Gain calibration of cryogenic amplification chains using normal-metal–insulator–superconductor junctions
To achieve a high-efficiency readout for a low-temperature microwave circuit using cryogenic and room temperature electronics, the signal has to go through one or more amplifiers to obtain a reasonable signal-to-noise ratio. In practice, the readout line has additional losses due to different microwave components which makes the gain estimation a non-trivial task.
Here, we present a gain calibration scheme that is based on a normal-metal–insulator–superconductor junction heterostructure coupled to a superconducting microwave resonator . Depending on the bias voltage applied on the heterostructure, the device can be utilized as a quantum circuit refrigerator  or as an incoherent photon source . For the latter case, we derive an analytic expression for the total gain that is based on only a single fitting parameter.
We present our experimental results where we reach 0.1 dB relative uncertainty of the total gain in a three-stage amplification chain.
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