  We suggest a system in which the amplitude of macroscopic flux tunneling can
be modulated via the Aharonov-Casher effect. The system is an rf-SQUID with the
Josephson junction replaced by a Bloch transistor -- two junctions separated by
a small superconducting island on which the charge can be induced by an
external gate voltage. When the Josephson coupling energies of the junctions
are equal and the induced charge is q=e, destructive interference between
tunneling paths brings the flux tunneling rate to zero. The device may also be
useful as a qubit for quantum computation.
