  Injection locking is a well-established technique widely used in optics as
well as solid-state devices for efficient suppression of noise. We present the
spectroscopic characterization of the effect of the injection-locking signal
(ILS) in mitigating the phase noise of a Josephson parametric oscillator (JPO),
whose output oscillating phase undergoes indeterministic switching between the
bistable states with symmetry $\theta \rightarrow{\theta+\pi}$. With the
injection of a weak locking signal, we measure the phase noise power spectral
density of the self-sustained oscillator output state for different locking
signal strengths. We observed suppression of phase noise by injection locking.
As the ILS strength surpasses more than a few photons, the output state stays
completely pinned to the locking phase of the ILS, and the random telegraphic
noise due to the switching of the states is significantly suppressed.
