  We use a scanning superconducting quantum interference device (SQUID) to
image the magnetic flux produced by a superconducting device designed for
quantum computing. The nanometer-scale SQUID-on-tip probe reveals the flow of
superconducting current through the circuit as well as the locations of trapped
magnetic flux. In particular, maps of current flowing out of a flux-control
line in the vicinity of a qubit show how these elements are coupled, providing
insight on how to optimize qubit control.
