Quantifying Entanglement with a Joint Readout of Superconducting Qubits

We employ a single channel as a joint readout of highly-entangled two-qubit states in a circuit quantum electrodynamics architecture. The measurement model for the readout is fully characterized using a set of Rabi experiments demonstrating a large sensitivity to two-qubit correlations. We quantify the high degree of entanglement by measuring the violation of a Clauser-Horne-Shimony-Holt inequality with a value of $2.61\pm0.04$, without any optimization for the target state. In its present form, this joint readout will be capable of resolving improvements to the fidelity of two-qubit operations and will be extendable to systems of three or four qubits.