Gaussian-Klauder coherent states are applied to the bound “quantum bouncer,” a gravitating particle above an infinite potential boundary. These Gaussian-Klauder states, originally created for Rydberg atoms, provide an overcomplete set of wave functions that mimic classical trajectories for extended times through the utilization of energy localization. For the quantum bouncer, analytic methods are applied presently to compute first and second moments of position and momentum operators, and from these results, at least two scalings of Gaussian-Klauder parameters are highlighted, one of which tends to remains localized for markedly more bounces than comparable states that are Gaussian in position (by an order of magnitude in some cases). We close with a connection that compares Gaussian-Klauder states and positional Gaussian states directly for the quantum bouncer, relating the two through a known energy-position duality of Airy functions. Our results, taken together, ultimately reemphasize the primacy of energy localization as a key ingredient for long-lived classical correspondence in systems with smooth spectra.

A scheme is presented for the long-distance teleportation of an unknown atomic state between two separated cavities. Our scheme works in the regime where the atom-cavity coupling strength is smaller than the cavity decay rate. Thus the requirement on the quality factor of the cavities is greatly relaxed. Furthermore, the fidelity of our scheme is not affected by the detection inefficiency and atomic decay. These advantages are important in view of experiments.

The use of $d$-state systems, or qudits, in quantum information processing is discussed. Three-state and higher-dimensional quantum systems are known to have very different properties from two-state systems—i.e., qubits. In particular there exist qudit states which are not equivalent under local unitary transformations unless a selection rule is violated. This observation is shown to be an important factor in the theory of decoherence-free, or noiseless, subsystems. Experimentally observable consequences and methods for distinguishing these states are also provided, including the explicit construction of new decoherence-free or noiseless subsystems from qutrits. Implications for simulating quantum systems with quantum systems are also discussed.

Recently a long distance and high key rate quantum key distribution (QKD) has become possible by the idea of the decoy state method. We show that a longer distance QKD is possible by utilizing a heralded single photon source (utilizing spontaneous parametric down-conversion) as a source instead of a weak coherent pulse (WCP) as proposed in the original decoy state method. Moreover, the key rate is improved by utilizing a presently available photon number resolving detector as a trigger detector of the heralded single photon source and it is shown to approach the key rate of the WCP.

Conventional quantum key distribution (QKD) protocols include a basis selection process for providing a secure secret key. In contrast, this paper proposes an entanglement-based QKD with no basis selection procedure. Entangled-photon pulse trains with an average photon number less than one per pulse are sent to two legitimate parties, from which a secret key is created utilizing the entanglement nature. Eavesdropping on a transmission line is prevented by a condition of less than one photon per pulse, and sending classically correlated coherent pulses instead of quantum correlated ones is revealed by monitoring coincident count rates.

We present a method for the solution of the behavior of an ensemble of qubits in a random time-dependent external field. The forward evolution in time is governed by a transfer matrix. The elements of this matrix determine the various decoherence times. The method provides an exact solution in cases where the noise is piecewise constant in time. We show that it applies, for example, to a realistic model of decoherence of electron spins in semiconductors. Results are obtained for the nonperturbative regimes of the models, and we see a transition from weak relaxation to overdamped behavior as a function of noise anisotropy.

The composite rotation approach has been used to develop a range of robust quantum logic gates, including single qubit gates and two qubit gates, which are resistant to systematic errors in their implementation. Single qubit gates based on the BB1 family of composite rotations have been experimentally demonstrated in a variety of systems, but little study has been made of their application in extended computations, and there has been no experimental study of the corresponding robust two qubit gates to date. Here we describe an application of robust gates to nuclear magnetic resonance studies of approximate quantum counting. We find that the BB1 family of robust gates is indeed useful, but that the related NB1, PB1, B4, and P4 families of tailored logic gates are less useful than initially expected.

We describe a simple model for electron-molecule collisions that has one nuclear and one electronic degree of freedom and that can be solved to arbitrarily high precision, without making the Born-Oppenheimer approximation, by employing a combination of the exterior complex scaling method and a finite-element implementation of the discrete-variable representation. We compare exact cross sections for vibrational excitation and dissociative attachment with results obtained using the local complex potential approximation as commonly applied in the “boomerang” model, and suggest how this two-dimensional model can be used to test the underpinnings of contemporary nonlocal approximations to resonant collisions.

