Behavior of Excess Noise in a Nondegenerate Josephson-Parametric Amplifier

We have performed noise measurements on a nondegenerate Josephson-parametric amplifier cooled to 0.25K and operated at 19.3 GHz. At this temperature and frequency the equilibrium noise emitted by losses consists almost entirely of vacuum fluctuations. 

Below the threshold for oscillation we have achieved power gains in excess of 5000 with a single-sideband noise temperature of 1.0 += .1K and all the noise is accounted for by the equilibrium fluctuations of the amplifier losses and the input termination.