Quantifying the power radiated to a qubit through its control lines

We report measurements of the power radiated from a qubit drive line to a quantum device within a dilution refrigerator. The measurements were recorded using an in situ hot electron microwave nanobolometer with a designed-50- Ω absorber [1] that is coupled to a coaxial input. First, the nanobolometer was calibrated by connecting a temperature-controlled blackbody source spanning the range 0.1 K –0.5 K [2]. Second, applying a microwave tone at 5.8 GHz, we measure the background radiated power and attenuation from the passive components in the drive line. Finally, we obtain the thermal latencies of those components. Our measurement results are crucial for understanding and suppressing qubit dephasing due to photon shot noise [3].

 

[1] Phys. Rev. Lett. 117, 030802 (2016)

[2] Review of Scientific Instruments 92, 034708 (2021)

[3] Phys. Rev. A 106, 042605 (2022)