Comment on "Time Crystal in a Single-mode Nonlinear Cavity"

Abstract

I argue that a single driven quantum Van der Pol oscillator should not be considered a dissipative time crystal, contrary to previous claims. In particular, I show that its phase is prone to randomly drift when considering dephasing or additional nonlinearities, and hence its oscillations are not robust in that sense. The arguments I provide are applicable to many other models studied in the literature for which the limit of persistent oscillations coincides with the classical limit. I hope that this comment will spark a discussion about what should or should not be considered a time crystal (at least in the context of open quantum systems) and clarify it to a point.