Observation of the tradeoff between internal quantum nonseparability and external classical correlations

TL;DR

This paper experimentally demonstrates a tradeoff between internal quantum nonseparability within a photon and external classical correlations between photons, highlighting that classical correlations can diminish internal entanglement.

Contribution

It reveals a novel tradeoff relation between quantum nonseparability and classical correlations, extending monogamy concepts beyond entanglement alone.

Findings

External classical correlations reduce internal quantum nonseparability.

Preserving internal entanglement requires minimizing external correlations.

Experimental evidence of the tradeoff in a photonic system.

Abstract

The monogamy relations of entanglement are highly significant. However, they involve only amounts of entanglement shared by different subsystems. Results on monogamy relations between entanglement and other kinds of correlations, and particularly classical correlations, are very scarce. Here we experimentally observe a tradeoff relation between internal quantum nonseparability and external total correlations in a photonic system and found that even purely classical external correlations have a detrimental effect on internal nonseparability. The nonseparability we consider, measured by the concurrence, is between different degrees of freedom within the same photon, and the external classical correlations, measured by the standard quantum mutual information, are generated between the photons of a photon pair using the time-bin method. Our observations show that to preserve the internal entanglement in a system, it is necessary to maintain low external correlations, including classical ones, between the system and its environment.