Characterization of an entangled system of two superconducting qubits   using a multiplexed capacitance readout
  We characterize a pair of Cooper-pair boxes coupled with a fixed capacitor using spectroscopy and measurements of the ground-state quantum capacitance. We use the extracted parameters to estimate the concurrence, or degree of entanglement between the two qubits. We also present a thorough demonstration of a multiplexed quantum capacitance measurement technique, which is in principle scalable to a large array of superconducting qubits. 
M. D. Shaw, J. F. Schneiderman, J. Bueno, B. S. Palmer, P. Delsing and   P. M. Echternach