Decoherence in a superconducting ring
  A superconducting ring has different sectors of states corresponding to different values of the trapped magnetic flux; this multitude of states can be used for quantum information storage. If a current supporting a nonzero flux is set up in the ring, fluctuations of electromagnetic field will be able to ``detect'' that current and thus cause a loss of quantum coherence. We estimate the decoherence exponent for a ring of a round type-II wire and find that it contains a macroscopic suppression factor $(\delta / R_{1})^{2}$, where $R_{1}$ is the radius of the wire, and $\delta$ is the London penetration depth. We present some encouraging numerical estimates based on this result. 
S. Khlebnikov