  Magnetic flux tunability is an essential feature in most approaches to
quantum computing based on superconducting qubits. Independent control of the
fluxes in multiple loops is hampered by crosstalk. Calibrating flux crosstalk
becomes a challenging task when the circuit elements interact strongly. We
present a novel approach to flux crosstalk calibration, which is circuit model
independent and relies on an iterative process to gradually improve calibration
accuracy. This method allows us to reduce errors due to the inductive coupling
between loops. The calibration procedure is automated and implemented on
devices consisting of tunable flux qubits and couplers with up to 27 control
loops. We devise a method to characterize the calibration error, which is used
to show that the errors of the measured crosstalk coefficients are all below
0.17%.
