Absolute power sensing with dispersively and waveguide coupled transmons: a comparison

Superconducting qubits are the building blocks of quantum computational devices. Due to their operation at milli-Kelvin temperatures requiring a series of cryogenic electrical 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 additional dedicated devices [1, 2]. Here, we present a superconducting qubit design with three transmons directly coupled to the central feedline and three transmons coupled via a readout resonator. We show a comparison between the two approaches in order to assess their viability for power sensing applications. Further, we show, with the hybrid device described above, how to sense the absolute power of RF signals at milli-Kelvin temperatures. The accessible power ranges from -134 dBm to -120 dBm for the waveguide transmons and from -70 dBm to -40 dBm for the transmons coupled via a readout resonator. Finally, we present a route towards uncertainty evaluation and propagation, which is crucial for a reliable comparison between power sensing techniques.





[1] Schneider et al., Phys. Rev. A 97, 062334 – “Local sensing with the multilevel ac Stark effect” (2018) DOI: https://doi.org/10.1103/PhysRevA.97.062334

[2] Hönigl-Decrinis et al., Phys. Rev. Applied 13, 024066 – “Two-Level System as a Quantum Sensor for Absolute Calibration of Power” (2020) DOI: https://doi.org/10.1103/PhysRevApplied.13.024066