Thermal fluctuations in macroscopic quantum memory

Abstract

We describe macroscopic quantum memory devices based on type-II toroidal superconductors and estimate in one case and compute in another the rates at which quantum information stored in these devices ``degrades'' because of thermal fluctuations. In the case when the entire solid torus is superconducting, the Boltzmann factor in the rate corresponds to a well-defined critical fluctuation, and the rate is suppressed exponentially with the linear size of the system. In the case when superconductivity is confined to the surface of the torus, the rate is determined by diffusive motion of vortices around the torus and does not depend exponentially on the linear size; we find, however, that when the two dimensions of the torus are comparable the rate does not contain the usual volume enhancement factor, i.e. it does not grow with the total surface area of the sample. We describe a possible way to write to and read from this quantum memory.