Development of a Josephson vortex two-state system based on a confocal annular Josephson junction

Abstract

We report theoretical and experimental work on the development of a Josephson vortex qubit based on a confocal annular Josephson tunnel junction (CAJTJ). The key ingredient of this geometrical configuration is a periodically variable width that generates a spatial vortex potential with bistable states. This intrinsic vortex potential can be tuned by an externally applied magnetic field and tilted by a bias current. The two-state system is accurately modeled by a one-dimensional sine-Gordon like equation by means of which one can numerically calculate both the magnetic field needed to set the vortex in a given state as well as the vortex depinning currents. Experimental data taken at 4.2 K on high-quality N b/Al-AlOx/N b CAJTJs with an individual trapped fluxon advocate the presence of a robust and finely tunable double-well potential for which reliable manipulation of the vortex state has been classically demonstrated. The vortex is prepared in a given potential by means of an externally applied magnetic field, while the state readout is accomplished by measuring the vortex-depinning current in a small magnetic field. Our proof of principle experiment convincingly demonstrates that the proposed vortex qubit based on CAJTJs is robust and workable.

Field cooled annular Josephson tunnel junctions

“…4 . The experimental findings of this Section with no trapped field ratify once again that the effects of a transverse magnetic field applied to a Lyngby-type confocal AJTJ are in all respect comparable to those of an in-plane magnetic field applied in the direction of the bias current 16,17 . In other words, the radial distribution of the magnetic field induced by the shielding currents circulating on the outer borders of the top and bottom junction's electrodes is qualitatively similar to that created by an uniform in-plane magnetic field in the Y -direction.…”

“…These effects have been investigated both theoretically and experimentally for rectangular as well as for annular junctions [13][14][15] ; furthermore, it has been demonstrated that for AJTJs made by specularly symmetric electrodes a transverse magnetic field is equivalent to an in-plane field applied in the direction of the current flow. The transverse critical field is much smaller than its in-plane analog 16,17 .…”

“…It will be shown that the passive magnetic field generated by the persistent current can efficiently replace the external field induced by coils, solenoids or control lines. The fabrication process and the geometrical layout can be found elsewhere 4,17,20,21 . All our samples were designed with the so-called Lyngby-type geometry 22 that refers to a specularly symmetric configuration in which the width of the current carrying electrodes matches one of the ellipse outer axis.…”

Confocal Annular Josephson Tunnel Junctions with Large Eccentricity

“…More details on the samples description end their fabrication were reported, respectively, in Ref. 28 and Ref. 43 .…”

“…It is this smooth periodic change of the width of the planar tunnel barrier that makes the physics of CAJTJs very rich and interesting especially since the modeling is very accurate 20,27 . Recently 28 , experiments have shown that a fluxon trapped in a long and narrow CAJTJ experiences a finely tunable DWP and both the preparation and readout of the vortex states in either the left or right state, that are important with respect to the possible realization of a vortex qubit, can be achieved by simple and robust procedures. The previously presented findings concerned CAJTJs with the moderate aspect ratio of 2 : 1 in which, as in Fig.…”