Alexey Legoshin

Cold atomic ensembles are among the most versatile tools in modern physics research. They play a key role in advancing next-generation frequency standards, qubit architectures, and quantum sensing technologies. Many experiments demand prolonged spatial confinement of atoms to facilitate extended interactions with electromagnetic fields, achievable through electric, magnetic, gravitational, or optical trapping mechanisms.

A cornerstone of atomic trapping is the magneto-optical trap (MOT), which relies on six counterpropagating laser beams and a pair of anti-Helmholtz coils to produce a radially symmetric quadrupole magnetic field with a central zero point.

Alternatively, optical dipole traps exploit the electric dipole interaction with a tightly focused, fardetuned high-power laser beam, offering weaker confinement than MOTs—typically below 1 mK. Unlike MOTs, these traps permit extremely weak optical excitation, circumventing limitations imposed by radiation pressure. Additionally, their trapping mechanism is largely insensitive to ground-state magnetic sublevels (neglecting tensor polarizability effects), enabling versatile configurations such as optical lattices.

Figure 1: Image of atoms trapped in the MOT. The atomic cloud is at the center of the image. The cloud dimensions are approximately 1 mm.

ble of confining Rb 87 atomic clouds at temperatures near 175 µ K. For dipole trapping, we adopted a farred-detuned (1012 nm) laser system, chosen for its dual functionality: we plan to use it both as a trapping beam and as one component in two-photon Rydberg excitation schemes. A further advantage of this configuration is its potential to confine Rydberg atoms while meeting the magic wavelength condition for ground-to-Rydberg transitions.

Magic-wavelength optical dipole traps offer particularly powerful advantages for cold atom manipulation. At this specific wavelength, the light shift for two atomic states becomes identical, effectively decoupling the internal atomic dynamics from the external motional states. This enables long coherence times for quantum operations while maintaining strong spatial confinement. Furthermore, such traps allow state-insensitive confinement, which is crucial for precision measurements and quantum information processing. The magic wavelength condition also facilitates efficient Rydberg excitation by providing identical trapping potentials for both ground and Rydberg states, minimizing decoherence during excitation processes.

We characterized the dipole trap intended for atom transfer, achieving a maximum depth of 3 mK under our experimental conditions—surpassing the Doppler cooling limit and ensuring efficient atomic confinement.

Доклад посвящен разработке сложной (состоящей из грубой и точной) системы наведения (компенсации угла) воздушной линии длиной 230 см для передачи высокостабильной оптической частоты. Активная компенсация фазовых шумов при полностью запущенной системе наведения позволяет подавить временную нестабильность частоты до значений менее 10–18 за время усреднения 10 с, при этом расширяя диапазон работы линии при угловом смещении потенциального объекта, принимающего излучение, с 0,3 до 3,15 градуса.

Nowadays optical frequency transfer has become an essential component for numerous quantum technology applications. There is a gradual increase in demand for such systems due to the rapid development of quantum technologies themselves, e. g. optical frequency transmission became crucial for high-precision timekeeping and communication systems relying on the frequency stability of quantum clocks that offer unparalleled accuracy, making ideal for metrology, navigation, and testing fundamental physical theories. Moreover, optical frequency transmission has already found popularity in such up-to-date topics as quantum communication and quantum cryptography.

For over 6 years, our laboratory has been developing various systems for the transmission of stabilized frequency signals. In 2017, we started with the elaboration of a phase noise compensation system in a 5-meter long optical fiber frequency transmission line and afterwards extended the line to 2.8 kilometers. Taking advantage of the result we also connected three of our laboratories with fiber optic cables with phase noise compensation systems to facilitate the process of comparing developed frequency standards. With an awareness of the fiber optic links applicability limitations (such as insufficient mobility and flexibility) we developed a 5-meter long free-space optical link with the same phase-noise compensation system in 2020 and increased its length to 17 meters with the addition of a precision pointing system in 2021.

Finally, in 2023, we introduced a 215-cm free-space optical transmission link with a flexible pointing system that enables dynamically stable tracking of moving objects: potentially drones or even satellites. The test scheme of this transfer system with defined upgrades is presented in Figure 1.

Phase noise compensation system for both fiber and free-space optical links The transmission link introduces phase noise into the signal. To compensate for corresponding frequency shifts, a laser beam used for transmission is split into two parts. The first part passes through a reference arm of an interferometer, while the second is transmitted to the receiver via an acousto-optic modulator (AOM1) and partly reflected back. The returned signal contains doubled link noise and is heterodyned with the reference beam. The resulting beat signal is used in a phase-locked loop that controls the frequency shift introduced by AOM1 and compensates the link noise.

Pointing system for free-space optical link To compensate for small high-frequency beam direction fluctuations, a precise (fast) pointing system was used, which includes a position-sensitive quadrant photodetector managing a mirror with two-coordinate galvanic control. To expand the pointing range, we implement a coarse (slow) pointing system by attaching the optical plate to an alt-azimuth telescope mount. The servo signal of the fast system is used to correct the angular velocity of the dynamic tripod rotation around two axes.

In the near future, we plan to extend the free-space link length and test the system in actual environments when pointing at a moving UAV. In addition, we plan to focus on processing the received noise signal in order to use it to obtain information about atmospheric parameters.

We strongly believe that the development of optical transmission systems is extremely useful for the expansion of quantum technologies.

В отличие от броуновской частицы в классической жидкости движение такой частицы в сверхтекучем гелии существенно зависит от наличия квантовых вихрей. Известно, что когерентное вращение сверхтекучей компоненты вокруг кора вихря приводит к эффективному захвату примесных частиц вихрями. В этом случае частица либо движется исключительно вдоль кора вихря, либо, если возмущения жидкости достаточно велики, только часть времени проводит в свободном движении между захватами в вихри.

Ситуация качественно изменяется если частица в сверхтекучем гелии оказывается активной, т.е. способной поглощать энергию извне. При достаточно интенсивном тепловыделении частицы у её поверхности формируется противоток нормальной (вязкой) и сверхтекучей компонент, в котором формируются вихри, плотность которых определяется взаимной скоростью нормальной и сверхтекучей компонент, т.е. интенсивностью тепловыделения. Такая частица способна взаимодействовать уже и со сверхтекучей компонентой. Данная работа посвящена исследованию движения активных броуновских частиц в трёхмерном пространстве. В качестве активных частиц были использованы сверхпроводящие частицы с характерным размером 40 мкм, левитирующие в поле магнитной ловушки и облучаемые интенсивным лазерным излучением (~ Вт/см2 ).