Entanglement and communication-reducing properties of noisy N-qubit states

TL;DR

This paper investigates how noisy multi-qubit states retain entanglement and Bell inequality violations, enabling quantum advantages in distributed tasks without additional processing, and identifies new entangled states with classical explanations.

Contribution

It introduces new families of multi-qubit states that are entangled yet admit local realistic descriptions, even under noise, and analyzes their Bell inequality violation properties.

Findings

Certain noisy states violate Bell inequalities, enabling quantum advantages.

Some entangled states allow classical explanations despite Bell violations.

Finite gaps between entanglement and Bell violation persist as qubit number grows.

Abstract

We consider properties of states of many qubits, which arise after sending certain entangled states via various noisy channels (white noise, coloured noise, local depolarization, dephasing and amplitude damping). Entanglement of these states is studied and their ability to violate certain classes of Bell inequalities. States which violate them allow for higher than classical efficiency of solving related distributed computational tasks with constrained communication. This is a direct property of such states -- not requiring their further modification via stochastic local operations and classical communication such as entanglement purification or distillation procedures. We identify novel families of multi-particle states which are entangled but nevertheless allow local realistic description of specific Bell experiments. For some of them, the "gap" between the critical values for entanglement and violation of Bell inequality remains finite even in the limit of infinitely many qubits.