We study analytically the photoionization of a coherent superposition of atomic and molecular electronic states by an ultrashort, attosecond x-ray pulse. We show that the broad photoelectron spectrum contains detailed information about the time-dependent electron wave packet. The asymmetry of the photoelectron momentum distribution measures the momentum asymmetry of the initial bound-state wave packet. We show that molecular interference modulates the time-dependent photoelectron spectrum and asymmetry. The modulation also depends on the internuclear separation. The photoelectron spectrum and its asymmetry monitors coherent electron motion and in principle electron transfer on the attosecond time scale.

We have studied the interference of a variable number of independently created $m_F=0$ microcondensates in a $\mathrm{C}{\mathrm{O}}_2$-laser optical lattice. The observed average interference contrast decreases with condensate number $N$. Our experimental results agree well with the predictions of a random walk model. While the exact result can be given in terms of Kluyver’s formula, for a large number of sources a $1∕\sqrt{N}$ scaling of the average fringe contrast is obtained. This scaling law is found to be of more general applicability when quantifying the decay of coherence of an ensemble with $N$ independently phased sources.

Observation of coherent population trapping (CPT) at ground-state Zeeman sublevels of ${\mathrm{Cr}}^{3+}$ ion in ruby at room temperature is reported. A mechanism of CPT, not owing to optical pumping, is revealed in a situation when the optical pulse duration is shorter than the population decay time from the excited optical state.

The entanglement dependence of the geometric phase is studied in the case of two interacting spins under a time-independent magnetic field. A general entanglement-dependence geometric phase is formulated. This unification scheme covers the analyses by Sjöqvist for precessing spins and for isotropically interacting spins. From the general formula, the geometric phase is determined explicitly for an isotropic interaction and nonzero magnetic field.

We report on a proposal aimed at measuring the Casimir force in the cylinder-plane configuration. The Casimir force is evaluated including corrections due to finite parallelism, conductivity, and temperature. The range of validity of the proximity force approximation is also discussed. An apparatus to test the feasibility of a precision measurement in this configuration has been developed, and we describe both a procedure to control the parallelism and the results of the electrostatic calibration. Finally we discuss the possibility of measuring the thermal contribution to the Casimir force and deviations from the proximity force approximation, both of which are expected at relatively large distances.

A scheme of universal quantum computation is described that does not require local control. All the required operations, an Ising-type interaction and spatially uniform simultaneous one-qubit gates, are translation invariant.

We investigate the collective excitations of a low-temperature dilute gas mixture that consists of a Bose-Einstein condensate (BEC) and a Fermi gas that is a normal (i.e., nonsuperfluid) Fermi liquid. We find that the BEC-mediated fermion-fermion interactions, as a consequence of retardation, can become repulsive and support a zero-sound mode that is essentially undamped. In addition, we find a damped zero-sound mode that can be described as a BEC sound mode modified by fermion-mediated boson-boson interactions, and we derive its decay rate caused by Landau damping. We study the mode structure of these excitations and find avoided-crossing behavior as well as a termination point. The collective-mode dynamics also reveals that phase separation sets in when the fermion-mediated boson-boson interaction destroys the stability of the homogeneous BEC. We estimate the time and length scales of the onset of the phase separation, and we discuss the feasibility of experimentally probing these consequences of mediated interactions.

In this paper we study the nonlinear behavior of an electromagnetically induced transparency (EIT) resonance subject to a coherent driving field. The EIT is associated with a $\Lambda$ three-level system where two hyperfine levels within an electronic ground state are coupled to a common excited state level by a coupling field and a probe field. In addition there is an radio-frequency (rf) field driving a hyperfine transition within the ground state. The paper contrasts two different situations. In one case the rf-driven transition shares a common level with the probed transition and in the second case it shares a common level with the coupled transition. In both cases the EIT resonance is split into a doublet and the characteristics of the EIT doublet are determined by the strength and frequency of the rf-driving field. The doublet splitting originates from the rf-field induced dynamic Stark effect and has close analogy with the Autler-Townes effect observed in three-level pump-probe spectroscopy study. The situation changes when the rf field is strong and the two cases are very different. One is analogous to two $\Lambda$ three-level systems with EIT resonance associated with each. The other corresponds to a doubly driven three-level system with rf-field-induced electromagnetically induced absorption resonance. The two situations are modeled using numerical solutions of the relevant equation of motion of density matrix. In addition a physical account of their behaviors is given in terms of a dressed state picture.

