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 investigated the absence of certain bright peaks in Autler-Townes laser excitation spectra of alkali metal atoms. Our research revealed that these dips in the spectra are caused by a specific architecture of adiabatic (or ``laser-dressed'') states in hyperfine (HF) components. The dressed states' analysis pinpointed several cases where constructive and destructive interference between HF excitation pathways in a two-photon excitation scheme limits the available two-photon transitions. This results in a reduction of the conventional two-photon selection rule for the total angular momentum $F$, from $\Delta F= 0,\pm 1$ to $\Delta F\equiv 0$. Our discovery presents practical methods for selectively controlling the populations of unresolvable HF $F$-components of $ns_{1/2}$ Rydberg states in alkali metal atoms. Using numerical simulations with sodium and rubidium atoms, we demonstrate that by blocking the effects of HF interaction with a specially tuned auxiliary control laser field, the deviations from the ideal selectivity of the HF components population can be lower than $0.01\%$ for Na and $0.001\%$ for Rb atoms.

Atomic and molecular (A&M) data and databases, which contain information about species, their identities, and processes, are critical and useful tools used in many fields of astrophysics, chemistry, and astro-informatics. Moreover methods of computational astrochemistry have become increasingly important in the last decades for the investigation of interaction and dynamics of small molecules enclosed in larger structures (Albert et al 2020, Srećković et al. 2020). In this contribution the role of some A&M processes has been studied. Acknowledgments The article is based upon work from COST Action CA21101, Confined molecular systems: from a new generation of materials to the stars (COSY) and Science Fund of the Republic Serbia [Grant no. 3108/2021, NOVA2LIBS4fusion]. Authors thank N. Pop for fruitful discussions.

The analysis of problems related to nonlocalized population transfer between quantum levels requires nontraditional mathematical approaches. Here we study the processes of irremovable nonadiabatic transitions in a (N -1)-dimensional system of completely degenerate so-called dark states, taking as a base model multilevel N-pod systems. It is shown that quantum dynamics of the dark states due to the operator of nonadiabatic coupling can be described in terms of the Riemannian parallel transport of their geometric counterparts in the space of control parameters. The results of mathematical modeling of nonadiabatic effects, presented in the paper for the case of four-level systems interacting with three laser fields (tripod systems), demonstrate full agreement between the quantum and geometric approaches. These results are of interest for experimental and laboratory plasma research and atomic and laser physics.

The features of the resonant and anomalous structure of the associative ionization (AI) spectra for the case of the dipole-dipole interaction of Rydberg atoms are analyzed using the stochastic approach. The use of this approach makes it possible to quantitatively describe the AI reaction with the formation of a positively charged molecular ion. It is found that the efficiency of asymmetric ionization processes for Auger transitions can exceed that of symmetric ones by orders of magnitude. The important role of this phenomenon for the development of modern applied quantum research and the concept about ionization processes occurring in the ionosphere is discussed. The obtained results can be used to solve a number of fundamental problems of ionospheric plasma physics. This especially concerns the time delay effect of signals of global navigation satellite systems (GNSS), which significantly affects the stability of GNSS operation and remote sensing of the Earth’s surface.

В рамках стохастического подхода выполнен анализ особенностей резонансной и аномальной структур спектров ассоциативной ионизации (АИ) для случая дипольно-дипольного взаимодействия ридберговских атомов. Использование такого подхода позволяет количественно описать реакцию АИ с образованием положительно заряженного молекулярного иона. Обнаружено, что эффективность несимметричных процессов ионизации для оже-переходов может на порядки превышать эффективность симметричных. Обсуждена важная роль этого явления для развития современных квантовых прикладных исследований и представлений о процессах ионизации, протекающих в ионосфере. Полученные результаты могут быть использованы для решения ряда принципиальных проблем физики плазмы ионосферы. Особенно это касается эффекта "временной задержки" сигналов глобальных навигационных спутниковых систем (ГНСС), что, в свою очередь, существенно влияет на устойчивость работы ГНСС и дистанционное зондирование поверхности Земли.

