# Optimal experimental demonstration of error-tolerant quantum witnesses

## TL;DR

This paper refines and experimentally demonstrates two error-tolerant quantum witnesses using photonic systems, providing robust tests of macroscopic realism with high statistical significance and minimal loopholes.

## Contribution

It introduces improved quantum witnesses for macroscopic realism and implements them in a microscopic photonic system, enhancing the reliability of quantum tests.

## Key findings

- Maximum violation of the first witness grows with system dimensionality.
- Second witness achieves maximum violation in any dimension.
- Experimental implementation confirms robustness and significance of the tests.

## Abstract

Testing quantum theory on macroscopic scales is a longstanding challenge that might help to revolutionise physics. For example, laboratory tests (such as those anticipated in nanomechanical or biological systems) may look to rule out macroscopic realism: the idea that the properties of macroscopic objects exist objectively and can be non-invasively measured. Such investigations are likely to suffer from i) stringent experimental requirements, ii) marginal statistical significance and iii) logical loopholes. We address all of these problems by refining two tests of macroscopic realism, or `quantum witnesses', and implementing them in a microscopic test on a photonic qubit and qutrit. The first witness heralds the invasiveness of a blind measurement; its maximum violation has been shown to grow with the dimensionality of the system under study. The second witness heralds the invasiveness of a generic quantum operation, and can achieve its maximum violation in any dimension -- it therefore allows for the highest quantum signal-to-noise ratio and most significant refutation of the classical point of view.