  A quantum computer that can be constructed based on a superconducting
nanocircuits has previously been proposed. We examine the effect of background
charge fluctuations on a coupled Josephson charge qubit system used in such a
computer. In previous work, the background charge fluctuations were found to be
an important dephasing channel for a single Josephson qubit. We investigate the
effect of fluctuations in the bias at the charge degeneracy point of a
Josephson charge qubit system. Evaluated quantities are gate fidelity and
diagonal elements of the qubit's density matrix. The fluctuation leads to gate
error, however quantum gate operation becomes more accurate with increasing
interaction between qubit systems.
