We suggest a scheme to slow and stop broadband laser pulses inside an atomic medium using electromagnetically induced transparency. Extending the suggestion of Harris et al. [Phys. Rev. Lett. 70, 552 (1993)], the key idea is to use matched Fourier components for the probe and coupling laser beams.

We describe an efficient theoretical criterion, suitable for indistinguishable particles to quantify the quantum correlations of any pure two-fermion state, based on the Slater rank concept. It represents the natural generalization of the linear entropy used to treat quantum entanglement in systems of nonidentical particles. Such a criterion is here applied to an electron-electron scattering in a two-dimensional system in order to perform a quantitative evaluation of the entanglement dynamics for various spin configurations and to compare the linear entropy with alternative approaches. Our numerical results show the dependence of the entanglement evolution upon the initial state of the system and its spin components. The differences with previous analyses accomplished by using the von Neumann entropy are discussed. The evaluation of the entanglement dynamics in terms of the linear entropy results to be much less demanding from the computational point of view, not requiring the diagonalization of the density matrix.

Bounds on the concurrence of the superposition state in terms of the concurrences of the states being superposed are found in this paper. The bounds on concurrence are quite different from those on the entanglement measured by von Neumann entropy [Linden et al., Phys. Rev. Lett. 97, 100502 (2006)]. In particular, a nonzero lower bound can be provided if the states being superposed are properly constrained.

We present both differential and total cross sections for the direct ionization of water vapor by protons in the incident energy range $0.1–100\mathrm{MeV}$. Different theoretical models are investigated within the framework of the Born approximation in order to evaluate the influence of each pairwise Coulomb interaction term among the ejected electron, the scattered proton, and the residual ionized target in the final state. In all these models, the ground state of the water molecule is described by means of an accurate molecular wave function proposed by Moccia [J. Chem. Phys. 40, 2186 (1964)]. The results of these full ab initio quantum-mechanical treatments are compared to experimental data. Good agreement is generally observed, showing that sophisticated Born models are sufficient to explain all the experimental data, including doubly differential, singly differential, and total cross sections.

Generally, conditions for deep sub-Doppler laser cooling do not match conditions for strong atomic localization, that takes place in a deeper optical potential and leads to higher temperature. Moreover, for a given detuning in a deep optical potential the secular approximation, which is frequently used for a quantum description of laser cooling, fails. Here we investigate the atomic localization in optical potential, using a full quantum approach for atomic density matrix beyond the secular approximation. It is shown that laser cooling in a deep optical potential, created by a light field with polarization gradients, can be used as an alternative method for the formation of high contrast spatially localized structures of atoms for the purposes of atom lithography and atomic nanofabrication. Finally, we analyze possible limits for the width and contrast of localized atomic structures that can be reached in this type of light mask.

We study the ground states of rotating atomic Bose-Einstein condensates with dipolar interactions. We present the results of numerical studies on a periodic geometry which show vortex lattice ground states of various symmetries: triangular and square vortex lattices, “stripe crystal,” and “bubble crystal.” We present the phase diagram (for systems with a large number of vortices) as a function of the ratio of dipolar to contact interactions and of the chemical potential. We discuss the experimental requirements for observing transitions between vortex lattice ground states via dipolar interactions. We finally investigate the stability of mean-field supersolid phases of a quasi-two-dimensional nonrotating Bose gas with dipolar interactions.

We consider localized modes (bright solitons) of the one-dimensional quintic nonlinear Schrödinger equation with a periodic potential, describing several mean-field models of low-dimensional condensed gases. In the case of attractive nonlinearity we deduce sufficient conditions for collapse. We show that there exist spatially localized modes with arbitrarily large numbers of particles. We study such solutions in the semi-infinite gap (attractive case) and in the first gap (attractive and repulsive cases), and show that a nonzero minimum value of the number of particles is necessary for a localized mode to be created. In the limit of large negative frequencies (attractive case) we observe quantization of the number of particles of the stationary modes. Such solutions can be interpreted as coupled Townes solitons and appear to be stable. The modes in the first gap have numbers of particles infinitely growing with frequencies approaching one of the gap edges, which is explained by the power decay of the modes. Stability of the localized modes is discussed.

We construct the list of all possible inert states of spin Bose-Einstein condensates for $S\le 4$. In doing so, we also obtain their symmetry properties. These results are applied to classify line defects of spin condensates in zero magnetic field.

We propose how to prepare four remote logical qubits, with each in a separate cavity and encoded in the decoherence-free subspace by two atoms, into various entangled states. By means of the cavity-assisted photon scattering, we can link the four remote logical qubits in different cavities to be, respectively, $W$ state, Greenberger-Horne-Zeilinger state, and cluster state, which are important in view of the distributed quantum information processing. The generation of the latter two entangled states can be theoretically generalized to many-qubit cases. Because our qubits are defined in dephasing-free subspace, our scheme is immune to dephasing during or even after the entanglement preparation.

Resonance enhanced multiphoton ionization of the $X_1{}^2\Pi _{1∕2}$ state of lead monofluoride $\left({}^{208}\mathrm{Pb}^{19}\mathrm{F}\right)$ via the $B{}^2\Sigma _{1∕2}$ state is demonstrated. The ionization potential is observed to be $7.54\left(1\right)\mathrm{eV}$. Limits on the lifetime of the $B$ state are found to be consistent with that reported by Chen et al. The transition dipole moment for the $X_1\to B$ transition is found to be $0.005\left(1\right)\mathrm{a.u.}$ Limits on the ionization cross section of the $B$ state are found.