The probabilities of various excitations or ionization processes are determined in a number of light atoms during elastic and nonelastic neutron-nucleus scattering. In particular, we have evaluated the final-state probabilities in the cases of elastic scattering of fast neutrons at the hydrogen and helium atoms. Atomic excitation or ionization probabilities have been determined numerically for the neon atom. This study is an important step in our understanding of numerous optical phenomena which can be observed when an intense flux of fast neutrons propagates in a dense medium.

An analytic study is presented of the asymptotic properties of the three-Coulomb-center problem $e{Z}_{1}ZZ$. The electron energies and wave functions of this system, where $e$ designates an electron and $Z$, $Z_1$ are bare nuclei, are calculated asymptotically exactly for large distances $L$ between the fragments of a quasimolecular system. The electron exchange interaction between $eZZ$ and $Z_1$ fragments is also calculated asymptotically exactly and used to estimate the electron-capture cross section in slow collisions of $Z=1$ and $Z_1=2,3,4$ systems.

The phenomenon of Coulomb explosion is studied through qualitative numerical simulations of clusters irradiated with intense ultrashort laser pulses. We introduce a semiquantum approach which allows us to model two different types of materials—akin to rare gases and dielectrics—and which is appropriate for both low- and high-energy domains, i.e., the thermodynamic regime and the Coulomb explosion regime. Through a detailed study of clusters submitted to laser pulses of various intensities, we demonstrate that Coulomb explosion is the process responsible for cluster explosion under femtosecond laser pulses. We examine the differences in the dynamics of explosion of rare-gas clusters as a function of the wavelength of the incident laser radiation. For dielectric clusters, our simulations reveal a fragmented explosion mechanism; the influence of the size of the cluster is also studied.

Recombination and bremsstrahlung radiation yields from strong field rescattering of $9\times {10}^{14}\mathrm{W}∕{\mathrm{cm}}^2\left({\mathrm{Ne}}^{+}\right)\text{to}3\times {10}^{17}\mathrm{W}∕{\mathrm{cm}}^2\left({\mathrm{Ne}}^{8+}\right)$ photoelectrons show the laser magnetic field alters the rescattering dynamics for fields beyond one-atomic unit of field $(3\times {10}^{16}\mathrm{W}∕{\mathrm{cm}}^2)$. At $3\times {10}^{16}\mathrm{W}∕{\mathrm{cm}}^2$, the deflection of the rescattering wave begins to exceed its transverse width and the yields of photons with energies above $100\text{hartree}$ of energy $\left(2.7\mathrm{keV}\right)$ are attenuated. Radiation at $3\times {10}^{17}\mathrm{W}∕{\mathrm{cm}}^2\left({\mathrm{Ne}}^{8+}\right)$ is suppressed by ${10}^2$ for the highest energy photons near $1000\text{hartree}$ and by ${10}^5$ for lower energy in the range of $10–100\text{hartree}$.

Adiabatic dressed state potentials are created when magnetic substates of trapped atoms are coupled by a radio-frequency field. We discuss their theoretical foundations and point out fundamental advantages over potentials purely based on static fields. The enhanced flexibility enables one to implement numerous configurations, including double wells, Mach-Zehnder, and Sagnac interferometers which even allows for internal state-dependent atom manipulation. These can be realized using simple and highly integrated wire geometries on atom chips.

Numerical simulations are presented for the output of a Ne-like Ti soft x-ray laser at $32.6\mathrm{nm}$ driven by the grazing-incidence pumping (GRIP) scheme, using a modified one-dimensional (1D) lagrangian hydrodynamic code MED103 coupled with an atomic physics data package and a 2D RAYTRACE code as a postprocessor. We predict that saturation can be achieved at a plasma length of $0.2\mathrm{cm}$ by an optimized driving configuration with an energy of only $200\mathrm{mJ}$. We also compare the output beam quality of the Ne-like Ti soft x-ray laser generated by an optimized pump energy of $200\mathrm{mJ}$ and a higher energy of $385\mathrm{mJ}$. Better spatial coherence is expected for a higher pump energy.

