Device-Independent Certification of a Nonprojective Qubit Measurement

TL;DR

This paper demonstrates a device-independent method to certify nonprojective measurements on qubits by violating a Bell-like inequality, removing the need for assumptions about the quantum system's internal workings.

Contribution

It introduces the first device-independent certification of nonprojective qubit measurements using Bell inequality violations in an experimental setup.

Findings

Violates a Bell-like inequality by over 8 standard deviations

Confirms the system is best described by two qubits

Achieves first device-independent certification of nonprojective measurements

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.