We propose a dynamical approach to quantum memories using an oscillator-cavity model. This overcomes the known difficulties of achieving high quantum input-output fidelity with long storage times compared to the input signal duration. We use a generic model of the memory response, which is applicable to any linear storage medium ranging from a superconducting device to an atomic medium. The temporal switching or gating of the device may either be through a control field changing the coupling, or through a variable detuning approach, as in more recent quantum memory experiments. An exact calculation of the temporal memory response to an external input is carried out. This shows that there is a mode-matching criterion which determines the optimum input and output pulse time evolution. This optimum pulse shape can be modified by changing the gate characteristics. In addition, there is a critical coupling between the atoms and the cavity that allows high fidelity in the presence of long storage times. The quantum fidelity is calculated both for the coherent state protocol, and for a completely arbitrary input state with a bounded total photon number. We show how a dynamical quantum memory can surpass the relevant classical memory bound, while retaining long storage times.

So far, various multiphoton entangled states have been observed experimentally by using different experimental setups. We present a scheme to realize many stochastic local operations and classical communication inequivalent states of three and four qubits via projective measurements on suitable entangled states. We demonstrate how these states can be observed experimentally in a single setup and study the feasibility of implementation with present-day technology.

We demonstrate two types of basic Bloch vector rotations in thulium ion ensemble embedded in yttrium aluminum garnet crystal based on our previous work that designed the appropriate Hamiltonians driving Bloch vector rotations through geometric evolution in two-level atom. The angles of the geometric rotations were controlled by the relative phases of the laser pulses driving the rotations. The rotation angles were measured using photon echoes and compared with the corresponding theoretical values. The operation fidelity was calculated for both demonstrated rotations that resulted in average values of 94.00 and 99.96%, respectively. The schemes to improve the operation fidelity will be discussed.

In the photoelectron spectrum of ${\mathrm{N}}_2$ the apparent ionization energy to form the $B{}^2\Sigma _u^{+}$ state increases linearly with the photon energy. Rotationally resolved measurements of the fluorescent decay of this state show a linear increase of rotational heating with increasing photon energy. These results are in quantitative agreement with the prediction of the theory of recoil-induced rotational excitation, indicating that the rotational heating that has been observed previously arises primarily from such recoil-induced excitation. Together with other results that have been reported they show that recoil-induced internal excitation is significant in many situations, including near threshold.

We present a configuration-interaction (CI) method based on Sturmian functions. The components of this CI basis are the solutions of a two-body Sturmian eigenproblem, where the eigenvalues are related to the interacting potential in the two-body equation. Our method accommodates any arbitrary, physically sound, central potential in the Sturmian equations and different adequate asymptotic conditions. Computation of eigenvalues and eigenfunctions is performed by direct numerical discretization of the Sturmian equation. We apply this method to obtain bound states for two-electron systems. We show the convergence of the partial-wave expansion for the ground-states energies of the He atom and the ${\mathrm{H}}^{-}$ ion, and obtain very accurate results that are compared with other recent CI calculations.

We study the coupling of resonant electronic excitations to vibrational center-of-mass motion of ultracold atoms in an optical lattice, when ground- and excited-state optical potentials differ. On-site excitation-vibration coupling can be approximately treated by renormalizing the effective transition energy, which appears, e.g., in lattice clock setups. In addition the dipole-dipole coupling between neighboring sites mediates emission and absorption of vibrational quanta in conjunction with optical transitions. This mechanism for excitation of vibrations can lead to significant heating of the lattice atoms motion during resonant light interaction. We calculate estimates of the corresponding parameters for a gas in the Mott-insulator state from a perturbation expansion in small atomic displacements.

The $KLL$ Auger spectrum of atomic aluminum has been recorded applying electron impact for the creation of $1s$ core hole states. The experimental results are compared to predictions obtained with ab initio calculations for spectral structures.

From an extensive calculation of static properties of a trapped Fermi superfluid at zero temperature using a density-functional formulation, we demonstrate a universal behavior of its observables, such as energy, chemical potential, radius, etc., over the crossover from the BCS limit to unitarity leading to scaling over many orders of magnitude in fermion number. This scaling allows us to predict the static properties of the system, with a large number $(\sim {10}^5)$ of fermions, over the crossover with an error of 1%–2%, from the knowledge of those for a small number $(\sim 10)$ of fermions.

We demonstrate that certain domain structures can be created both in ferromagnetic and antiferromagnetic spinor condensates if the initial phase is spatially modulated. Meanwhile, spin dynamics of the condensate with modulated phases exhibits exotic features in comparison with those of a condensate with a uniform phase. We expect that these phenomena could be observed experimentally using a phase-imprinting method.

A quantum theory of light propagation in two optical channel waveguides tunneling coupled to a common continuum of modes (such as those of a slab waveguide) is presented, and classical and quantum interference effects are investigated. For classical light, the photonic system realizes an optical analog of coherent population trapping in the continuum encountered in atomic physics, where destructive interference between different light leakage channels leads to the appearance of a trapped state embedded in the continuum. For nonclassical light, two-photon interference effects are predicted, such as the tendency of photon pairs to bunch when decaying into the continuum.

We propose and analyze a physical implementation of two-qubit parity measurements as required for continuous error correction, assuming a setup in which the individual qubits are strongly coupled to separate optical cavities. A single optical probe beam scatters sequentially from the two cavities, and the continuous parity measurement is realized via fixed quadrature homodyne photodetection. We present models based on quantum stochastic differential equations (QSDEs) for both an ideal continuous parity measurement and our proposed cavity quantum electrodynamics (cavity QED) implementation; a recent adiabatic elimination theorem for QSDEs is used to assert strong convergence of the latter to the former in an appropriate limit of physical parameters. Performance of the cavity QED scheme is studied via numerical simulation with experimentally realistic parameters.

