A new method of polarization of atoms and nuclei by pulsed bichromatic resonance radio-frequency fields is proposed to produce an initial state of a quantum computer constructed with the use of cubits representing impurity atoms in a solid matrix having the hyperfine structure. It is shown that this method can provide strong polarization without using ultralow temperatures. The problem is considered for the interaction of three-and four-level systems with bichromatic and two-phase fields.

We propose a new method for the amplification of high-order harmonics of laser radiation based on stimulated emission accompanying the propagation of a probe pulse of a high-order harmonic through a gas medium irradiated by a high-power pump wave. It is demonstrated that such an amplification becomes noticeable in the case when a probe wave of a high-order harmonic enters the interaction area with some delay time with respect to the pump wave. Our estimates for the gain of high-order harmonics show that this gain is proportional to the delay time, reaching unity even for rather short delay times (corresponding to several optical cycles of the pump wave).

We consider spontaneous generation of a high-order harmonic by an atomic beam passing through a focus produced by two coherent pump waves propagating at a small angle with respect to each other. It is demonstrated that, due to phase matching, the use of two coherent waves in a gas dispersive medium allows the efficiency of high-order harmonic generation to be considerably increased as compared with the case of a single wave with a doubled amplitude. Conditions permitting the optimization of harmonic generation are analyzed.

A new method of atomic and nuclear orientation and polarization is suggested. The method is based on the use of two coherent resonant phase-shifted monochromatic electromagnetic waves. It is demonstrated that a profound phase-shift-dependent level repopulation can be accomplished in a pulsed mode at room temperature. The possible application of this effect to quantum computers is briefly discussed.

A new method is proposed for cooling polarized neutrons by thermalizing neutrons in spherical and torroidal magnetic traps with elastic triplet scattering by cold, double spin-polarized, hydrogen atoms.

The problem of the amplification of high harmonics generated during the above-threshold ionization of atoms in a high-power laser wave field is examined for the first time. An estimate of the gain coefficient as a function of the parameters of the atom beam and the pump wave is given.

The coherent repopulation of a quantum system consisting of three nonequidistant levels in the field of a resonant bichromatic rf wave is studied. The atoms are assumed to have an impulsive interaction with the rf wave in which the pulse duration is less than any of the relaxation times. The hyperfine structure of gas atoms and a system of atomic oscillator levels in a magnetic trap are considered as examples of such a quantum system. It is shown that in the second case, the coherent repopulation effect can be used to cool neutral atoms in magnetic traps.

For phase-locked emitters, provided a certain relation exists between the pump wave and atomic beam parameters, a saturation effect is shown to be possible for which the intensity of the high-order harmonics ceases to depend on the atomic density. By means of a simple model that includes variations in the intensity of the pump wave in the plane transverse to the focal axis, an expression is obtained for the optimum atomic density of the medium corresponding to intensity saturation. The dependence of the optimum atomic density on the laser power and harmonic number obtained is found to be in qualitative agreement with recently published experimental data.

We consider the resonant interaction between atoms with hyperfine energy levels and a bichromatic radio-frequency field. Nuclear Zeeman levels of an impurity center in a magnetic host form a structure of this kind. Using the spin-density-matrix formalism, we solve the problem of coherent repopulation of a system of three of these levels under the action of a bichromatic resonant radio-frequency wave, taking into account transverse relaxation, and note the connection between this effect and the well-known phenomenon of coherent population capture when a laser bichromatic field interacts resonantly with a three-level system. We discuss various possibilities for observation of this effect experimentally.

A multiphoton photoelectric effect at a metal in an intense laser field is analyzed. The light is propagating along the surface of the metal. An interpretation is offered for the above-threshold peaks in the spectrum of photoelectrons. The Coulomb interaction of these electrons with the image potential of the electron cloud in the metal is taken into account in this interpretation. The observed electron spectrum can be explained on the basis of a multiple scattering of an electron by the image potential, with a field photon being captured in each scattering event. It is also shown that the above-threshold peaks arise in fields far weaker than the corresponding fields in the case of atoms (in above-threshold ionization).

Specific features of harmonic generation in a strong laser field are analytically considered. The proposed approach assumes a direct connection between the above-threshold ionization of atoms and harmonic generation. An expression governing harmonic intensities is derived in terms of the probability amplitudes of the above-threshold ionization of atoms corresponding to the model of multiple Coulomb scattering of a photoelectron on the potential of the residual ion (the MCS model). The results of calculations are compared with the experimental data.

The inner shell Auger effect arising as a result of the resonance excitation of a valence electron with the weak electromagnetic wave is considered. The residual Coulomb interaction between a valence electron and frame electrons lies in the background of the mechanism. In the assumption of the adiabatic setting up the wave field the probability for the Auger effect of inner electrons is found.