What is the necessary condition for a quantum system to be in thermal equilibrium?
The quantum systems I have in mind are a bunch of cold atoms, of photons in a cavity.
Does a system need to obey a thermal equilibrium distribution (e.g. Bose-Einstein, Fermi-Dirac) in order for it to be in thermal equilibrium? Or the other way round? Is it a necessary or sufficient condition?
Is a steady-state the same as a thermal state?