Here, I discuss the propagation of an ultrashort pulse through a collection of harmonic multilevel systems. In the limit of weak excitation and a large number of excited states, I show that the amplitude of the input driving pulse decays exponentially with propagation distance. The absorption coefficient associated with this decay is determined by the characteristic time of the manifold of excited states, instead of the polarization decay time as in the conventional absorption coefficient of a two-level atom. The input ultrashort pulse creates in the excited states a wave packet, which oscillates emitting secondary pulses in the process. Analytic solutions are obtained that describe the propagation of individual wave-packet re-emission pulses, and it is shown that their phase depends on the detuning of the input pulse.

We examine the effect of spin-orbit coupling on geometric phases in hydrogenlike atoms exposed to a slowly varying magnetic field. The marginal geometric phases associated with the orbital angular momentum and the intrinsic spin fulfill a sum rule that explicitly relates them to the corresponding geometric phase of the whole system. The marginal geometric phases in the Zeeman and Paschen-Back limits are analyzed. We point out the existence of nodal points in the marginal phases that may be detected by topological means.

We present a scheme that makes use of quantum mechanical entanglement to significantly enhance the measurement precision accessible by interferometers. This is achieved by making use of a special type of interferometer composed of two “quantum beam splitters,” which each transform the input particles into a macroscopic superposition at the outputs. We show that this technique not only enables phase measurements to be made with Heisenberg limited precision, but also overcomes the major practical problems of detector inefficiencies and imperfect beam splitters. It may provide a promising route to implementing sub-shot-noise limited measurement schemes in the laboratory.

We study the role of the information deposited in the environment of an open quantum system in the course of the decoherence process. Redundant spreading of information—the fact that some observables of the system can be independently read off from many distinct fragments of the environment—is investigated as the key to effective objectivity, the essential ingredient of classical reality. This focus on the environment as a communication channel through which observers learn about physical systems underscores the importance of quantum Darwinism—selective proliferation of information about “the fittest states” chosen by the dynamics of decoherence at the expense of their superpositions—as redundancy imposes the existence of preferred observables. We demonstrate that the only observables that can leave multiple imprints in the environment are the familiar pointer observables singled out by environment-induced superselection (einselection) for their predictability. Many independent observers monitoring the environment will therefore agree on properties of the system as they can only learn about preferred observables. In this operational sense, the selective spreading of information leads to appearance of an objective classical reality from within the quantum substrate.

We derive the optimal input states and the optimal quantum measurements for estimating the unitary action of a given symmetry group, showing how the optimal performance is obtained with a suitable use of entanglement. Optimality is defined in a Bayesian sense, as minimization of the average value of a given cost function. We introduce a class of cost functions that generalizes the Holevo class for phase estimation, and show that for states of the optimal form all functions in such a class lead to the same optimal measurement. As a first application of the main result is the complete proof of the optimal efficiency in the transmission of a Cartesian reference frame. As a second application, we derive the optimal estimation of a completely unknown two-qubit maximally entangled state, provided that $N$ copies of the state are available. In the limit of large $N$, the fidelity of the optimal estimation is shown to be $1-3∕\left(4N\right)$.

We study the robustness of the Greenberger-Horne-Zeilinger (“cat”) class of multipartite states under decoherence. The noise model is described by a general completely positive map for qubits independently coupled to the environment. In particular, the robustness of $N$-party entanglement is studied in the large $N$ limit when (a) the number of spatially separated subsystems is fixed but the size of each subsystem becomes large and (b) the size of the subsystems is fixed while their number becomes arbitrarily large. We obtain conditions for entanglement in these two cases. Among our other results, we show that the parity of an entangled state (i.e., whether it contains an even or odd number of qubits) can lead to qualitatively different robustness of entanglement under certain conditions.

