| Charge fluctuations originating from gate bias and background traps can adversely affect a Cooper-pair Box (CPB) charge qubit. It has been suggested that two strongly capacitively coupled CPBs can help significantly suppress decoherence. Here we study the charge noise induced decoherence in such a coupled CPBs system. We first calculate the two lowest-energy states of this coupled system and then evaluate the relaxation rate and pure dephasing rate around its double degeneracy point. Our results show that the double degeneracy point is the optimal operating point for such a logical qubit, where pure dephasing vanishes and relaxation can be greatly suppressed by maximizing the capacitive coupling between the two boxes. Away from the double degeneracy point, pure dephasing increases rapidly even along the optimal operating line we have found. |
