Chain of Hardy-type local reality constraints for $n$ qubits

TL;DR

This paper introduces a new set of local realistic inequalities and Hardy-type constraints for $n$ qubits, extending non-locality arguments beyond Bell correlations and providing tools for entanglement analysis.

Contribution

It derives novel local realistic inequalities and Hardy-type constraints for $n$ qubits, generalizing previous two-qubit results and exploring quantum violations.

Findings

Quantum violations of the inequalities are maximized by specific states.

New inequalities involve joint probabilities beyond Bell correlations.

Results offer systematic tools for entanglement investigation.

Abstract

Non-locality without inequality is an elegant argument introduced by L. Hardy for two qubit systems, and later generalised to $n$ qubits, to establish contradiction of quantum theory with local realism. Interestingly, for $n=2$ this argument is actually a corollary of Bell-type inequalities, viz. the CH-Hardy inequality involving Bell correlations, but for $n$ greater than 2 it involves $n$-particle probabilities more general than Bell-correlations. In this paper, we first derive a chain of completely new local realistic inequalities involving joint probabilities for $n$ qubits, and then, associated to each such inequality, we provide a new Hardy-type local reality constraint without inequalities. Quantum mechanical maximal violations of the chain of inequalities and of the associated constraints are also studied by deriving appropriate Cirel'son type theorems. These results involving joint probabilities more general than Bell correlations are expected to provide a new systematic tool to investigate entanglement.