The inner-shell photoionization of highly charged, many-electron ions and their subsequent radiative decay are studied theoretically within the multiconfiguration Dirac-Fock approach. Special attention is paid to the linear polarization of the characteristic x-ray radiation. Detailed calculations have been carried out, in particular for the $2p^{5}3s\to 2p^6$ radiative transitions following ionization of the $2p$ electron of the sodiumlike iron ${\mathrm{Fe}}^{15+}$ and uranium ${\mathrm{U}}^{81+}$ ions. For these elements, the inner-shell photoionization was found to induce the (relatively) low linear polarization of the subsequently emitted photons which is strongly affected, moreover, by the higher-order (nondipole) effects in the electron-photon interaction.

We report first-principles calculations on the elastic scattering of low-energy electrons by ethylene, ${\mathrm{C}}_2{\mathrm{H}}_4$. We introduce a simple but effective method of representing polarization effects and show that it provides differential cross sections in close agreement with experiment.

An experimental study is presented on scattering of laser light from argon gas clusters irradiated by multipicosecond Nd:glass laser pulses at moderate intensity of ${10}^{15}\mathrm{W}∕{\mathrm{cm}}^2$. Space-resolved side-scattered laser light has a predominantly blueshifted and broadened spectrum (up to $\sim 8\mathrm{nm}$). The scattered signal intensity and average blueshift exhibit a marked dependence on the backing pressure of the gas. The results are explained by self-phase modulation of the laser radiation due to changing polarizability as the heated clusters pass through resonance at 3 times the critical density during which intense absorption and scattering occurs. The observed blueshift may be useful in diagnostics of this important phase of laser-cluster interactions.

A bichromatic near-resonant dark optical lattice (DOL) with rubidium atoms is demonstrated, which provides confinement in the Lamb-Dicke regime in all spatial dimensions. We apply spatially phase-matched optical potentials for each hyperfine ground state in order to enable improved Sisyphus cooling, undisturbed by optical hyperfine pumping processes. We also explore a method to increase the occupation of the DOL. Initially $2\times {10}^9$ rubidium atoms with a temperature of $70\mu \mathrm{K}$ and a density of $5\times {10}^{10}\text{atoms}∕{\mathrm{cm}}^3$ are prepared in a magneto-optic trap (MOT) and a fraction of $5\times {10}^7$ atoms is loaded into a far detuned one-dimensional optical lattice (FOL). Subsequently, the MOT is replaced by the DOL, and the atoms become well localized within the microscopic light-shift potentials at a temperature of $10\mu \mathrm{K}$ with a typical density of $3\times {10}^{11}\text{atoms}∕{\mathrm{cm}}^3$. We then apply alternating cycles of free evolution in the FOL and cooling and trapping in the DOL, obtaining a fourfold density increase to $1.2\times {10}^{12}\text{atoms}∕{\mathrm{cm}}^3$—i.e., 7.5% occupation—while maintaining a temperature of $10\mu \mathrm{K}$. In a final adiabatic cooling step we reduce the well depth to 75 times the single-photon recoil energy, which leads to a temperature of $2.8\mu \mathrm{K}$ and a phase-space density of $1.7\times {10}^{-3}$. Despite the increased density, no excess heating or collisional losses are observed.

Based on the results of a previous communication [Dimitrovski et al., Phys. Rev. Lett. 93, 083003 (2004)], we study ionization and excitation of a hydrogenic atom from the ground and first excited states in short electric field pulses of several cycles. A process of ionization and recombination which occurs periodically in time is identified, for both small and extremely large peak electric field strengths. In the limit of large electric peak fields closed-form analytic expressions for the population of the initial state after single- and few-cycle pulses are derived. These formulas, strictly valid for asymptotically large momentum transfer from the field, give excellent agreement with fully numerical calculations for all momentum transfers.