Local quasi hidden variable modelling and violations of Bell-type inequalities by a multipartite quantum state

TL;DR

This paper introduces a new local quasi hidden variable model that preserves locality but relaxes positivity, providing a unified framework to analyze Bell inequality violations in multipartite quantum states and establishing new bounds on such violations.

Contribution

The paper develops a general LqHV simulation model for quantum correlations, deriving analytical bounds on Bell inequality violations for multipartite states, including infinite-dimensional cases.

Findings

Every quantum correlation scenario admits an LqHV simulation.

Derived a new upper bound (2S-1)^{N-1} on Bell violations for N-partite states.

Provided exact numerical estimates for Bell violations in quantum information applications.

Abstract

We introduce for a general correlation scenario a new simulation model, a local quasi hidden variable (LqHV) model, where locality and the measure-theoretic structure inherent to an LHV model are preserved but positivity of a simulation measure is dropped. We specify a necessary and sufficient condition for LqHV modelling and, based on this, prove that every quantum correlation scenario admits an LqHV simulation. Via the LqHV approach, we construct analogs of Bell-type inequalities for an N-partite quantum state and find a new analytical upper bound on the maximal violation by an N-partite quantum state of S_{1}x...xS_{N}-setting Bell-type inequalities - either on correlation functions or on joint probabilities and for outcomes of an arbitrary spectral type, discrete or continuous. This general analytical upper bound is expressed in terms of the new state dilation characteristics introduced in the present paper and not only traces quantum states admitting an S_{1}x...xS_{N}-setting LHV description but also leads to the new exact numerical upper estimates on the maximal Bell violations for concrete N-partite quantum states used in quantum information processing and for an arbitrary N-partite quantum state. We, in particular, prove that violation by an N-partite quantum state of an arbitrary Bell-type inequality (either on correlation functions or on joint probabilities) for S settings per site cannot exceed (2S-1)^{N-1} even in case of an infinite dimensional quantum state and infinitely many outcomes.