Loss and decoherence due to stray infrared light in superconducting quantum circuits

TL;DR

Stray infrared light from the 4 K stage significantly degrades superconducting qubit and resonator performance, but effective shielding can greatly mitigate this loss, improving device quality and coherence times.

Contribution

This study identifies the impact of stray infrared light on superconducting circuits and demonstrates effective shielding methods to reduce loss and decoherence.

Findings

Stray infrared light causes quasiparticle generation and loss in superconducting devices.

Adding a black shield at the sample temperature improves device performance by an order of magnitude.

Proper shielding can reduce stray light effects, achieving quality factors up to 10^8.

Abstract

We find that stray infrared light from the 4 K stage in a cryostat can cause significant loss in superconducting resonators and qubits. For devices shielded in only a metal box, we measured resonators with quality factors Q = 10^5 and qubits with energy relaxation times T_1=120 ns, consistent with a stray light-induced quasiparticle density of 170-230 \mu m^{-3}. By adding a second black shield at the sample temperature, we found about an order of magnitude improvement in performance and no sensitivity to the 4 K radiation. We also tested various shielding methods, implying a lower limit of Q = 10^8 due to stray light in the light-tight configuration.