Detecting highly entangled states with a joint qubit readout
Jerry M. Chow, L. DiCarlo, Jay Gambetta, Andreas Nunnenkamp, Lev S. Bishop, Luigi Frunzio, Michel Devoret, S. M. Girvin, Robert J. Schoelkopf · Physical Review A · 2010
A single-channel joint readout is used to analyze highly entangled two-qubit states in a circuit quantum electrodynamics architecture. The measurement model for the readout is fully characterized, demonstrating a large sensitivity to two-qubit correlations. We quantify the high degree of entanglement by measuring a violation of the Clauser-Horne-Shimony-Holt inequality with a value of $2.61\ifmmode\pm\else\textpm\fi{}0.04$, without optimizing the preparation of the two-qubit state. In its present form, this joint readout can resolve improvements to the fidelity of two-qubit operations and be extended to three or four qubits.