Design of a ballistic fluxon qubit readout

Journal article, 2007

A detailed design is given for a flux qubit readout using ballistic fluxons. In this scheme, fluxons propagate through an underdamped Josephson transmission line (JTL) coupled to the qubit, whose state affects the fluxon propagation time. For strong qubit–JTL coupling, and far from the symmetry point, a qubit can be measured with fidelity greater than 99% and measurement time of 4 ns. The readout circuit requires additional rapid single flux quantum (RSFQ) interface circuitry to launch and receive the delayed flux solitons. The parameters of this driver and receiver have been optimized to produce low fluxon speed at launch and impedance matching at the receiver. The delayed solitons are compared to a reference line using a detector with time resolution of better than 16 ps. Both the JTL and RSFQ interface were designed for the Nb 30 A cm^-2 process developed at VTT, Finland, with postdeposition of the Al qubit at IPHT, Germany