  An experimental scheme is proposed to test Bell's inequality by using
superconducting nanocircuits. In this scheme, quantum entanglement of a pair of
charge qubits separated in a sufficient long distance may be created by cavity
quantum electrodynamic techniques; the population of qubits is experimentally
measurable by dc currents through the probe junctions, and one measured outcome
may be recorded for every experiment. Therefore, both locality and detection
efficiency loopholes should be closed in the same experiment. We also propose a
useful method to measure the amount of entanglement based on the concurrence
between Josephson qubits. The measurable variables for Bell's inequality as
well as the entanglement are expressed in terms of a useful phase-space Q
function.
