Finally, our optical spin-wave measurement work is on PRL!

Congratulations to everyone, in particular Lingjing and Yizun (finally) who have been working so hard on this project, since as early as 2020. 

Special thanks to Dr. Stefano Grava and Prof. Darrick Chang for many communications from which the Fudan team learned a lot, and for making extremely nice theoretical contributions. 

Special thanks to Prof. Zhou Lei team for many important discussions along the way.

Now, some physics (by saijun and darrick).

When light interacts with two-level atoms, the microscopic quantum interactions mediated by exchange of photons are often hidden by collective radiation that drives macroscopic light propagation. By freezing and then restarting the collective interactions, the microscopic quantum dynamics can be unveiled.

While laser pulses naturally excite atomic spin waves that are phase-matched to radiation, we employ a geometric phase patterning technique to coherently convert between phase-matched and mismatched excitations. By shutting off macroscopic propagation, the microscopic interactions can accumulate. As one example, we apply our technique to observe a density-dependent decay of spin-wave order arising from near-field pairwise interactions. This fundamental mechanism limits the optical depth per unit length of atomic ensembles, which has important implications for quantum technologies.

Our observation is in direct analogue to the dipolar relaxation for near-field samples regularly observed in the microwave domain. Here, in the optical domain, the coherent control of light propagation becomes essential for isolating the microscopic interaction with far-field control and measurements. Our method should be a powerful tool to unveil many-body aspects of cold-atom-based light-matter interactions in the future. For example, our technique should allow for the detection and coherent control of long-lived subradiant atomic states in optical lattices.

Now, we are looking for new members to join the optical spin-wave project.  Please drop an email to saijunwu@fudan.edu.cn to discuss possibilities.