Violation of Bell Inequality with Unentangled Photons

TL;DR

This paper demonstrates a violation of Bell inequality using unentangled photons, revealing that quantum indistinguishability can produce quantum correlations traditionally attributed to entanglement, thus challenging conventional understanding.

Contribution

It shows Bell inequality violation arising from quantum indistinguishability rather than entanglement, expanding the understanding of quantum correlations.

Findings

Bell inequality violated by more than four standard deviations

Quantum indistinguishability can generate quantum correlations

Insights into the fundamental origin of quantum phenomena

Abstract

Violation of local realism via Bell inequality - a profound and counterintuitive manifestation of quantum theory that conflicts with the prediction of local realism - is viewed to be intimately linked with quantum entanglement. Experimental demonstrations of such a phenomenon using quantum entangled states are among the landmark experiments of modern physics and paved the way for quantum technology. Here we report the violation of the Bell inequality that cannot be described by quantum entanglement in the system but arises from quantum indistinguishability by path identity, shown by the multi-photon frustrated interference. By analyzing the measurement of four-photon frustrated interference within the standard Bell-test formalism, we find a violation of Bell inequality by more than four standard deviations. Our work establishes a connection between quantum correlation and quantum indistinguishability, providing insights into the fundamental origin of the counterintuitive characteristics observed in quantum physics.