  We study quantum fluctuations of persistent current in a small
superconducting ring. Based on a microscopic model of the ring we argue that
under certain conditions such ring will exhibit coherent quantum phase slips,
similar to those in a flux qubit. We evaluate the frequency of such coherent
oscillations and find that it is strongly dependent on wire's diameter
primarily due to the large momentum released by the condensate as a result of a
phase slip event. We also find that the value of such frequency is not a
self-averaging quantity, that is, it depends on a particular realization of the
static impurity potential.
