Reducing the impact of radioactivity on quantum circuits in a deep-underground facility

L Cardani,M Clemenza,G Pettinari,M Martinez,A Cruciani,M Weber,A V Ustinov,L Gironi,Gianluigi Catelani ,M Vignati,N Casali,Francesco Valenti ,Marc Lagoin ,Fábio Henriques ,Daria Gusenkova ,C Rusconi,Oliver Sander ,C Tomei,Thibault Charpentier ,Giulia D'imperio ,Wolfgang Wernsdorfer ,S Pirro,I Colantoni,Lukas Grünhaupt ,I Pop

doi:10.1038/s41467-021-23032-z

Abstract

As quantum coherence times of superconducting circuits have increased from nanoseconds to hundreds of microseconds, they are currently one of the leading platforms for quantum information processing. However, coherence needs to further improve by orders of magnitude to reduce the prohibitive hardware overhead of current error correction schemes. Reaching this goal hinges on reducing the density of broken Cooper pairs, so-called quasiparticles. Here, we show that environmental radioactivity is a significant source of nonequilibrium quasiparticles. Moreover, ionizing radiation introduces time-correlated quasiparticle bursts in resonators on the same chip, further complicating quantum error correction. Operating in a deep-underground lead-shielded cryostat decreases the quasiparticle burst rate by a factor thirty and reduces dissipation up to a factor four, showcasing the importance of radiation abatement in future solid-state quantum hardware.

Highlights

As quantum coherence times of superconducting circuits have increased from nanoseconds to hundreds of microseconds, they are currently one of the leading platforms for quantum information processing
A daunting technological challenge, macroscopic components, such as capacitors, inductors, and Josephson junctions can be interconnected and assembled in complex quantum circuits, as recently proven by the operation of processors consisting of tens of quantum bits[4,5,6,7]. While these pioneering implementations showcase the advantages of solid-state platforms, one of their main challenges for future development, increasing quantum coherence, stems from the difficulty in decoupling from various noisy environments[2]; be that dielectric defects, magnetic moments, trapped charges and vortices, spurious electromagnetic modes, or excess quasiparticles (QPs)
QPs, which can be viewed as broken Cooper pairs, degrade the performance of superconducting circuits in two ways[8]: their presence introduces dissipation, and fluctuations in their numbers give rise to noise

Summary

Introduction

As quantum coherence times of superconducting circuits have increased from nanoseconds to hundreds of microseconds, they are currently one of the leading platforms for quantum information processing. In the case of high kinetic inductance materials, such as grAl, where the geometric inductance can be neglected[10], the measured relative frequency shift informs on the corresponding change in the number of QPs with respect to the number of Cooper pairs: δxQP = 2δL/L = −4δf0/f0.

Results

Conclusion