Experimental non-classicality of an indivisible quantum system

TL;DR

This paper provides experimental evidence that a single three-state quantum system (qutrit) exhibits non-classical behavior incompatible with classical models, highlighting fundamental quantum properties without involving entanglement.

Contribution

It demonstrates the first experimental violation of classical explanations for a single qutrit, emphasizing non-classicality in indivisible quantum systems without entanglement.

Findings

Single qutrit cannot be described by a classical joint probability distribution.

Experimental violation of classical models for pairwise compatible measurements.

Non-classicality arises in indivisible quantum systems without entanglement.

Abstract

Quantum theory demands that, in contrast to classical physics, not all properties can be simultaneously well defined. The Heisenberg Uncertainty Principle is a manifestation of this fact. Another important corollary arises that there can be no joint probability distribution describing the outcomes of all possible measurements, allowing a quantum system to be classically understood. We provide the first experimental evidence that even for a single three-state system, a qutrit, no such classical model can exist that correctly describes the results of a simple set of pairwise compatible measurements. Not only is a single qutrit the simplest system in which such a contradiction is possible, but, even more importantly, the contradiction cannot result from entanglement, because such a system is indivisible, and it does not even allow the concept of entanglement between subsystems.