Nonlocality activation in a photonic quantum network

TL;DR

This paper demonstrates experimentally that nonlocal correlations can be activated in a quantum network from states that are initially Bell-local, even under noisy conditions, without assumptions about the states or channels.

Contribution

It introduces a method to activate nonlocality in quantum networks from Bell-local states using a quantum channel, without assumptions about the system or theory.

Findings

Nonlocality can be activated from Bell-local states in a network setting.

Nonlocal correlations are certified without assumptions about the states or channels.

The approach is robust to noise and practical for real-world applications.

Abstract

Bell nonlocality refers to correlations between two distant, entangled particles that challenge classical notions of local causality. Beyond its foundational significance, nonlocality is crucial for device-independent technologies like quantum key distribution and randomness generation. Nonlocality quickly deteriorates in the presence of noise, and restoring nonlocal correlations requires additional resources. These often come in the form of many instances of the input state and joint measurements, incurring a significant resource overhead. Here, we experimentally demonstrate that single copies of Bell-local states, incapable of violating any standard Bell inequality, can give rise to nonlocality after being embedded into a quantum network of multiple parties. We subject the initial entangled state to a quantum channel that broadcasts part of the state to two independent receivers and certify the nonlocality in the resulting network by violating a tailored Bell-like inequality. We obtain these results without making any assumptions about the prepared states, the quantum channel, or the validity of quantum theory. Our findings have fundamental implications for nonlocality and enable the practical use of nonlocal correlations in real-world applications, even in scenarios dominated by noise.