Testing Bell's inequality in a constantly coupled Josephson circuit by effective single-qubit operations

In superconducting circuits with interbit untunable (e.g., capacitive) couplings, ideal local quantum operations cannot be exactly performed on individual Josephson qubits. Here we propose an effective dynamical decoupling approach to overcome the fixed-interaction difficulty for effectively implementing elemental logical gates for quantum computation. The proposed single-qubit operations and local measurements should allow testing Bell's inequality with a pair of capacitively coupled Josephson qubits. This provides a powerful approach, besides spectral-analysis [Nature 421, 823 (2003); Science 300, 1548 (2003)], to verify the existence of macroscopic quantum entanglement between two fixed-coupling Josephson qubits.