We present $\mathrm{}(e,2e)\mathrm{}$ measurements of the autoionizing region of Xe between the ${}^2{P}_{3/2}$ and ${}^2{P}_{1/2}$ ionic thresholds, which correspond to ejected-electron energies of less than 1.3 eV. In an $\mathrm{}(e,2e)\mathrm{}$ simulation of photoelectron spectroscopy we obtain the photoabsorption spectrum and the β-parameter spectrum. The results are in fair agreement with the true photoelectron data.

We study ionization processes of atomic hydrogen by antiproton impact in the energy range of 0.1–500 keV by the close-coupling method based on the B-spline expansion. Superposition of piecewise B-spline functions enables us to express the continuum wave functions more flexibly than the traditional pseudostate representation, in which overall functions such as the Sturmian are used for the expansion. The present expansion also remedies the defect of the traditional one-center expansion that the ionization cross section is underestimated at low energies owing to the finite range of the pseudostates. Our ionization cross sections agree excellently with recent two-center calculations at all the energies. The electron probability densities are also presented in both the coordinate and the momentum spaces.

Dynamics of the inner and outer above-barrier ionization and of the Coulomb explosion are calculated for large hydrogen iodide clusters irradiated by superintense ultrashort laser pulses. We have found that the Coulomb forces predominate in the expansion of these clusters in comparison with the hydrodynamic forces. The energy distribution of the iodine multiple atomic ions in laser focal volume is derived. Results of our calculations are in a good agreement with the recent experimental data of Tisch et al. [Phys. Rev. A 60, 3076 (1999)].

Starting from the two-mode Bose-Hubbard model, we derive an exact version of the standard Mathieu equation governing the wave function of a Josephson junction. For a finite number of particles N, we find an additional $\cos 2\phi$ term in the potential. We also find that the inner product in this representation is nonlocal in $\phi .$ Our model exhibits phenomena, such as $\pi$ oscillations, which are not found in the standard phase model, but have been predicted from Gross-Pitaevskii mean-field theory.

A destructive interference mechanism that protects atomic dark states from entanglement with undesirable motional degrees of freedom for atoms trapped inside an optical cavity is described in a number of examples. Estimates of the motional decoherence rate for dark states are given, and the rates are shown to be slower than naive Lamb-Dicke estimates would suggest. This may be practically useful in the implementation of quantum logic with trapped atoms in optical cavity QED using adiabatic passage techniques.

We consider one copy of a quantum system prepared in one of two nonorthogonal pure product states of multipartite distributed among separated parties. We show that there exist protocols that achieve optimal conclusive discrimination by means of local operations and classical communications as good as by global measurements. Also, we show a protocol that minimizes the average number of local operations. Our result implies that two pure product multipartite states might not have the nonlocal property although more than two can have.

We introduce a procedure to measure the density matrix of a material system. The density matrix is addressed locally in this scheme by applying a sequence of delayed-light pulses. The procedure is based on the stimulated Raman adiabatic passage technique. It is shown that a series of population measurements on the target state of the population transfer process yields unambiguous information about the populations and coherences of the addressed states, which, therefore, can be determined.

We present a technique for direct measurement of the Wigner distribution function of a traveling field, based on a Mach-Zehnder interferometer with a phase-sensitive element. In our scheme, the problem arising from the non-unity efficiency of the photodetectors is circumvented by the optical intensity of the input field for interferometry.

We demonstrate the formation of cold ground-state ${\mathrm{Cs}}_2$ molecules (presumably in a single well-defined rovibrational level) by stimulated Raman photoassociation (PA) in a Cs vapor cell magneto-optical trap. Autler-Townes measurements allow us to precisely define the experimental conditions. The efficiency of the process is comparable to that of the formation of cold molecules obtained after spontaneous deexcitation of photoassociated molecules. Stimulated Raman PA should provide samples where the molecules are cold not only translationally, but are also in a selected rovibrational level.

Using singlet S states of the helium atom as an example, precise calculation of energy levels in few-electron atoms is described. In particular, a complete set of effective operators is derived, which generates $O\left(m{\alpha }^6\right)$ relativistic and radiative corrections to the Schrödinger energy. Average values of these operators can be calculated using a variational Schrödinger wave function.

Although the conditions for performing arbitrary unitary operations to simulate the dynamics of a closed quantum system are well understood, the same is not true of the more general class of quantum operations (also known as superoperators) corresponding to the dynamics of open quantum systems. We propose a framework for the generation of Markovian quantum dynamics and study the resources needed for universality. For the case of a single qubit, we show that a single nonunitary process is necessary and sufficient to generate all unital Markovian quantum dynamics, whereas a set of processes parametrized by one continuous parameter is needed in general. We also obtain preliminary results for the unital case in higher dimensions.

In this paper, we consider the problem of trying to make an image of an object while minimizing the number of photons absorbed by the object. We call protocols that achieve this goal “minimal absorption measurements.” Such imaging techniques are particularly relevant in situations where the object may be damaged by the radiation used to make the image. Our main result is a bound that relates the minimum number of absorbed photons to the sensitivity of the measurement. This bound holds where $\epsilon ,$ half the difference between the transparencies, is sufficently small. In the case where the object consists of a single pixel, we show that the dependency of our bound on $\epsilon$ may be reproduced by simply counting the number of photons absorbed by the object or by a simple interferometric setup, depending on the details of the problem. In the case where the object consists of many pixels, we give an example where our bound may be approached when all the pixels are addressed collectively, whereas addressing each pixel individually implies an increase in the number of absorbed photons by a factor logarithmic in the number of pixels. Finally, we consider some special situations where our bound does not apply, and where interferometric methods may make large gains.

