Period Tripling due to Parametric Down-Conversion in Circuit QED.

Discrete time-translation symmetry breaking can be observed in periodically driven systems oscillating at a fraction of the frequency of the driving force. However, with the exception of the parametric instability in period doubling, multiperiodic driving does not lead to an instability threshold. In this Letter, we point out that quantum vacuum fluctuations can be generically employed to induce period multiplication. In particular, we discuss the period-tripled states in circuit QED and propose a microwave setup. We show that for weak dissipation or strong driving, the system exhibits a nonequilibrium phase transition in the sense that the timescale over which the period-tripled state is generated can be arbitrarily separated from the timescale of the subsequent dephasing.