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Bulletin of the American Physical Society

Reducing Thermal Photon Dephasing in Transmon Qubits: Microwave Attenuator Design

Jen-Hao Yeh, Rui Zhang, Jay C. LeFebvre, F. C. Wellstood, B. S. Palmer

Published Mar 15, 2017• 0 citations
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To reduce the level of thermally generated electrical noise transmitted to superconducting quantum devices operating at 20 mK, we have developed thin-film microwave power attenuators operating from 1 to 10 GHz. The 20 and 30 dB attenuators are built on a quartz substrate and use 75 nm thick films of nichrome for dissipative components and 1 μm thick silver films as hot electron heat sinks. The noise temperature of the attenuators was quantified by connecting the output to a 3D cavity containing a transmon qubit and extracting the dephasing rate of the qubit as a function of temperature and dissipated power Pd in the attenuator. The minimum noise temperature Tn of the output from the 20 dB attenuator was Tn≤53 mK for no additional applied power and Tn≈120 mK when dissipating 30 nW. In the limit of large dissipated power (Pd>1 nW), we find Tn∝Pd1/5.4, consistent with detailed thermal modeling of heat flow in the attenuators.
Improving qubit coherence times is fundamental to the development of quantum computing technology. In most experiments in circuit quantum electrodynamics, dephasing induced by residual thermal photons in the readout resonator limits the coherence times of superconducting qubits. The authors design and test a type of microwave cavity attenuator that can be well thermalized to the base stage of a dilution refrigerator. With these cavity attenuators, they reproducibly measure enhanced qubit coherence times and report very low thermal-photon populations in the readout mode. This invention is a useful addition to the toolbox for state-of-the-art quantum circuitry.