  We have measured flux-noise correlations in a tunable superconducting flux
qubit. The device consists of two loops that independently control the qubit's
energy splitting and tunnel coupling. Low frequency flux noise in the loops
causes fluctuations of the qubit frequency and leads to dephasing. Since the
noises in the two loops couple to different terms of the qubit Hamiltonian, a
measurement of the dephasing rate at different bias points provides a way to
extract both the amplitude and the sign of the noise correlations. We find that
the flux fluctuations in the two loops are anti-correlated, consistent with a
model where the flux noise is generated by randomly oriented unpaired spins on
the metal surface.
