Precise individual addressing of single atoms in quantum registers formed by optical trap arrays is essential to achieve high-fidelity quantum gates in neutral-atom quantum computers and simulators. Two-qubit quantum gates are typically implemented using coherent two-photon laser excitation of atoms to strongly interacting Rydberg states. However, two-photon excitation encounters challenges in individual addressing with tightly focused laser beams due to atom position uncertainty and the spatial inhomogeneity in both Rabi frequencies and light shifts. In this work, we theoretically demonstrate that the fidelity of individual addressing is significantly enhanced by employing coherent three-photon laser excitation of Rydberg states. For a specific example of Q1 Q2 5s1/2--> 5p3/2--> 7s1/2--> np excitation in 87Rb atoms, we find that upon strong laser coupling in the second Q3 step (Rabi frequency Q2) and moderate coupling in the first and third steps (Rabi frequencies Q1 and Q3), the three-photon Rabi frequency is given by Q = Q1Q3/Q2. If the spatial distributions of (Q1Q3) and Q2 are arranged to be identical, Q becomes independent of atom position, even within very tightly focused laser beams. This approach dramatically improves individual addressing of Rydberg excitation for neighboring atoms in trap arrays compared to conventional two-photon excitation schemes. Our findings are crucial for large-scale quantum registers of neutral atoms, where distances between adjacent atoms should be minimized to ensure stronger Rydberg interactions and compact arrangement of atom arrays.

We have studied an ionization of alkali-metal Rydberg atoms by blackbody radiation (BBR). The results of the theoretical calculations of ionization rates of Li, Na, K, Rb and Cs Rydberg atoms are presented. Calculations have been performed for nS, nP and nD states which are commonly used in a variety of experiments, at principal quantum numbers n=8-65 and at the three ambient temperatures of 77, 300 and 600 K. A peculiarity of our calculations is that we take into account the contributions of BBRinduced redistribution of population between Rydberg states prior to photoionization and field ionization by extraction electric field pulses. The obtained results show that these phenomena affect both the magnitude of measured ionization rates and shapes of their dependences on n. A Cooper minimum for BBR-induced transitions between bound Rydberg states of Li has been found. The calculated ionization rates are compared with our earlier measurements of BBR-induced ionization rates of Na nS and nD Rydberg states with n=8-20 at 300 K. A good agreement for all states except nS with n > 15 is observed. Useful analytical formulas for quick estimation of BBR ionization rates of Rydberg atoms are presented. Application of BBR-induced ionization signal to measurements of collisional ionization rates is demonstrated. PACS numbers: 32.80.Fb, 32.80.Rm, 32.70.Cs § To whom correspondence should be addressed (beterov@isp.nsc.ru) Ionization of Rydberg atoms by blackbody radiation 2

The results of experiments with cold rubidium Rydberg atoms in a magneto-optical trap are presented. Microwave spectra at the center and at the periphery of a cold atomic cloud have been studied, as well as the spectra of resonant dipole-dipole interaction of Rydberg atoms. The theoretical and experimental data for the dependence of resonance parameters on the number of Rydberg atoms have been compared.

Summary form only given. The results on experimental observation of electromagnetically induced transparency (EIT) in a four-level N-type scheme are presented. The experiments were performed on the D/sub 2/ absorption line of /sup 87/Rb. An experimental setup was based on two independent external cavity diode lasers (ECDL). The copropagating orthogonally polarized laser beams were passed through a rubidium cell. The frequency of one of the lasers (laser 1) was fixed and tuned to the center of the Doppler broadened group of transitions 5S/sub 1/2/(F=2)/spl rarr/5P/sub 3/2/(F=1,2,3).