We describe a mathematical solution for the generation of entangled N-photon states in two field modes. A simple and compact solution is presented for a two-mode Jaynes-Cummings model by combining the two field modes in a way that only one of the two resulting quasimodes enters in the interaction term. The formalism developed is then applied to calculate various generation probabilities analytically. We show that entanglement, starting from an initial field and an atom in one defined state may be obtained in a single step. We also show that entanglement may be built up in the case of an empty cavity and excited atoms whose final states are detected, as well as in the case when the final states of the initially excited atoms are not detected.

How useful is a quantum dynamical operation for quantum information processing? Motivated by this question, we investigate several strength measures quantifying the resources intrinsic to a quantum operation. We develop a general theory of such strength measures, based on axiomatic considerations independent of state-based resources. The power of this theory is demonstrated with applications to quantum communication complexity, quantum computational complexity, and entanglement generation by unitary operations.

Effects of the interchannel coupling on the spin polarization of energetic photoelectrons emitted from atomic Ne valence subshells are examined. Like previously obtained results for cross sections and angular distributions, the photoelectron spin polarization parameters too are found considerably influenced by the coupling. The result completes a series of studies to finally conclude that the independent particle description is inadequate for the entire range of photoionization dynamics over the full spectral energy domain.

We present a scheme that is able to protect the quantum states of a cavity mode against the decohering effects of photon loss. The scheme preserves quantum states with a definite parity, and improves previous proposals for decoherence control in cavities. It is implemented by sending single atoms, one by one, through the cavity. The atomic state gets first correlated to the photon number parity. The wrong parity results in an atom in the upper state. The atom in this state is then used to inject a photon in the mode via adiabatic transfer, correcting the field parity. By solving numerically the exact master equation of the system, we show that the protection of simple quantum states could be experimentally demonstrated using presently available experimental apparatus.

We develop a Green’s function method to evaluate the exact equilibrium particle-density profiles of noninteracting Fermi gases in external harmonic confinement in any spatial dimension and for arbitrary trap anisotropy. While in a spherically symmetric configuration the shell effects are negligible in the case of a large number of particles, we find that for very anisotropic traps the quantum effects due to single-level occupancy and the deviations from the Thomas-Fermi approximation are also visible for mesoscopic clouds.

In this paper, we discuss the insecurity with present implementations of the Ekert protocol for quantum-key distribution based on the Wigner inequality. We propose a modified version of this inequality which guarantees safe quantum-key distribution.

Light-induced nonlinear terms in the Gross-Pitaevskii equation arise from the stimulated coherent exchange of photons between two atoms. For atoms in an optical dipole trap this effect depends on the spatial profile of the trapping laser beam. Two different laser beams can induce the same trapping potential but very different nonlinearities. We propose a scheme to measure light-induced nonlinearities which is based on this observation.

We present a theory of the geometric phase based logically on the Bargmann invariant of quantum mechanics, and null phase curves in ray space, as the fundamental ingredients. Null phase curves are themselves defined entirely in terms of the (third order) Bargmann invariant, and it is shown that these are the curves natural to geometric phase theory, rather than geodesics used in earlier treatments. The natural symplectic structure in ray space is seen to play a crucial role in the definition of the geometric phase. Logical consistency of the formulation is explicitly shown, and the principal properties of geometric phases are deduced as systematic consequences.

We present an experiment testing quantum correlations with frequency shifted photons. We test Bell inequality with two-photon interferometry where we replace the beam splitters with acousto-optic modulators, which are equivalent to moving beam splitters. We measure the two-photon beats induced by the frequency shifts, and we propose a cryptographic scheme in relation. Finally, setting the experiment in a relativistic configuration, we demonstrate that the quantum correlations are not only independent of the distance but also of the time ordering between the two single-photon measurements.

The constraints imposed by symmetry on the open channels of dipositronium has been studied, and the symmetry-adapted lowest open channel of each quantum state has been identified. Based on this study, the existence of two more $0^{+}$ bound states has been theoretically confirmed, and a $0^{+}$ narrow resonance has been predicted. A variational calculation has been performed to evaluate the critical strength of the repulsive interaction. Two $0^{-}$ states are found to have their critical strengths very close to 1, they are considered as candidates of new narrow resonances or loosely bound states.

Hyperspherical partial-wave theory has been applied here in a new way in the calculation of the triple differential cross sections for the ionization of hydrogen atoms by electron impact at low energies for various equal-energy-sharing kinematic conditions. The agreement of the cross section results with the recent absolute measurements of [J. Röder, M. Baertschy, and I. Bray, Phys. Rev. A 45, 2951 (2002)] and with the latest theoretical results of the ECS and CCC calculations [J. Röder, M. Baertschy, and I. Bray, Phys. Rev. A (to be published)] for different kinematic conditions at 17.6 eV is very encouraging. The other calculated results, for relatively higher energies, are also generally satisfactory, particularly for large ${\Theta }_{\mathrm{ab}}$ geometries. In view of the present results, together with the fact that it is capable of describing unequal-energy-sharing kinematics [J. N. Das, J. Phys. B 35, 1165 (2002)], it may be said that the hyperspherical partial-wave theory is quite appropriate for the description of ionization events of electron–hydrogen-type systems. It is also clear that the present approach in the implementation of the hyperspherical partial-wave theory is very appropriate.