We study the properties of atomic entanglement in the eigenstates spectrum of the inhomogeneous Tavis-Cummings model. The inhomogeneity is present in the coupling among the atoms with quantum electromagnetic field. We calculate analytical expressions for the concurrence, and we find that this exhibits a strong dependence on the inhomogeneity.

We consider the question of whether it is possible to use the entanglement between spatially separated modes of massive particles to observe nonlocal quantum correlations. Mode entanglement can be obtained using a single particle, indicating that it requires careful consideration before concluding whether experimental observation—e.g., violation of Bell inequalities—is possible or not. In the simplest setups analogous to optics experiments, that observation is prohibited by fundamental conservation laws. However, we show that using auxiliary particles, mode entanglement can be converted into forms that allow the observation of quantum nonlocality. The probability of successful conversion depends on the nature and number of auxiliary particles used. In particular, we find that an auxiliary Bose-Einstein condensate allows the conversion arbitrarily many times with a small error that depends only on the initial state of the condensate.

We study a quantum state transfer between spins interacting with an arbitrary network of spins coupled by uniform $XX$ interactions. It is shown that in such a system under fairly general conditions, we can expect a nearly perfect transfer of states. Then we analyze a generalization of this model to the case of many network users, where the sender can choose which party he wants to communicate with by appropriately tuning his local magnetic field. We also remark that a similar idea can be used to create an entanglement between several spins coupled to the network.

We consider information transmission through a noiseless quantum channel, where the information is encoded into massive indistinguishable particles: bosons or fermions. We study the situation in which the particles are noninteracting. The encoding input states obey a set of physically motivated constraints on the mean values of the energy and particle number. In such a case, the determination of both classical and quantum capacity reduces to a constrained maximization of entropy. In the case of noninteracting bosons, signatures of Bose-Einstein condensation can be observed in the behavior of the capacity. A major motivation for these considerations is to compare the information-carrying capacities of channels that carry bosons with those that carry fermions. We show analytically that fermions generally provide higher channel capacity, i.e., they are better suited for transferring bits as well as qubits, in comparison to bosons. This holds for a large range of power-law potentials, and for moderate to high temperatures. Numerical simulations seem to indicate that the result holds for all temperatures. Also, we consider the low-temperature behavior for the three-dimensional box and harmonic trap, and again we show that the fermionic capacity is higher than the bosonic one for sufficiently low temperatures.

In a recent paper [Phys. Rev. Lett. 97, 070401 (2006)] the transition rate of magnetic spin-flip of a neutral two-level atom trapped in the vicinity of a thick superconducting body was studied. In the present paper we will extend these considerations to a situation with an atom at various distances from a dielectric film. Rates for the corresponding electric dipole-flip transition will also be considered. The rates for these atomic flip transitions can be reduced or enhanced, and in some situations they can even be completely suppressed. For a superconducting film or a thin film of a perfect conducting material various analytical expressions are derived that reveals the dependence of the physical parameters at hand.

We study the properties of a trapped interacting three component Fermi gas. We assume that one of the components can have a different mass from the other two. We calculate the different phases of the three component mixture and find a rich variety of different phases corresponding to different pairing channels, and simple ways of tuning the system from one phase to another. In particular, we predict coexistence of several different superfluids in the trap, forming a shell structure, and phase transitions from this mixture of superfluids to a single superfluid when the system parameters or temperature is varied. Such shell structures realize superfluids with a nontrivial spatial topology and leave clear observable signatures in the density profile of the gas.

We reassess some mathematical aspects of the theory by Keldysh [Sov. Phys. JETP 20, 1307 (1965)]. In particular, we discuss in detail what closed contour is associated with the symbol $\oint$ introduced by Keldysh and we expose how the contour involved is conveniently deformed to calculate the integral by the saddle-point method. The integral cannot be reduced to a contribution of a single pole that could be evaluated via the residue theorem, contrary to the statement in the recent work by Mishima et al. [Phys. Rev. A 66, 033401 (2002)].

In response to the preceding Comment, we present the intermediate steps skipped in our mathematical derivation of the Keldysh integral using the residue integration method in the complex domain. We describe in detail the pole structures and the closed contour for performing the residue calculation. We analyze and discuss the implications of the analytic formula together with representative numerical calculations. The present analysis completes and validates our mathematical analysis of the Keldysh integral.

It is well known that the density of the target can have a crucial impact on charge-changing collisions of partially ionized heavy ions. However, the basic understanding of this experimental observation is hampered by the difficulty in knowing the charge-state evolution of projectiles inside solids. Therefore, the present experiments with $200\mathrm{MeV}∕\mathrm{u}$ bare and H-like nickel ions were performed to study charge-changing cross sections in different monatomic and compound gases and solids. The experimental results clearly demonstrate that the electron-loss cross sections in solids increase by about 40% compared to gases. The results support the Bohr-Lindhard model which predicts this gas-solid difference originating from enhanced ionization of excited ions. The experimental results are compared with recent theoretical estimates.