  We introduce a technique to control the macroscopic quantum state of an rf
SQUID qubit. We propose to employ a stream of single flux quantum (SFQ) pulses
magnetically coupled to the qubit junction to momentarily suppresses its
critical current. This effectively lowers the barrier in the double-well
rf-SQUID potential thereby increasing the tunneling oscillation frequency
between the wells. By carefully choosing the time interval between SFQ pulses
one may accelerate the interwell tunneling rate. Thus it is possible to place
the qubit into a chosen superposition of flux states and then effectively to
freeze the qubit state. We present both numerical simulations and analytical
time-dependent perturbation theory calculations that demonstrate the technique.
Using this strategy one may control the quantum state of the rf-SQUID in a way
analogous to the \pi pulses in other qubit schemes. Research supported in part
by ARO grant # DAAG55-98-1-0367.
