Realistic limits on the nonlocality of an N-partite single-photon superposition

TL;DR

This paper investigates the practical limitations of demonstrating nonlocality with N-partite single-photon superpositions, showing that real-world imperfections prevent scalable violations of Bell inequalities.

Contribution

It demonstrates that decoherence and measurement inefficiencies negate the scalability advantages of W states for nonlocality tests in realistic experiments.

Findings

Decoherence diminishes nonlocality violations as N increases.

Measurement inefficiencies prevent scalable nonlocality demonstrations.

Nonlocality cannot be conclusively demonstrated in realistic conditions for large N.

Abstract

A recent paper [L. Heaney, A. Cabello, M. F. Santos, and V. Vedral, New Journal of Physics, 13, 053054 (2011)] revealed that a single quantum symmetrically delocalized over N modes, namely a W state, effectively allows for all-versus-nothing proofs of nonlocality in the limit of large N. Ideally, this finding opens up the possibility of using the robustness of the W states while realizing the nonlocal behavior previously thought to be exclusive to the more complex class of Greenberger-Horne-Zeilinger (GHZ) states. We show that in practice, however, the slightest decoherence or inefficiency of the Bell measurements on W states will degrade any violation margin gained by scaling to higher N. The non-statistical demonstration of nonlocality is thus proved to be impossible in any realistic experiment.