  Entanglement verification and measurement is essential for experimental tests
of quantum mechanics and also for quantum communication and information
science. Standard methods of verifying entanglement in a bipartite mixed state
require detection of both particles and involve coincidence measurement. We
present a method that enables us to verify and measure entanglement in a
two-photon mixed state without detecting one of the photons, i.e., without
performing any coincidence measurement or postselection. We consider two
identical sources, each of which can generate the same two-photon mixed state
but they never emit simultaneously. We show that one can produce a set of
single-photon interference patterns, which contain information about
entanglement in the two-photon mixed state. We prove that it is possible to
retrieve the information about entanglement from the visibility of the
interference patterns. Our method reveals a distinct avenue for verifying and
measuring entanglement in mixed states.
