Characterizing and Optimizing Qubit Coherence based on SQUID Geometry*

Magnetic flux noise is the dominant source of dephasing in tunable superconducting qubits. While the mechanism behind this 1/f flux noise is poorly understood, it has been proposed that it originates from random fluctuations of spin impurities located on the surface of SQUIDs. A previously proposed microscopic model predicts that flux noise increases with the perimeter and decreases with the wire width of the SQUID loop. We discuss a refinement of the previous model for superconducting films of finite thickness which we validate by measuring the flux noise amplitudes of about 50 capacitively shunted flux qubits over a wide range of geometric SQUID parameters. Our results may therefore serve as a guide on how to improve SQUID designs to minimize 1/f flux noise.