In this manuscript, the chemi-ionization (CI) processes in atom - Rydberg atom collisions are examined. We obtained the crosssections, as well as rate coefficients for CI processes in potassium and sodium collisions for principal quantum numbers 5 n 25 in the 500 K T 1500 K temperature region. The presented data could be useful for the improvement of analysis and modelling of weakly ionized layers of different atmospheres and cosmic objects where these and other CI processes could change excited state populations and the free electron density, consequently affecting the optical characteristics. (c) 2022 COSPAR. Published by Elsevier B.V. All rights reserved.

This work focuses on visualization of dark states arising in systems of hyperfine states upon two-photon excitation. In our experiment, levels 3S(1/2) and 3P(1/2) of sodium atoms are coupled by a strong S-laser field forming the laser-dressed adiabatic states, which are then monitored by scanning a weak probe P-field across the 3P(1/2) -> 7D(3/2) transition. The excitation spectrum of the 7D(3/2) state exhibits one main peak and several side peaks. The latter shift farther apart with increasing S-laser intensity, while the position of the main peak is hardly affected, which associates its origin with a class of dark states decoupled from laser interactions. The results of numerical simulation of the observed spectra based on the solution of the optical Bloch equations are in good agreement with the experimental data. The results are of interest for atomic and laser physics.

Photoionization processes involving polarized Xe atoms in the presence of an external magnetic field were analyzed in our previous studies. It was shown that the modulation of quantum beats in the observed electronic photo signals could not be explained by the Paschen-Back effect accounting for fine-structure levels only. In the current work, ionized Xe+ isotopes are used as the output channel of photoionization. By comparing the temporal partial signals in the mass spectra of the Xe+ isotopes, it was possible to explain the modulation of quantum beats in ion fluxes as a consequence of the presence of the hyperfine structure in the Zeeman components of the 129Xe and 131Xe isotopes .

We present an analysis of the stimulated Raman adiabatic passage processes based on the methods of differential geometry. The present work was inspired by an excellent article by Bruce W. Shore et al. (R. G. Unanyan, B. W. Shore, and K. Bergmann Phys. Rev. A \textbf{59}, 2910 (1999)). We demonstrate how a purely geometric interpretation of the adiabatic passage in quantum tripod systems as a Riemannian parallel transport of the dark state vector along the Bloch sphere allows describing the evolution of the system for a given sequence of Stokes, pump and control laser excitation pulses. In combination with the Dykhne-Davis-Pechukas adiabaticity criterion and the minimax principle for circles on a sphere, this approach allows obtaining the analytical form of the optimal laser pulse sequences for a high fidelity tripod fractional STIRAP. In contrast to the conventional STIRAP in $\Lambda$-systems, the Gaussian approximations of the optimal laser pulse sequences allow reaching the infidelity of $10^{-7}$ for the adiabaticity parameter of $300$ without noticeable oscillatory or other detrimental effects on population transfer accuracy.

We have investigated the formation of laser-dressed states in HF components of sodium atoms and their manifestations in Autler-Townes spectra. An explicit procedure is proposed for constructing a specific Morris-Shore wave function basis, within which two-photon excitation schemes are reduced to sets of mutually orthogonal 3- and 2-level excitation sequences, and single uncoupled states. It has been demonstrated that the Morris-Shore basis states correspond to bright and dark states, along with chameleon states introduced in our recent work. Numerical simulations of the Autler-Townes spectra reveal that chameleon states can be further categorized as "fast" or "slow." Our extended classification of dressed states enables more accurate reconstruction of excited state properties in optical methods for atomic media diagnostics.

Photoionization processes involving polarized Xe atoms in the presence of an external magnetic field were analyzed in our previous studies. It was shown that the modulation of quantum beats in the observed electronic photo signals could not be explained by the Paschen-Back effect accounting for fine-structure levels only. In the current work, ionized Xe+ isotopes are used as the output channel of photoionization. By comparing the temporal partial signals in the mass spectra of the Xe+ isotopes, it was possible to explain the modulation of quantum beats in ion fluxes as a consequence of the presence of the hyperfine structure in the Zeeman components of the 129Xe and 131Xe isotopes. Keywords: quantum beats, hyperfine structure, Zeeman effect, two-photon excitation, femtosecond ionization, "pump-probe"-experiment.

