  We have observed period-tripling subharmonic oscillations, in a
superconducting coplanar waveguide resonator operated in the quantum regime,
$k_B T \ll \hbar\omega$. 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, $\omega/2\pi \sim
5\,$GHz, when the resonator was driven by a current at $3\omega$ with an
amplitude exceeding an instability threshold. The output radiation was
red-detuned from the fundamental mode. We observed three stable radiative
states with equal amplitudes and phase-shifted by $120^\circ$. The
downconversion from $3\omega$ to $\omega$ is strongly enhanced by 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.
