title: Device-Independent Certification of a Nonprojective Qubit Measurement.
abstract: Quantum measurements on a two-level system can have more than two independent outcomes, and in this case, the measurement cannot be projective. Measurements of this general type are essential to an operational approach to quantum theory, but so far, the nonprojective character of a measurement can only be verified experimentally by already assuming a specific quantum model of parts of the experimental setup. Here, we overcome this restriction by using a device-independent approach. In an experiment on pairs of polarization-entangled photonic qubits we violate by more than 8 standard deviations a Bell-like correlation inequality that is valid for all sets of two-outcome measurements in any dimension. We combine this with a device-independent verification that the system is best described by two qubits, which therefore constitutes the first device-independent certification of a nonprojective quantum measurement.
authors.affiliation: Departamento de Física, Universidad de Concepción, 160-C Concepción, Chile.|Center for Optics and Photonics, Universidad de Concepción, 160-C Concepción, Chile.|MSI-Nucleus for Advanced Optics, Universidad de Concepción, 160-C Concepción, Chile.
authors.forename: Esteban S
authors.initials: ES
authors.lastname: Gómez
authors.affiliation: Departamento de Física, Universidad de Concepción, 160-C Concepción, Chile.|Center for Optics and Photonics, Universidad de Concepción, 160-C Concepción, Chile.|MSI-Nucleus for Advanced Optics, Universidad de Concepción, 160-C Concepción, Chile.
authors.forename: Santiago
authors.initials: S
authors.lastname: Gómez
authors.affiliation: Departamento de Física, Universidad de Concepción, 160-C Concepción, Chile.|Center for Optics and Photonics, Universidad de Concepción, 160-C Concepción, Chile.|MSI-Nucleus for Advanced Optics, Universidad de Concepción, 160-C Concepción, Chile.
authors.forename: Pablo
authors.initials: P
authors.lastname: González
authors.affiliation: Departamento de Física, Universidad de Concepción, 160-C Concepción, Chile.|Center for Optics and Photonics, Universidad de Concepción, 160-C Concepción, Chile.|MSI-Nucleus for Advanced Optics, Universidad de Concepción, 160-C Concepción, Chile.
authors.forename: Gustavo
authors.initials: G
authors.lastname: Cañas
authors.affiliation: Departamento de Física, Universidad de Concepción, 160-C Concepción, Chile.|Center for Optics and Photonics, Universidad de Concepción, 160-C Concepción, Chile.|MSI-Nucleus for Advanced Optics, Universidad de Concepción, 160-C Concepción, Chile.
authors.forename: Johanna F
authors.initials: JF
authors.lastname: Barra
authors.affiliation: Departamento de Física, Universidad de Concepción, 160-C Concepción, Chile.|Center for Optics and Photonics, Universidad de Concepción, 160-C Concepción, Chile.|MSI-Nucleus for Advanced Optics, Universidad de Concepción, 160-C Concepción, Chile.
authors.forename: Aldo
authors.initials: A
authors.lastname: Delgado
authors.affiliation: Center for Optics and Photonics, Universidad de Concepción, 160-C Concepción, Chile.|MSI-Nucleus for Advanced Optics, Universidad de Concepción, 160-C Concepción, Chile.|Departamento de Ingeniería Eléctrica, Universidad de Concepción, 160-C Concepción, Chile.
authors.forename: Guilherme B
authors.initials: GB
authors.lastname: Xavier
authors.affiliation: Departamento de Física Aplicada II, Universidad de Sevilla, E-41012 Sevilla, Spain.
authors.forename: Adán
authors.initials: A
authors.lastname: Cabello
authors.affiliation: Department of Theoretical Physics, University of the Basque Country UPV/EHU, P.O. Box 644, E-48080 Bilbao, Spain.
authors.forename: Matthias
authors.initials: M
authors.lastname: Kleinmann
authors.affiliation: Institute for Nuclear Research, Hungarian Academy of Sciences, H-4001 Debrecen, P.O. Box 51, Hungary.
authors.forename: Tamás
authors.initials: T
authors.lastname: Vértesi
authors.affiliation: Departamento de Física, Universidad de Concepción, 160-C Concepción, Chile.|Center for Optics and Photonics, Universidad de Concepción, 160-C Concepción, Chile.|MSI-Nucleus for Advanced Optics, Universidad de Concepción, 160-C Concepción, Chile.
authors.forename: Gustavo
authors.initials: G
authors.lastname: Lima