Fig. 3
Frequency spectra of voltage noise and noise equivalent power (NEP). a, b Spectral density of the noise in the signal quadrature of the down-converted probe tone with the Josephson parametric amplifier (JPA) off (a) and on (b) as functions of the noise frequency fn. The bottommost curve shows the spectral density far off resonance at fp = 539.275 MHz and Pp = −132.5 dBm, whereas the green, blue, orange, and yellow curves are measured at Pp = −126 dBm (JPA off) and Pp = −126.5 dBm (JPA on), and span a narrow frequency range near the resonance. Specifically, the probe frequencies are fp = 540.6125 MHz − δ, where the values of δ are indicated in a. For clarity, the curves have been offset vertically in increments of 10 dBm/Hz. The two peaks above 1 kHz in b are due to the aliased JPA idler. The excess noise at multiples of 1.4 Hz is attributed to the pulse tube (PT) cryocooler. The dashed line indicates a first-order RC filter response with a time constant identical to that in the δ = 0 trace in b. c NEP with the JPA on as a function of the noise frequency at Pp = −126.5 dBm and fp = 541.9625 MHz. These data yield 0.3 zJ for the energy resolution estimate of the detector (see text). Discontinuity in the data on all panels near 2 kHz is caused by the fact that we measure the high- and low-frequency noise separately with different time steps
