A flux-driven Josephson parametric amplifier for experiments with propagating quantum microwaves*

For the detection of propagating quantum microwaves in circuit QED linear amplifiers are key ingredients. Phase sensitive amplifiers [e.g., Josephson parametric amplifiers (JPA)] in principle allow for the amplification of one signal quadrature without adding noise. In practice, however, internal losses often introduce a finite amount of noise. We have recently shown that, despite such a residual noise, signals on the quantum level can be fully characterized using two amplification chains and suitable correlations [E.P. Menzel et al., PRL 105, 100401 (2010)]. In this work, we characterize a flux-driven JPA. At 5.64\,GHz the maximum degenerate gain is 25.5\,dB and the signal bandwidth is 1.8\,MHz. Phase-insensitive measurements yield a noise temperature of 100$\pm$20\,mK, which is below the standard quantum limit of 135\,mK.