Abstract
The magnetic stability of E′ centers and the peroxy radical on the surface of α-quartz is investigated with first-principles calculations to understand their role in magnetic flux noise in superconducting qubits (SQs) and superconducting quantum interference devices (SQUIDs) fabricated on amorphous silica substrates. Paramagnetic E′ centers are common in both stoichiometric and oxygen deficient silica and quartz, and we calculate that they are more common on the surface than the bulk. However, we find the surface defects are magnetically stable in their paramagnetic ground state and thus will not contribute to 1/f noise through fluctuation at millikelvin temperatures.
Highlights
Defects in silica are known to degrade the performance of many devices, including conventional electronic[1] and quantum[2,3] circuits
The energy scale for the change of magnetization of these intrinsic defects on the silica surface is much higher than the effective temperatures of operation of devices of interest, including superconducting quantum interference devices (SQUIDs) and superconducting qubits
It is possible that E centers contribute to decoherence in these superconducting devices, since they have magnetic ground states and the 1+ charge states are at least metastable; experiments support this view since higher concentrations of E centers in plasma-enhanced chemical vapor deposited (PECVD) silica compared to thermal silica[71] correlate with a higher loss tangent.[9,42]
Summary
Defects in silica are known to degrade the performance of many devices, including conventional electronic[1] and quantum[2,3] circuits. First-principles calculations have enabled the assignment of atomistic defect structures to the measured electron paramagnetic resonance (EPR) and optical signatures of a wide variety of E centers and ODCs in silica and α-quartz.[15,16,17] Calculations of the EPR of E centers remains an active field.[18] Most calculations have focused on bulk silica, but similar defects exist on silica and α-quartz surfaces.[19,20,21] Surface defects are relevant in circuit processing because they can be buried at interfaces by overlaying other materials on top of the silica, and because their concentrations and properties can be controlled through the fabrication process
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