Figure 2

Microwave noise power is calibrated in a narrow band centered at the frequency${\mathit{f}}_{\mathit{c}}\mathbf{=}\mathbf{7.62}\mathbf{\text{ GHz}}$, by varying the temperature of the 50 Ω termination on the cold circulator. A one-parameter fit to Eq. (5), adjusting only the overall gain factor, allows for determination of the HEMT amplifiers noise temperature $T_N^{\mathrm{HEMT}}=2.62\text{ K}$ and observation of the quantum noise $T_Q=0.18\text{ K}$. The data points in the inset are the quantum noise at different center frequencies in the band of the HEMT amplifier, and the straight line is the expected behavior $T_Q=\frac{h{f}_c}{2k_{\mathrm{B}}}$.