Violation of a Leggett-Garg inequality with ideal non-invasive measurements

TL;DR

This paper demonstrates a violation of the Leggett-Garg inequality using ideal negative result measurements in a microscopic system, supporting the non-classical nature of quantum mechanics.

Contribution

It introduces a general protocol for ideal negative result measurements with an ancillary system and reports the first experimental realization in a solid-state spin system.

Findings

Violation of Leggett-Garg inequality observed

Non-classical behavior confirmed in microscopic system

Protocol applicable to various quantum systems

Abstract

The quantum superposition principle states that an entity can exist in two different states simultaneously, counter to our 'classical' intuition. Is it possible to understand a given system's behaviour without such a concept? A test designed by Leggett and Garg can rule out this possibility. The test, originally intended for macroscopic objects, has been implemented in various systems. However to-date no experiment has employed the 'ideal negative result' measurements that are required for the most robust test. Here we introduce a general protocol for these special measurements using an ancillary system which acts as a local measuring device but which need not be perfectly prepared. We report an experimental realisation using spin-bearing phosphorus impurities in silicon. The results demonstrate the necessity of a non-classical picture for this class of microscopic system. Our procedure can be applied to systems of any size, whether individually controlled or in a spatial ensemble.