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We have observed period-tripling subharmonic oscillations in a driven superconducting coplanar waveguide resonator operated in the quantum regime, k₁T. 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, /250. 16em{0ex}GHz, when driving the resonator with a current at 3, with amplitude exceeding an instability threshold. We observed three stable radiative states with equal amplitudes, phase shifted by 2/3 rad, red detuned from the fundamental mode. The down-conversion from 3 to 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.
Svensson et al. (Wed,) studied this question.