Bell Inequality Tests with Macroscopic Entangled States of Light

TL;DR

This paper explores methods to perform loophole-free Bell tests using macroscopic entangled states of light, aiming to overcome detection challenges and reveal quantum nonlocality more effectively.

Contribution

It analyzes a preselection approach to enhance macroscopic entanglement usefulness in Bell tests, offering tools applicable to various filtering schemes.

Findings

Preselection can improve the detection of macroscopic entanglement.

Filtering methods are crucial for practical Bell tests with macroscopic states.

The tools developed can analyze different state preprocessing schemes.

Abstract

Quantum correlations may violate the Bell inequalities. Most of the experimental schemes confirming this prediction have been realized in all-optical Bell tests suffering from the detection loophole. Experiment which closes this loophole and the locality loophole simultaneously is highly desirable and remains challenging. A novel approach to a loophole-free Bell tests is based on amplification of the entangled photons, i.e.\@ on macroscopic entanglement, which optical signal should be easy to detect. However, the macroscopic states are partially indistinguishable by the classical detectors. An interesting idea to overcome these limitations is to replace the postselection by an appropriate preselection immediately after the amplification. This is in the spirit of state preprocessing revealing hidden nonlocality. Here, we examine one of possible preselections, but the presented tools can be used for analysis of other schemes. Filtering methods making the macroscopic entanglement useful for Bell test and quantum protocols are the subject of an intensive study in the field nowadays.