We have studied the decoherence properties of adiabatic quantum computation (AQC) in the presence of in general non-Markovian, e.g., low-frequency, noise. The developed description of the incoherent Landau-Zener transitions shows that the global AQC maintains its properties even for decoherence larger than the minimum gap at the anticrossing of the two lowest-energy levels. The more efficient local AQC, however, does not improve scaling of the computation time with the number of qubits $n$ as in the decoherence-free case. The scaling improvement requires phase coherence throughout the computation, limiting the computation time and the problem size $n$.

We employ reduced-density-matrix-functional theory in the calculation of the fundamental gap of open-shell systems. The formula for the calculation of the fundamental gap is derived with special attention to the spins of the neutral and the charged systems. We discuss the effects of different functionals as well as the changes due to different basis sets. Also, we investigate the importance of varying the natural orbitals for the calculation of the fundamental gap.

The autoionizing Rydberg states of krypton close to the ${}^{2}P_{1∕2}$ ionization limit were measured using a tunable narrow-bandwidth vacuum-ultraviolet laser system. The hyperfine structure of the high odd-parity autoionizing Rydberg states of ${}^{83}\mathrm{Kr}$ was observed and analyzed. The measurements and their analysis by multichannel quantum defect theory provided information on the role of the nuclear spin in the photoionization of ${}^{83}\mathrm{Kr}$ and on the hyperfine structure of the ${}^{2}P_{1∕2}$ state of ${}^{83}\mathrm{Kr}^{+}$. The ${}^{2}P_{1∕2}$ ionization energies of all naturally occurring Kr isotopes were determined with great precision and accuracy, improving the existing literature values of the isotope-averaged ionization energy by more than one order of magnitude.

With a Stark decelerator, beams of neutral polar molecules can be accelerated, guided at a constant velocity, or decelerated. The effectiveness of this process is determined by the six-dimensional (6D) volume in phase space from which molecules are accepted by the Stark decelerator. Couplings between the longitudinal and transverse motion of the molecules in the decelerator can reduce this acceptance. These couplings are nearly absent when the decelerator operates such that only every third electric-field stage is used for deceleration, while extra transverse focusing is provided by the intermediate stages. For many applications, the acceptance of a Stark decelerator in this so-called $s=3$ mode significantly exceeds that of a decelerator in the conventionally used $(s=1)$ mode. This has been experimentally verified by passing a beam of OH radicals through a $2.6\text{−}\mathrm{m}$-long Stark decelerator. The experiments are in quantitative agreement with the results of trajectory calculations, and can qualitatively be explained with a simple model for the 6D acceptance. These results imply that the 6D acceptance of a Stark decelerator in the $s=3$ mode of operation approaches the optimum value, i.e., the value that is obtained when any couplings are neglected.

In a recent experiment [Phys. Rev. Lett. 100, 040404 (2008)], an analog of photon-assisted tunneling was observed for a Bose-Einstein condensate in an optical lattice subject to a constant force plus a sinusoidal shaking. Contrary to previous theoretical predictions, the width of the condensate was measured to be proportional to the square of the effective tunneling matrix element, rather than a linear dependence. For a simple model of two interacting bosons in a one-dimensional optical lattice, both analytical and numerical calculations indicate that such a transition from a linear to a quadratic dependence can be interpreted through the ballistic transport and the corresponding exact dispersion relation of bound boson pairs.

Ultracold atom experiments offer unprecedented potential for the quantitative comparison of quantum many-body effects with theory. This comparison is hindered by the lack of numerical methods capable of dealing with these large inhomogeneous three-dimensional systems. In this paper we demonstrate the applicability of a highly efficient numerical method based on the Gutzwiller approximation for calculating the ground state of the Bose-Hubbard model in the regimes of the recent cold atom experiments. The numerical method is applied to the experimental regime employed by Campbell et al. [Science 314, 281 (2006)], where Mott-insulator shells were directly imaged via atomic clock shifts. We calculate several quantities that are closely related to the current experimental measurement methods: average number distributions, momentum distributions, two-photon spectra, and three-body recombination loss rates. Our results shed light on a number of features of the experimental observations and clarify both the crossover behavior around the superfluid–Mott-insulator transition point and the experimentally observed three-body decay rate of the Mott-insulator shells.

We investigate decoherence in atom interferometry due to scattering from a background gas and show that the supposition that residual coherence is due to near-forward scattering is incorrect. In fact, the coherent part is completely unscattered, although it is phase shifted. This recoil-free process leaves both the atom and the gas in an unchanged state, but allows for the acquisition of a phase shift. This is essential to understanding decoherence in a separated-arm atom interferometer, where a gas of atoms forms a refractive medium for a matter wave. Our work elucidates the actual microscopic, many-body, quantum-mechanical scattering mechanism that gives rise to prior phenomenological results for the phase shift and decoherence.

We examine the operation of a device for a public quantum key distribution network. The recipients attempt to determine whether or not their individual key copies, which are a sequence of coherent states, are identical. To quantify the success of the protocol we use a fidelity-based figure of merit and describe a method for increasing this in the presence of noise and imperfect detectors. We show that the fidelity may be written as the product of two factors: one that depends on the properties of the device setup and another that depends on the detectors used. We then demonstrate the effect various parameters have on the overall effective operation of the device.

We demonstrate coherent diffraction imaging using multiple harmonics from a high-harmonic generation source. An algorithm is presented that builds the known incident spectrum into the reconstruction procedure with the result that the useable flux is increased by more than an order of magnitude. Excellent images are obtained with a resolution of $(165\pm 5)\mathrm{nm}$ and compare very well with images from a scanning electron microscope.