Reducing qubit decoherence in 3D circuit quantum electrodynamics with cold cavity attenuators

Dephasing induced by residual thermal photons in the readout resonator is one of the leading factors limiting the coherence time of transmon qubits in the circuit QED architecture. This residual thermal population of the order of 10^-1-10^-3 is suspected to arise from noise impinging on the resonator from the input and output transmission lines. We have designed and tested a new type of narrowband microwave attenuator made of a brass or OFHC copper cavity that is well thermalized to the mixing chamber stage of a dilution refrigerator. By adding such a cavity attenuator inline with a 3D superconducting cavity housing a transmon qubit, we have reproducibly measured increased qubit coherence times below 100 mK. At base temperature, through Hahn echo experiment, we measured T_2E / 2T₁ = 0.98(+0.02/-0.13). We thus obtained a upper bound on the residual photon population close to 10^-4 in the fundamental mode of the readout cavity, which to our knowledge is the lowest value reported so far.