title: Revealing genuine optical-path entanglement.
abstract: How can one detect entanglement between multiple optical paths sharing a single photon? We address this question by proposing a scalable protocol, which only uses local measurements where single photon detection is combined with small displacement operations. The resulting entanglement witness does not require postselection, nor assumptions about the photon number in each path. Furthermore, it guarantees that entanglement lies in a subspace with at most one photon per optical path and reveals genuinely multipartite entanglement. We demonstrate its scalability and resistance to loss by performing various experiments with two and three optical paths. We anticipate applications of our results for quantum network certification.
authors.affiliation: Group of Applied Physics, University of Geneva, CH-1211 Geneva 4, Switzerland.
authors.forename: F
authors.initials: F
authors.lastname: Monteiro
authors.affiliation: Group of Applied Physics, University of Geneva, CH-1211 Geneva 4, Switzerland.
authors.forename: V
authors.initials: V
authors.lastname: Caprara Vivoli
authors.affiliation: Group of Applied Physics, University of Geneva, CH-1211 Geneva 4, Switzerland.
authors.forename: T
authors.initials: T
authors.lastname: Guerreiro
authors.affiliation: Group of Applied Physics, University of Geneva, CH-1211 Geneva 4, Switzerland.
authors.forename: A
authors.initials: A
authors.lastname: Martin
authors.affiliation: Centre for Quantum Technologies, National University of Singapore, 3 Science Drive 2, Singapore 117543, Singapore.
authors.forename: J-D
authors.initials: JD
authors.lastname: Bancal
authors.affiliation: Group of Applied Physics, University of Geneva, CH-1211 Geneva 4, Switzerland.
authors.forename: H
authors.initials: H
authors.lastname: Zbinden
authors.affiliation: Group of Applied Physics, University of Geneva, CH-1211 Geneva 4, Switzerland.
authors.forename: R T
authors.initials: RT
authors.lastname: Thew
authors.affiliation: Department of Physics, University of Basel, CH-4056 Basel, Switzerland.
authors.forename: N
authors.initials: N
authors.lastname: Sangouard