We propose a quantum computer scheme using spin waves for quantum-information exchange. We demonstrate that spin waves in the antiferromagnetic layer grown on silicon may be used to perform single-qubit unitary transformations together with two-qubit operations during the cycle of computation. The most attractive feature of the proposed scheme is the possibility of random access to any qubit and, consequently, the ability to recognize two qubit gates between any two distant qubits. Also, spin waves allow us to eliminate the use of a strong external magnetic field and microwave pulses. By estimate, the proposed scheme has as high as ${10}^4$ ratio between quantum system coherence time and the time of a single computational step.

A set of quantum states can be unambiguously discriminated if and only if they are linearly independent. However, for a linearly dependent set, if C copies of the state are available, then the resulting C particle states may form a linearly independent set, and be amenable to unambiguous discrimination. We obtain one necessary and one sufficient condition for the possibility of unambiguous discrimination among N states given that C copies are available and that the single copies span a D-dimensional space. These conditions are found to be identical for qubits. We then examine in detail the linearly dependent trine ensemble. The set of $C>\mathrm{}1\mathrm{}$ copies of each state is a set of linearly independent lifted trine states. The maximum unambiguous discrimination probability is evaluated for all $C>\mathrm{}1\mathrm{}$ with equal a priori probabilities.

We study the interrelation between high-order harmonic generation (HHG) and above-threshold ionization (ATI) in the frequency domain. HHG can be described simply as an ATI followed by laser assisted recombination (LAR). The plateau reflects mainly the characteristics of LAR. We also study the correspondence between frequency-domain and time-domain pictures of HHG.

The Volkov solutions for the Dirac equation are used to describe the dynamics of an electronic wave packet interacting with a very strong laser pulse in the relativistic regime. By means of a projection method we explore the regimes of long and short laser wavelength compared to the wave packet dimensions. Comparisons with classical electron trajectories show that the behavior of the expectation value of the operators becomes different as we enter to the short-wavelength regime. In addition, a study of the wave packet evolution shows unexpected features in this regime. The calculations are completed with an analysis of the total wave function in terms of the dynamics of its positive and negative energy components.

We study the effect of interatomic correlations beyond the mean-field approximation on the ground-state properties of a trapped Bose gas interacting through a repulsive potential. To this end we employ a variational procedure that is based on the energy functional derived, in the local-density approximation, using the low-density expansion for the ground-state energy of the homogeneous gas. We also make analytic estimates of the corrections due to the higher-order terms in the expansion. We find that corrections are of the order of $1\%.$ However, there is a qualitative change in the density profile for large N (number of atoms) due to presence of a logarithmic term in the expansion.

We introduce a linear, canonical transformation of the fundamental single-mode field operators a and $a^{†}$ that generalizes the linear Bogoliubov transformation familiar in the construction of the harmonic oscillator squeezed states. This generalization is obtained by adding a nonlinear function of any of the fundamental quadrature operators $X_1$ and $X_2$ to the linear transformation, thus making the original Bogoliubov transformation quadrature dependent. Remarkably, the conditions of canonicity do not impose any constraint on the form of the nonlinear function, and lead to a set of nontrivial algebraic relations between the c-number coefficients of the transformation. We examine in detail the structure and the properties of the quantum states defined as eigenvectors of the transformed annihilation operator b. These eigenvectors define a class of multiphoton squeezed states. The structure of the uncertainty products and of the quasiprobability distributions in phase space shows that besides coherence properties, these states exhibit a squeezing and a deformation (cooling) of the phase-space trajectories, both of which strongly depend on the form of the nonlinear function. The presence of the extra nonlinear term in the phase of the wave functions has also relevant consequences on photon statistics and correlation properties. The nonquadratic structure of the associated Hamiltonians suggests that these states be generated in connection with multiphoton processes in media with higher nonlinearities. We give a detailed description of the quadratic nonlinear transformation, which defines four-photon squeezed states. In particular, the behaviors of the second-order correlation function $g^{\mathrm{}\left(2\right)\mathrm{}}\mathrm{}\left(0\right)\mathrm{}$ and of the fourth-order correlation function $g^{\mathrm{}\left(4\right)\mathrm{}}\mathrm{}\left(0\right)\mathrm{}$ are studied. The former exhibits super-Poissonian statistics, while the latter indicates photon bunching in the four-photon emissions.

An optical Einstein-Podolsky-Rosen (EPR) state is produced in the spontaneous parametric down-conversion of an ultrashort classical pump pulse. It is shown both theoretically and experimentally that this results in the violation of a Bell-type inequality of the kind proposed by Grangier et al. The experiment is based on measuring interference between the light in the EPR state and weak light pulses in a coherent state. The maximum observed visibility of the interference pattern was $(89\mathrm{}\pm 4)\%.$ This interference can be regarded as a manifestation of nonlocality in the sense described by Banaszek and Wodkiewicz, even though the EPR state has a positive Wigner function. We develop the theory of two-channel homodyne detection with ultrafast pulses and then apply it to the EPR state generated in a type-II collinear geometry. Particular attention is paid to the investigation of how dispersion and phase matching influence the violation of local realism.

We give a (remote) quantum-gambling scheme that makes use of the fact that quantum nonorthogonal states cannot be distinguished with certainty. In the proposed scheme, two participants Alice and Bob can be regarded as playing a game of making guesses on identities of quantum states that are in one of two given nonorthogonal states: if Bob makes a correct (an incorrect) guess on the identity of a quantum state that Alice has sent, he wins (loses). It is shown that the proposed scheme is secure against the nonentanglement attack. It can also be shown heuristically that the scheme is secure in the case of the entanglement attack.