  For a quantum double well system interacting with a mesoscopic bath, it is
shown that a single particle in the bath is sufficient to substantially reduce
tunneling between the two wells. This is demonstrated by considering an ammonia
molecule in the center of a ring; in addition to halving the maser line
frequency, there is an increase in intensity by four orders of magnitude. The
tunneling varies non-monotonically with the number N of electrons in the ring,
reflecting the changing electronic correlations. Although the tunneling is
reduced for small N, it turns around and grows to its free value for large N.
This is shown to not violate Anderson's orthogonality theorem. Experimental
implementations are discussed.