Исследована ионизация когерентно двухфотонно возбужденной суперпозиции 4f-состояний атома Xe фемтосекундным импульсом пробного лазера в сверхзвуковом пучке с регулируемой задержкой между импульсами накачки и пробным. Регистрируемая колебательная структура в сигналах фотоионизации связана с когерентными биениями суперпозиции возбужденных состояний. Высокая эффективность предложенной схемы регистрации квантовых биений обусловлена практически 100%-й эффективностью сбора фотоэлектронов в отличие от ранее используемой регистрации флуоресценции. Ключевые слова: квантовые биения, двухфотонное возбуждение, фемтосекундная ионизация.

Ionization of coherently two-photon-excited superposition of 4 f states of Xe in a supersonic atomic beam is studied by means of femtosecond pump–probe spectroscopy. Oscillatory structure in the photoionization signal is related to coherent beats of a superposition of excited states. High efficiency of the proposed scheme of registration of quantum beats was achieved due to nearly 100% efficiency of collecting photoelectrons, in contrast to detection of fluorescence used traditionally.

Ionization of polarized states of Ar and Xe atoms by femtosecond probe pulse in a supersonic beam in the presence of magnetic field was studied theoretically and experimentally. The revealed oscillation structure in the photoionization signals occurs due to the Larmor precession of exited atomic states in a magnetic field. We derived analytical formulas for the photoelectron current and explained the detected oscillations in terms of photon and atomic polarization moments. Our results indicate the possibility of implementing Doppler-free spectroscopy involving bound-free transitions.

The measurements of Montenbrook et al. to determine the water surface level of the Walchensee alpine lake in Bavaria (Germany) are a vivid example of the manifestation of the peculiarities of the relationship between the GPS satellite system as a source and remote sensing of the Earth’s surface. The experiments were conducted in 2007 in the framework the GORS (GPS Occultation, Reflectometry and Scatterometry) program. The authors described the observed features in detail but did not provide their physical justification. In this study, it is shown that the observed effects are caused by the resonant interaction of electromagnetic waves with a medium containing Rydberg molecular complexes. They are the main reason for the delay of satellite constellation signals at altitudes of 60 to 110 km.

Penning ionisation (PI) processes involving pairs of Rydberg alkali-metal atoms, excited to different quantum states and experiencing dipole–dipole interactions, have a wide range of important properties in atomic physics. Within the framework of the semiclassical approximation, we have used both numerical and analytical approaches to examine the Penning autoionisation width dependence on the state quantum numbers in a quasi-molecule formed by the interacting partner atoms. We described the characteristics of optimal quantum numbers that lead to enhanced PI widths for the interacting Rydberg atom pairs of all alkali-metal atoms. The excited states of atoms in these pairs are asymmetric, resulting in a large atomic shell size difference: inspired by Efimov et al (2016 J. Phys. B: At. Mol. Opt. Phys. 49 125302), we call such a pair ‘Tom’ and ‘Jerry’ (for ‘big’ and ‘small’). Compared to symmetric pairs, the optimal asymmetric pairs display a significant (by several orders of magnitude) increase in the PI rate. This property makes PI a relevant source for producing charged particles in cold Rydberg systems that spontaneously evolve into cold plasma. Contrary to hydrogen atoms examined in (Efimov et al 2016 J. Phys. B: At. Mol. Opt. Phys. 49 125302), the difference in quantum defects in alkali-metal atoms results in a strong Penning width dependence on the orbital quantum numbers l of the quasi-molecule. In particular, alkali-metal atoms exhibit two PI channels associated with bound–bound optical transitions showing Δ l = ±1—individual and closely spaced (doublet-like) configurations of optimal pairs. Furthermore, we demonstrate that the presence of Förster resonances can lead to a notable (up to five times) increase in the PI efficiency.