $7<~n<~93$ were measured by electric field deflection of a supersonic cluster beam. Results for the smaller clusters are in overall good agreement with previous experiments. Polarizabilities of the larger clusters continue the downward trend with increasing size, but remain considerably above the bulk limit. There is a degree of correlation between the polarizability values and electronic shell structure. Furthermore, there is an indication that the polarizability is sensitive to the source conditions, possibly reflecting the influence of internal cluster temperature.