Single-Photon Measurement of Two-Qubit States

Quantum state tomography is a process to construct the density matrix of an unknown quantum state. Standard methods to perform state tomography of a two-photon mixed state require the detection of both photons via coincidence or an equivalent measurement technique. Recent applications of quantum interferometry to entanglement measurement problems have revealed that it is possible to characterize entanglement in certain types of two-photon mixed states by detecting only one of the photons. We show here that quantum state tomography of an arbitrary two-qubit photonic mixed state can be performed without detecting one of the photons. Our method is based on quantum interferometry and does not require any coincidence measurement.