  Charge fluctuations from gate bias and background traps severely limit the
performance of a charge qubit in a Cooper-pair box (CPB). Here we present an
experimentally realizable method to control the decoherence effects of these
charge fluctuations using two strongly capacitively coupled CPBs. This
coupled-box system has a low-decoherence subspace of two states. Our results
show that the inter-box Coulomb correlation can help significantly suppress
decoherence of this two-level system, making it a promising candidate as a
logical qubit, encoded using two CPBs.
