Measurement of Two-qubit Entanglement with Single-photon Detection

Standard methods of measuring entanglement in two-photon quantum states require detection of both photons when the quantum state is not pure. Recent applications of quantum interferometry to entanglement measurement problems have revealed that it is possible to measure entanglement in two-qubit mixed states without detecting one of the photons. These new type of measurement techniques do not require any coincidence measurement or postselection. We discuss two types of approaches that can be taken for such measurements. In the first approach, two identical sources producing the same quantum state is used. In the second approach, one of the two sources generates the state to be measured and the other source generates a known quantum state. These measurement techniques are useful when one of the photons in the two-photon state has a wavelength for which adequate detectors are not available.