Detection of thermal noise dynamics in quantum computer interconnects

We report a systematic study of thermal noise in five typical qubit drive lines in 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. We demonstrate new methodology of calibrating power with a temperature variable noise source spanning the range 0.1 K –0.5 K [2]. In steady state, we measure radiation temperatures of 50 mK – 70 mK without heating and up to 200 mK with tens of nanowatts of applied rf power. Subsequent lock-in measurements exhibit thermal latencies of 3 ms -- 30 ms crucially revealing thermal heat capacities in the nJ/K range. Our measurement results are essential for understanding and suppressing qubit dephasing due to photon shot noise [3] and the methodology is directly applicable in studies of future interconnects for quantum computers.



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

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

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