Classical to quantum in large number limit

TL;DR

This paper develops a quantumness witness based on the AvR test, showing that multiple copies of a system can reveal quantum properties even when individual systems appear classical, especially in large, mixed systems like NMR.

Contribution

It reformulates the AvR quantumness test for quantum states, demonstrating how multiple copies can detect quantumness in large, mixed systems contrary to classical expectations.

Findings

Multiple copies reveal quantumness in mixed systems.

Detection resources grow rapidly with system number.

Large systems can exhibit quantum features despite classical appearance.

Abstract

We construct a quantumness witness following the work of Alicki and van Ryn (AvR) in "A simple test of quantumness for a single system" [J. Phys. A: Math. Theor., vol. 41, 062001 (2008)]. The AvR test is designed to detect quantumness. We reformulate the AvR test by defining it for quantum states rather than for observables. This allows us to identify the necessary quantities and resources to detect quantumness for any given system. The first quantity turns out to be the purity of the system. When applying the witness to a system with even moderate mixedness the protocol is unable to reveal any quantumness. We then show that having many copies of the system leads the witness to reveal quantumness. This seems contrary to the Bohr correspondence, which asserts that in the large number limit quantum systems become classical, while the witness shows quantumness when several non-quantum systems, as determined by the witness, are considered together. However, the resources required to detect the quantumness increase dramatically with the number of systems. We apply the quantumness witness for systems that are highly mixed but in the large number limit, which resembles nuclear magnetic resonance (NMR) systems. We make several conclusion about detecting quantumness in NMR-like systems.