Measurements of ${\mathrm{Kr}}^{q+}$ yields in coincidence with K-shell fluorescence, as incident x-ray energy is varied across the K-shell threshold, are reported. Near threshold, we observe slight variations in the branching ratios as a function of energy, which are connected with the different behaviors of the flux-normalized partial yields for each q. The lower-q yields show a resonance peak near threshold superimposed on a smoothly rising edge, whereas the higher-q yields show only a smooth rise. A simple model is developed which accounts for these features, incorporating both the threshold photoexcitation and the cascade behavior of the spectator electrons.

The presence of a static electric field substantially enhances the dielectronic recombination (DR) rate below the classical field ionization limit and suppresses DR entirely above it. The addition of a small microwave field polarized orthogonally to the static field leads to a substantial DR signal above the classical ionization limit. We have examined DR from a continuum of finite bandwidth in static fields from 0 to 130 V/cm and with 8–11 GHz microwave fields of amplitudes up to 5 V/cm. We attribute the microwave-induced DR above the classical limit to the transfer of atoms by the microwave field from autoionizing the $m=0\mathrm{}$ Rydberg intermediate states to higher m states. Since higher m states are more stable against field ionization, these states are more likely to decay radiatively to stable bound Stark states, thereby completing DR. This m-changing process most likely occurs by resonantly driving the $\Delta n=0,$ $\Delta m=1\mathrm{}$ transitions of the autoionizing Rydberg-Stark states with the microwave field.

We show that an optical multilayer system comprising a thin metallic film sandwiched between two different half-space dielectrics generates remarkable quantum correlations for dipole emitters embedded in the structure, both in the far zone and in the near zone. For a pair of such dipole emitters localized in the same region of the structure, the correlations display super-radiance and subradiance phenomena, but the system allows for an unusual and seldom considered scenario in which the emitters are located on different sides and so they are separated by the metallic screen. We explore the quantum correlations in this situation and find that they are sensitive to the type of metallic screen as well as dipole orientation and dielectric mismatch across the screen. We point out the high-symmetry features and attribute the underlying physics to a subtle interplay between image and screening effects in the presence of dielectric mismatch.

Using first principles, it is demonstrated that radiative damping alone cannot lead to a nonvanishing linear electro-optic effect in a chiral isotropic medium. This conclusion is in contrast with that obtained by a calculation in which damping effects are included using the standard phenomenological model. We show that these predictions differ because the phenomenological damping equations are valid only in regions where the frequencies of the applied electromagnetic fields are nearly resonant with the atomic transitions. We also show that collisional damping can lead to a nonvanishing linear electro-optic effect, but with a strength sufficiently weak, it is unlikely to be observable under realistic laboratory conditions.

We introduce a protocol for quantum secret sharing based on reusable entangled states. The entangled state between the sender and the receiver acts only as a carrier to which data bits are entangled by the sender and disentangled from it by the receivers, all by local actions of simple gates. We also show that the interception by Eve or the cheating of one of the receivers introduces a quantum bit error rate larger than 25% which can be detected by comparing a subsequence of the bits.

We propose a molecular close-coupling expansion in terms of prolate spheroidal wave functions confined in an ellipsoidal box. We first implement the method for ionization of ${\mathrm{H}}_2^{+}$ molecular ions, by linearly polarized strong and short laser pulses, in the nonperturbative regime and within the (fixed nuclei) Born-Oppenheimer approximation. We further analyze the adequacy of the method to reproduce both the bound and the continuum nonadiabatic processes in ion-atom collisions.

The validity of the two-level approximation (TLA) in the interaction of atoms with few-cycle light pulses is studied by investigating a simple $\mathsf{V}$-type three-level atom model. Even the transition frequency between the ground state and the third level is far away from the spectrum of the pulse; this additional transition can make the TLA inaccuracy. For a sufficiently large transition frequency or a weak coupling between the ground state and the third level, the TLA is a reasonable approximation and can be used safely. When decreasing the pulse width or increasing the pulse area, the TLA will give rise to non-negligible errors compared with the precise results.

We trace the resistance to entanglement generation of spin-coherent states when passed through a beam splitter as we vary S through $S=1/\overrightarrow{2}\infty .$ In the infinite S limit the spin-coherent states are equivalent to the high-amplitude limit of the optical coherent states. These states generate no entanglement and are completely distinguishable. This transition is discussed in terms of the classicality of the states. The decline of the generated entanglement, and in this sense increase in classicality with S, is very slow and dependent on the amplitude z of the state. Surprisingly, we find that, for $|z|>1,$ there is an initial increase in entanglement followed by an extremely gradual decline to zero. Other aspects of classicality are also discussed over the transition in S, including the distinguishability, which decreases quickly and monotonically. We illustrate the distinguishability of spin-coherent states using the representation of Majorana.