title: Witnessing trustworthy single-photon entanglement with local homodyne measurements.
abstract: 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 to say whether a given state is path entangled and 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 trustworthy method for verifying the proper functioning of future quantum networks.
authors.affiliation: Laboratoire Kastler Brossel, Université Pierre et Marie Curie, Ecole Normale Supérieure, CNRS, 4 Place Jussieu, 75252 Paris Cedex 05, France.
authors.forename: Olivier
authors.initials: O
authors.lastname: Morin
authors.forename: Jean-Daniel
authors.initials: JD
authors.lastname: Bancal
authors.forename: Melvyn
authors.initials: M
authors.lastname: Ho
authors.forename: Pavel
authors.initials: P
authors.lastname: Sekatski
authors.forename: Virginia
authors.initials: V
authors.lastname: D'Auria
authors.forename: Nicolas
authors.initials: N
authors.lastname: Gisin
authors.forename: Julien
authors.initials: J
authors.lastname: Laurat
authors.forename: Nicolas
authors.initials: N
authors.lastname: Sangouard