Suppressing 1/f Magnetic Flux Noise in Superconducting Qubits with Weak Magnetic Fields (Part 2)

1/f magnetic flux noise is known to limit the coherence of superconducting qubits, yet its microscopic origin has remained an open question for decades. While many experimental and theoretical studies have sought to elucidate the physical mechanism, one missing but critical characterization is the noise response to magnetic fields. Here we present the first study of flux-noise-limited dephasing and 1/f noise spectroscopy in superconducting flux qubits subjected to in-plane magnetic fields up to B = 100 G. Notably, in addition to an increase in noise below 1 Hz, we observe a suppression of flux noise in the MHz range accompanied by an enhancement of echo T2 up to 100 G. We also report initial observations of interesting features including magnetic-field-specific avoided crossings in the qubit spectrum and a tri-fold splitting of the qubit frequency at certain magnetic fields. This study presents a new experimental angle into the exploration of surface magnetic defects, and ultimately should help inform a complete microscopic theory of 1/f flux noise in superconducting circuits.