DC SQUID Magnetometry

Synopsis

Circuits made from superconducting components offer many exciting features.  These features include extremely fast switching, very low power operation and quantum accuracy.  Additionally, the quantum mechanical nature of superconductivity can be harnessed, as in D-Wave Systems quantum processors.

The operation of these superconducting components can be affected by local magnetic fields. This can be a boon, allowing tunability of individual circuit elements.  But it can also be undesirable, particularly in the case of unwanted biasing from nearby trapped magnetic flux.  For large-scale integrated superconducting circuits, trapped flux mitigation techniques must be employed to ensure proper circuit operation.

I will present the techniques used to make accurate absolute measurements of local magnetic field across a D-Wave Systems Washington generation processor using a set of on-chip multiplexed unshunted DC SQUID magnetometers.  These measurements are used in conjunction with passive and active field compensation to minimize the magnetic field present during the superconducting transition of the chip in order to limit the number of magnetic flux lines trapped on chip.  This maximizes the operability of the superconducting quantum processor.  Detail will be given on the procedures necessary to measure the field when the SQUIDs are multiplexed and share signal lines.  A method will be given for measuring accurate magnetic fields even in the presence of asymmetries in the DC SQUID magnetometers.  Experimental results will be presented for measurements performed on a D-Wave Washington generation processor.