Spin noise measures collisions

Collision phenomena are ubiquitous in physics, chemistry and thermodynamics, and of importance in determining the microscopic structures and intermolecular interactions of atoms and molecules. To date, collision phenomena are mostly analyzed by scattering experiments, which usually require high vacuum degree and low temperature. These requirements place challenges in realistic applications, such as in situ measuring collisions inside of encapsulated devices under relatively high pressures that have many useful applications in atomic vapor magnetometers and electrometers. Therefore, a broad concern is how to investigate collisions in situ under wide-ranging experimental conditions. Recently, measuring spin noise in thermodynamic equilibrium by optical rotation is becoming a mainstream approach for non-perturbative studies of energy structures and correlated states in diverse systems. Generally speaking, colliding particles may exhibit new properties compared to free particles, such as collision-induced energy shifts, therefore collision phenomena can be analyzed through optical spin-noise spectroscopy by measuring energy structures of colliding particles. However, current optical spin-noise techniques are not suitable due to the trade-off between the bandwidth and the spectral resolution. Therefore, it is highly desirable to develop spin- noise techniques that have high bandwidth still with high resolution.

To this note, University of Science and Technology of China scientists: Shiming Song (PhD candidate), Dr. Min Jiang, Yushu Qin, Yu Tong, Wenzhe Zhang, Dr. Xi Qin and Prof. Xinhua Peng together with their colleague Prof. Ren-Bao Liu at The Chinese University of Hong Kong designed a new spin-noise technique that is based on the frequency down-conversion technique. They demonstrate that the application of the down-conversion technique to spin-noise techniques would help overcome the trade-off between the bandwidth and the spectral resolution of spin-noise techniques, and help realize in situ measurement of atomic collisions under wide-ranging conditions. Their work is currently published in the research journal, Physical Review Applied.

The research team considered investigation of alkali atomic collisions by measuring collision- induced frequency shifts (~100 kHz) of alkali atomic hyperfine levels ranging from 3 GHz to 7 GHz in a vapor cell. As such, they demonstrated gigahertz spin-noise spectra with 7 kHz half-height half- width (1 part per million) that is mainly limited by the transit-time broadening. In spite of the transit- time broadening, the demonstrated resolution is still at least 2 orders of magnitude better than existing gigahertz spin-noise techniques, and is high enough to measure collisional shifts and therefore collision phenomena.

With high-resolution gigahertz spin-noise spectra, the authors were able to in situ measure hyperfine shifts of colliding alkali atoms under ambient conditions, and obtain key collision parameters, such as the collision diameter, the well depth and the dominant interaction type. Overall, by improving the spectral resolution of gigahertz spin-noise techniques, they demonstrated the capability of characterizing collisions phenomena by spin-noise techniques.

In summary, the study presented an experimental demonstration of in situ measuring collisions under ambient conditions by a gigahertz-bandwidth and one-part-per-million-resolution spin-noise technique. Following detailed investigations as presented, optical spin-noise techniques are non- perturbative and beneficial for cold and ultracold atoms, with the capability of avoiding extra heating effects and atom loss caused by laser pumping. This provides a promising probe of cold and ultracold collisions. Moreover, the presented technique significantly improves the experimental efficiencies in ongoing experimental efforts to measure quantum noise, opening a feasible route towards quantum noise-based applications, for example, non-perturbative structural analysis for diverse spin systems, determining the fundamental precision of microwave quantum devices and the degree of squeezing and entanglement, and researching many-body phase transitions.