We present a brief overview of the current state of the experimental research on the development of the element base of quantum computers with qubits based on single neutral atoms trapped in optical traps. The discussion focuses on the requirements for qubits, peculiarities of single neutral atoms as qubits, methods for the quantum register development and for the implementation of single-qubit quantum logic operations in the laser and microwave fields, and two-qubit operations through the dipole–dipole interaction after a short laser excitation of atoms to the Rydberg states. The results of the experiments on the observation of the interaction of two and three Rydberg atoms by a Förster resonance controlled by dc and radio-frequency electric field are presented.
We have implemented a simple digital system for long-term frequency stabilization and locking to an arbitrary wavelength of the single-frequency ring CW Ti:Sapphire laser. This system is built using two confocal Fabry-Pérot cavities, one of which is used to narrow the short-term linewidth of the laser and the other to improve the long-term stability of the laser frequency. The length of the second cavity is stabilized using the radiation from an external-cavity diode laser locked to an atomic transition. Our system is an improvement of a commercial Tekhnoscan laser lock. This system has been successfully used in our experiments on high-resolution laser spectroscopy of ultracold rubidium Rydberg atoms.
We have developed a simple theoretical model describing multi-atom signals that could be measured in experiments on
resonant energy transfers in an ensemble of a few Rydberg atoms. We have shown that an efficiency of the selective field-ionization
detector, which is less than 1, leads to the mixing up of the spectra of the resonant energy transfer registered for
various numbers of detected Rydberg atoms. This may impede the possible observations of dipole blockade or coherent two-atom
interaction required to perform basic quantum gates. The formulae are presented, which help to estimate an actual mean
Rydberg atom number in an excitation volume per one exciting laser pulse at a given detection efficiency. We have also
found that a measurement of relationship of the amplitudes of resonances observed in the one- and two-atom signals provides
a straightforward determination of the absolute detection efficiency and actual mean Rydberg atom number. This novel
method is advantageous as it is independent of the specific experimental conditions.
We also performed a testing experiment on the resonant energy transfers in a small excitation volume of the velocity selected
Na atomic beam. The observed one- and two-atom resonances were analyzed and compared with the theoretical predictions.
A good agreement between experiment and theory in the width and amplitudes of the resonances has confirmed the validity
of simple approximations used in the developed theoretical model.
Experimental aspects of an application of Rydberg atoms to quantum computing are studied. A single neutral atom trapped in an antinode of the optical lattice can represent a quantum bit. Laser excitation of two atoms in neighboring antinodes allows for obtaining of quantum entanglement of the atoms via dipole-dipole interaction which is strong for high Rydberg states. A two-qubit operation could be realized in this way. The optimal values of a principal quantum number, an interatomic distance, time of a single two-qubit operation and other parameters have been estimated. The estimates were done for <sup>23</sup>Na and <sup>87</sup>Rb atoms. Also experimental results of microwave spectroscopy of a few sodium Rydberg atoms at the one-photon 37S<sub>1/2</sub> -> 37P<sub>1/2</sub> and two-photon 37S<sub>1/2</sub> -> 38S<sub>1/2</sub> transitions are presented. Microwave spectroscopy can be used to detect dipole-dipole interaction between a few Rydberg atoms. The calculations showing an influence of dipole-dipole interaction on two-atom spectra are also presented. A noticeable broadening of the five-atom
spectrum was observed in the experiment due to the dipole-dipole interaction.
The results of the experimental and theoretical study on associative ionization of laser excited Na Rydberg atoms in collisions with ground-state atoms and on thermal ionization by blackbody radiation in single and crossed effusive atomic beams are reported and discussed.
Spontaneous coherence transfer has been studied experimentally using two-color laser polarization spectroscopy of <SUP>87</SUP>Rb. Two classes of optical transitions were investigated for which the theory predicts presence and absence of anisotropy of a hyperfine sublevel of the ground state populated via spontaneous decay. The anisotropy was induced by a strong linearly polarized laser beam. It was controlled by the polarization rotation of another probe laser beam. A qualitative agreement with the theoretical predictions has been found.
Microwave Hanle effect has been studied in sodium Rydberg atoms for the first time. Spontaneous emission of the microwave transition 37P<SUB>3/2</SUB> yields 37S<SUB>1/2</SUB> at 70.166 GHz was replaced by an induced transition from a pulsed microwave source. Good agreement with the theoretical calculations has been found. The widths and shapes of observed resonances were defined by the spectral widths of the pulsed microwave radiation and parameters of laser excitation of the initial 37P<SUB>3/2</SUB> state. The interference occurred in the scheme of transitions similar to the Mach- Zehnder optical interferometer.
We report about an experimental observation of alignment induced resonance in Doppler free spectrum of D<SUB>2</SUB> absorption line of <SUP>87</SUP>Rb ((lambda) equals 780 nm). The resonance appeared as a peak of absorption instead of transmission at the position of crossover resonance between the 5S<SUB>1/2</SUB>(F equals 1) yields 5P<SUB>3/2</SUB>(F' equals 0) and the 5S<SUB>1/2</SUB>(F equals 1) yields 5P<SUB>3/2</SUB>(F' equals 1) transitions.
Review of experiments on microwave spectroscopy of sodium Rydberg atoms carried out during last several years by our group, is presented. Most of the experiments have been done for the first time with Rydberg atoms. These are: static and dynamics Stark spectroscopy, two-photon dynamic Stark effect, double Stark resonance, scattering of Rydberg atomic beam by strong resonant microwave radiation. Also an experiment on pure amplification without the inversion in two-level Rydberg micromaser is discussed.
The experiments on transient phenomena in pulsed optical orientation of Rb atoms by the emissions of semiconductor lasers are presented. With using probe field method it was found that macroscopic magnetic momentum induced by strong pulsed laser field is processing in weak magnetic field causing Larmor oscillations of the absorption coefficient of probe field. The same effect was observed at fast changing of direction of the magnetic field.
Results of the experimentation observation of changing the profile of Rydberg sodium atomic beam at strong resonance multiphoton interaction with microwave radiation are presented. The influence of kinetic phenomena and of rapid damping of Rydberg states in the resonance standing wave on the beam profile is discussed Theoretical estimates for the process of deflection of Rydberg atoms have been done.