All states are nonclassical: entanglement of joint statistics

TL;DR

This paper demonstrates that all quantum states exhibit nonclassical correlations when joint measurements are considered, except the maximally mixed state, challenging the classical intuition about certain states.

Contribution

It proves that every quantum state, including classical-like states, displays nonclassical joint statistics under suitable measurements, revealing a universal nonclassicality.

Findings

All states are nonclassical under joint measurements.

Classical-like states such as SU(2) and Glauber coherent states are also nonclassical.

Existence of non-separable joint measurement statistics for all states.

Abstract

Joint measurements of two observables reveal that every state is nonclassical, with the only trivial exception of the state with density matrix proportional to the identity. This naturally includes states considered universally as classical-like, such as SU(2) and Glauber coherent states. We show that this holds because we can always find a joint measurement whose statistics is not separable.