Transmon qubit as an absolute power sensor at milli-Kelvin temperatures

Superconducting qubits are the building blocks of quantum computational devices. Due to their operation at milli-Kelvin temperatures with a series of cryogenic components, the exact attenuation of the electrical lines is not known a priori and thus the microwave power in the cryogenic environment is not accurately predictable. However, qubits can be used as power sensors in the GHz range, yielding the microwave power in the milli-Kelvin environment without the need for further dedicated devices. In addition, this power sensing scheme can be used to evaluate the noise of parametric amplifiers. Here, we present a transmon-resonator circuit, commonly used for quantum computing, and demonstrate its application for power sensing. We show the measurement scheme and experimental results of our device. Moreover, we present a route towards uncertainty evaluation and propagation, also taking advantage of higher order transitions, with the aim to realize an in-situ microwave power standard operating at milli-Kelvin temperatures.