Josephson Vortex Qubit based on a Confocal Annular Josephson Junction

We report theoretical and experimental work on the development of a Josephsonvortex qubit based on a confocal annular Josephson tunnel junction (CAJTJ). Thekey ingredient of this geometrical configuration is a periodically variablewidth that generates a spatial vortex potential with bistable states. Thisintrinsic vortex potential can be tuned by an externally applied magnetic fieldand tilted by a bias current. The two-state system is accurately modeled by aone-dimensional sine-Gordon like equation by means of which one can numericallycalculate both the magnetic field needed to set the vortex in a given state aswell as the vortex depinning currents. Experimental data taken at 4.2K onhigh-quality Nb/Al-AlOx/Nb CAJTJs with an individual trapped fluxon advocatethe presence of a robust and finely tunable double-well potential for whichreliable manipulation of the vortex state has been classically demonstrated.The vortex is prepared in a given potential by means of an externally appliedmagnetic field, while the state readout is accomplished by measuring thevortex-depinning current in a small magnetic field. Our proof of principleexperiment convincingly demonstrates that the proposed vortex qubit based onCAJTJs is robust and workable.