Period-Tripling Subharmonic Oscillations in a Driven Superconducting Resonator*

We have observed period-tripling subharmonic oscillations in a driven superconducting coplanar waveguide resonator operated in the quantum regime, k_BT << hω/2π. The resonator is terminated by a tunable inductance that provides a Kerr-type nonlinearity. We detected the output field quadratures at frequencies near the fundamental mode, ω/2π ∼ 5 GHz, when current driving the resonator with an external signal at 3ω, with amplitude exceeding an instability threshold. We observed three stable radiative states with equal amplitudes, phase-shifted by 2π/3 radians, red-detuned from the fundamental mode. The downconversion from 3ω to ω is strongly enhanced by near-resonant excitation of the second mode of the resonator, and the cross-Kerr effect. Our experimental results are in quantitative agreement with a model for the driven dynamics of two coupled modes.