Quantum bit behavior of pinned fluxes on volume defects in superconductor

Abstract

We studied a qubit based on flux-pinning effects in \(\Delta H=\Delta B\) region of a superconductor. In the state that volume defects are many enough in a superconductor, \(\Delta\)H=\(\Delta\)B region on M–H curve would be evolved, which is the region that increased applied magnetic field (\(\Delta\)H) is the same as increasing magnetic induction (\(\Delta\)B). Magnetization (M) is constant in the region by 4\(\pi\)M = B - H. Here we show the behavior of fluxes in \(\Delta\)H = \(\Delta\)B region can be a candidate of qubit. Pinned fluxes on volume defects would move as a bundle in the region by repeating flux-pinning and pick-out depinning process from the surface to the center of the superconductor. During the process, magnetic fluxes would exist as one of states that are flux-pinning state at volume defects and depinning state from the defect. A difference of diamagnetic property occurs between pinning state at volume defects and depinning state from the volume defects. Thus, diamagnetic properties of the superconductor would oscillate in \(\Delta\)H=\(\Delta\)B region and the behavior would be observed in M-H curve. The oscillation can be used for a qubit by setting the pinning state at volume defects as \(\mathinner {|{1}\rangle }\) and the depinning state as \(\mathinner {|{0}\rangle }\). It is determined that the operating temperature as a qubit is up to 25 K if \(\hbox {MgB}_2\) is used as a base superconductor.