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, kappa T-B << hw. 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, omega/2 pi similar to 5 GHz, when driving the resonator with a current at 3 omega, with amplitude exceeding an instability threshold. We observed three stable radiative states with equal amplitudes, phase shifted by 2 pi/3 rad, red detuned from the fundamental mode. The down-conversion from 3 omega to omega 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.