Recently several techniques demonstrated the production of elliptically polarized high harmonics. One of these techniques consists of the interaction in a noble gas of two-color laser beams having orthogonal linear polarizations. Here we present the theoretical explanation of such a result observed in Lambert et al. [ Nat. Commun. 6, 6167 (2015)]. Numerical calculations based on the nonperturbative light-atom interaction theory reproduce well the experimental data. The degree of polarization is analyzed for different harmonic orders and found to be high. With the help of a simplified theoretical model it is shown that the degree of harmonic ellipticity depends mainly on the population of atomic state sublevels with different angular momentum projections.

A quantum memristor combines the memristive dynamics with the quantum behavior of the system. We analyze the idea of a quantum memristor based on ultracold ions trapped in a Paul trap. Corresponding input and output memristor signals are the ion electronic levels populations. We show that under certain conditions the output/input dependence is a hysteresis curve similar to classical memristive devices. This behavior becomes possible due to the partial decoherence provided by the feedback loop, which action depends on previous state of the system (memory). The feedback loop also introduces nonlinearity in the system. Ion-based quantum memristor possesses several advantages comparing to other platforms—photonic and superconducting circuits—due to the presence of a large number of electronic levels with different lifetimes as well as strong Coulomb coupling between ions in the trap. The implementation of the proposed ion-based quantum memristor will be a significant contribution to the novel direction of “quantum neural networks”.

We present the results of the numerical study of the generation of the elliptical terahertz (THz) radiation by an extended argon gas media interacting with a two-color laser field. On the base of a single-atom response to the action of the two-color laser field with a linear polarization of each of the components of the laser field and different angles between them, two angles (90° and 45°) and radiation at two frequencies (56 and 29 THz) corresponding to them and having relatively high values of ellipticities have been chosen for the calculation of the extended gas response. The results obtained for different widths and lengths (up to 20 cm) of the gas demonstrate the cone structure of the generated radiation together with a non-monotonical distribution of the ellipticity. Methods of control of the value of the ellipticity are presented and discussed. The single-atom investigations have been carried out with the help of the non-perturbative theory for the ionization-free regime of interaction.

Excitation of the isomeric nuclear state ${}^{229\mathrm{m}}\mathrm{Th}$ in the process of thorium atom irradiation by a two-color femtosecond pulse of a Ti:Sa laser at the fundamental wavelength and second harmonic ($\omega +2\omega$) is analyzed. It is shown that the rate of isomeric state excitation can be enhanced significantly with respect to other nucleus excitation processes in laser plasma or by an external coherent source at the resonance wavelength. This enhancement is due to the process of “nonlinear laser nuclear excitation,” which consists in excitation of a nuclear transition by e.m. radiation, generated by an electronic shell of the atom interacting with an intense laser field, with the photon energy coinciding with the nuclear transition energy. The process is based on the following. First, the atomic current associated with the motion of valence electrons in a strong $\omega +2\omega$ field is a nonlinear function of the field amplitude. Second, the energy of photons at the frequency of the Ti:Sa laser fifth harmonic coincides with the energy of the ${}^{229\mathrm{m}}\mathrm{Th}$ state. The yield of this harmonic is much higher if the two-color pulse (first and second harmonics of the fundamental) is used. Third, the intensity of the field produced by valence electrons at the atomic nucleus reaches the near-atomic value, which exceeds significantly the field strength associated with any other mechanism of laser plasma emission.