Bell inequalities with three ternary-outcome measurements - from theory to experiments (PLUS A CORRIGENDUM)

TL;DR

This paper introduces new Bell inequalities for bipartite systems with three-outcome measurements, showing they can be maximally violated by two-qubit states and demonstrating their experimental violation with photon pairs, challenging common assumptions about measurement outcomes.

Contribution

The work presents novel facet-defining Bell inequalities for the {[3 3 3] [3 3 3]} scenario, including the first example requiring non-projective measurements for maximal violation.

Findings

All inequalities can be maximally violated by two-qubit entangled states.

Experimental violation of these inequalities was successfully demonstrated.

Characterization of quantum resources can be achieved despite finite-size effects.

Abstract

We explore quantum nonlocality in one of the simplest bipartite scenarios. Several new facet-defining Bell inequalities for the {[3 3 3] [3 3 3]} scenario are obtained with their quantum violations analyzed in details. Surprisingly, all these inequalities involving only genuine ternary-outcome measurements can be violated maximally by some two-qubit entangled states, such as the maximally entangled two-qubit state. This gives further evidence that in analyzing the quantum violation of Bell inequalities, or in the application of the latter to device-independent quantum information processing tasks, the commonly-held wisdom of equating the local Hilbert space dimension of the optimal state with the number of measurement outcomes is not necessarily justifiable. In addition, when restricted to the minimal qubit subspace, it can be shown that one of these Bell inequalities requires non-projective measurements to attain maximal quantum violation, thereby giving the first example of a facet-defining Bell inequality where a genuine positive-operator-valued measure is relevant. We experimentally demonstrate the quantum violation of this and two other Bell inequalities for this scenario using energy-time entangled photon pairs. Using the obtained measurement statistics, we demonstrate how characterization of the underlying resource in the spirit of device-independence, but supplemented with auxiliary assumptions, can be achieved. In particular, we discuss how one may get around the fact that, due to finite-size effects, raw measurement statistics typically violate the non-signaling condition.