Josephson phase qubit with an optimal point

Current fluctuations in a Josephson phase qubit are considered to be a source of decoherence, especially for pure dephasing. One possible way of evading such decoherence is to employ an optimal operation point as used in flux and charge qubits, where the qubit is insensitive to the bias fluctuations. However, there is no optimal point in a phase qubit since qubit energy splitting becomes monotonically smaller with increasing the bias current. Here we propose a phase qubit with an optimal point by introducing qubit energy splitting that depends nonmonotonically on the current bias realized in capacitively coupled Josephson junctions. The effect of junction asymmetry on the optimal point is also investigated.