Suppressing 1/f Magnetic Flux Noise in Superconducting Qubits with Weak Magnetic Fields (Part 1)*

The microscopic origin of 1/f magnetic flux noise in superconducting circuits has remained an open question for several decades, despite extensive experimental and theoretical investigation. One crucial missing piece to develop a microscopic theory of the noise mechanism is the response to external magnetic fields. We apply in-plane magnetic fields to flux qubits, and study flux-noise-limited dephasing. We observe an enhancement of spin-echo T2 in fields up to B = 100 G. We measure the flux noise spectrum directly over several decades, and observe the suppression of flux noise in the MHz range accompanied by an enhancement of flux noise below 1 Hz. These results constitute the first characterization of flux noise limited dephasing in applied magnetic fields, which we hope will help to inform a complete microscopic theory of 1/f flux noise in superconducting circuits.