The quantum characteristics of sum-frequency process in an optical cavity with an input signal optical beam, which is a half of entangled optical beams, are analyzed. The calculated results show that the quantum properties of the signal beam can be maintained after its frequency is conversed during the intracavity nonlinear optical interaction. The frequency-conversed output signal beam is still in an entangled state with the retained other half of the initial entangled beams. The resultant quantum correlation spectra and the parametric dependences of the correlations on the initial squeezing factor, the optical losses and the pump power of the sum-frequency cavity are calculated. The proposed system for the frequency conversion of the entangled state can be used in quantum communication network and the calculated results can provide direct references for the design of experimental systems.

We present a theoretical model describing the magnetic-state population dynamics of spin-degenerate two-level atoms interacting with a narrowband, on-resonance, partially polarized electromagnetic field. The field is allowed to have three uncorrelated orthogonal vector components. The model is applied to a four-magnetic-state atom system with a single excited and three ground states. Even if the field is narrowband, the population dynamics may be completely predicated by the fluctuating polarization of light. In our examples, the fluctuation effects are mainly governed by a single parameter, the degree of polarization of the field.

Given a quantum pure state chosen from a set with some a priori probabilities, what is the optimal measurement needed to correctly guess the given state? We show that a good choice is the family of square-root or “pretty good” measurements, as each measurement in the family is optimal for at least one discrimination problem with the same quantum states but possibly different a priori probabilities. Furthermore, the map from measurement to discrimination problems can be explicitly described. In fact, for linearly independent states, every pair of discrimination problem and optimal measurement can be explicitly generated this way.

It is demonstrated how data for internal-conversion coefficients may be decomposed with good accuracy into terms scaling with different powers of the atomic number. The individual terms are identified as a near-field contribution, a contribution due to photon emission and subsequent atomic photoionization, and interference. The photoeffect channel is generally important for high atomic numbers. The ratio of the photoeffect component to the near-field contribution is consistent with a localization of the relativistic photoeffect to distances from the nucleus not substantially exceeding the Compton wavelength of the electron irrespectively of atomic number and shell. Such localization is in accordance with results of an impact-parameter study. As a by-product a fitting formula is produced for electric dipole transitions involving the $K$ shell. The formula which is accurate to the level of $\simeq 1\%$ covers a variation of the conversion coefficient over nearly four decades.

We present calculations of cross sections and rate coefficients for the dissociative recombination of $\mathrm{Ne}_2{}^{+}$, following collisions with low-energy electrons $(<1\mathrm{eV})$. The resonance energies and autoionization widths for the doubly excited states of ${\mathrm{Ne}}_2$ lying between the ground and first excited states of the ion are computed from electron scattering calculations using the complex Kohn variational method. The dynamics of the dissociative recombination process is investigated using multichannel quantum defect theory. The calculated absolute rate coefficient at $300\mathrm{K}$ is in good agreement with the existing measurement. The rate coefficient is found to decrease with electron temperature as $T^{-0.5}$ in the low-electron-temperature region $(T_e<1000\mathrm{K})$, where the process is dominated by the lowest ${}^{1,3}\Sigma _g^{+}$ dissociative states. For the high-temperature region $(T_e>1000\mathrm{K})$, where the contribution from remaining dissociative states (lowest ${}^{1,3}\Sigma _u^{+}$, ${}^{1,3}\Pi _g$, ${}^{1,3}\Pi _u$ and second resonance states of these symmetries) become more important, the rate coefficients decrease faster and eventually behave approximately as $T^{-1.5}$. We also present cross sections for competing electron-impact vibrational excitation and deexcitation of $\mathrm{Ne}_2{}^{+}$ obtained from the same calculations.

In order to probe nanostructures on a surface we present a microscope based on the quantum recurrence phenomena. A cloud of atoms bounces off an atomic mirror connected to a cantilever and exhibits quantum recurrences. The times at which the recurrences occur depend on the initial height of the bouncing atoms above the atomic mirror, and vary following the structures on the surface under investigation. The microscope has inherent advantages over existing techniques of scanning tunneling microscope and atomic force microscope. Presently available experimental technology makes it possible to develop the device in the laboratory.