Broadband Cryogenic milli-Kelvin Attenuators: Cooling Power Improvements

Dephasing due to fluctuations in the number of photons in a read-out cavity resonator can cause significant decoherence in superconducting transmon qubits.[1] To reduce the number of thermal photons injected into a read-out cavity by an input line that goes to higher temperature stages and is subjected to heating from applied signals, we have designed cryogenic microwave attenuators for operation below 100 mK and up to 10 GHz. To improve the transfer of heat from the dissipative elements in our attenuator we have interleaved thick conducting heat sinks with distributed dissipative NiCr elements. Our 20 dB attenuators have a minimum noise temperature T_n < 50 mK with no applied power.[2] When the dissipated power P_d is increased, this design shows an improvement in the cooling power proportional to the thickness t of the heat sink, and we find T_n ∝ (P_d/t)^1/5. With thickness t = 10 μm, the attenuator can dissipate up to P_d = 100 nW and keep T_n ~ 100 mK.

[1] A. P. Sears, et al., Phys. Rev. B 86, 180504(R) (2012).
[2] J-H. Yeh, et al., J. Appl. Phys. 121, 224501 (2017).