  Accurate and precise detection of multi-qubit entanglement is key for the
experimental development of quantum computation. Traditionally, non-classical
correlations between entangled qubits are measured by counting coincidences
between single-shot readouts of individual qubits. We report entanglement
metrology using a single detection channel with direct access to
ensemble-averaged correlations between two superconducting qubits. Following
validation and calibration of this joint readout, we demonstrate full quantum
tomography on both separable and highly-entangled two-qubit states produced on
demand. Using a subset of the measurements required for full tomography, we
perform entanglement metrology with ~95% accuracy and ~98% precision despite
~10% fidelity of single measurements. For the highly entangled states, measured
Clauser-Horne-Shimony-Holt operators reach a maximum value of 2.61+/-0.04 and
entanglement witnesses give a lower bound of ~88% on concurrence. In its
present form, this detector will be able to resolve future improvements in the
production of two-qubit entanglement and is immediately extendable to 3 or 4
qubits.
