  Conventional methods of measuring entanglement in a two-qubit photonic mixed
state require the detection of both qubits. We generalize a recently introduced
method which does not require the detection of both qubits, by extending it to
cover a wider class of entangled states. Specifically, we present a detailed
theory that shows how to measure entanglement in a family of two-qubit mixed
states - obtained by generalizing Werner states - without detecting one of the
qubits. Our method is interferometric and does not require any coincidence
measurement or postselection. We also perform a quantitative analysis of
anticipated experimental imperfections to show that the method is
experimentally implementable and resistant to experimental losses.

Quantum Physics