Relaxation to Negative Temperatures in Double Domain Systems

In recent years, quantum technologies have shown exceptional progress in their development that we can now engineer hybrid quantum systems using elements ranging from solids to atomic-molecule and optical systems. The investigation of hybrid quantum physics is an interesting and important focus as we can explore counterintuitive phenomena such as many-body and non-equilibrium physics inherent in the hybridization.
In this work, we investigate the relaxation processes of two spin ensembles consisting of identical spin one-half particles: the double spin domain systems, as collective phenomena characterized by the two spin-domain sizes. These spin ensembles couple equally to a common bosonic reservoir. Initially, we prepare the double domains so that the first spin ensemble is in the excited state while the second spin ensemble is in its ground state. By controlling the two spin sizes, we analyze the various collective spin relaxations.
We have found novel phenomena such that when the first spin size is much larger than the second one, the double spin domains relax to steady states where the spin population of the second domain exceeds 50%. That is, they relax to the negative temperature states [1].
Reference: [1]. Yusuke Hama, William J. Munro, and Kae Nemoto, arXiv:1612.08963.