  Single-photon entangled states, i.e. states describing two optical paths
sharing a single photon, constitute the simplest form of entanglement. Yet they
provide a valuable resource in quantum information science. Specifically, they
lie at the heart of quantum networks, as they can be used for quantum
teleportation, swapped and purified with linear optics. The main drawback of
such entanglement is the difficulty in measuring it. Here, we present and
experimentally test an entanglement witness allowing one not only to say
whether a given state is path-entangled but also that entanglement lies in the
subspace where the optical paths are each filled with one photon at most, i.e.
refers to single-photon entanglement. It uses local homodyning only and relies
on no assumption about the Hilbert space dimension of the measured system. Our
work provides a simple and trustful method for verifying the proper functioning
of future quantum networks.
