Quantum Bit Behavior of Pinned Fluxes on Volume Defects in a Superconductor

Abstract

We studied a qubit based on flux-pinning effects in $\Delta$H=$\Delta$B region of a superconductor. When volume defects are many enough in a superconductor, $\Delta$H=$\Delta$B region on M-H curve is formed, 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 that 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 pick-out depinning state in which fluxes are moving in the superconductor. 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 qubit by setting the pinning state at volume defects as $\ket{1}$ and the depinned state as $\ket{0}$. This method can operate at higher temperatures than that of using Josephson Junctions. In addition, it is expected that the device is quite simple and decoherences can be almost negligible.