  Low-capacitance Josephson junctions, where Cooper pairs tunnel coherently
while Coulomb blockade effects allow the control of the total charge, provide
physical realizations of quantum bits (qubits), with logical states differing
by one Cooper-pair charge on an island. The single- and two-bit operations
required for quantum computation can be performed by applying a sequence of
gate voltages. A basic design, described earlier [cond-mat/9706016], is
sufficient to demonstrate the principles, but requires a high precision time
control, and residual two-bit interactions introduce errors. Here we suggest a
new nano-electronic design, close to ideal, where the Josephson junctions are
replaced by controllable SQUIDs. This relaxes the requirements on the time
control and system parameters substantially, and the two-bit coupling can be
switched exactly between zero and a non-zero value for arbitrary pairs. The
phase coherence time is sufficiently long to allow a series of operations.
