A black hole is not just an abyss where everything falls. Right at its horizon, in the so-called quantum atmosphere, a faint glow is born — Hawking radiation. This is not calm light but a churning stream of particles that can influence the most delicate quantum connections. The authors took a pair of particles in a special Werner state (a type of quantum entanglement) and placed them near a Schwarzschild black hole — the simplest, non-rotating kind.
They tracked geometric quantum discord — a measure of quantum correlations more subtle than ordinary entanglement. In the region accessible to an external observer, the discord behaved capriciously: as it moved away from the horizon, it first dropped, then grew again. The peak of this change occurred exactly where Hawking radiation is brightest. In the region hidden behind the horizon, the picture was reversed: there the discord first grew and then fell.
Two parameters govern this dance of correlations. The Hartle-Hawking constant (a quantity that sets the evaporation rate) acts as an amplifier: the larger it is, the stronger the redistribution of quantum correlations between the accessible and inaccessible regions. The horizon radius, on the other hand, acts as a suppressor: the more massive the hole, the weaker the effect. Thus, a black hole does not destroy quantum order but reshapes it, following its own geometry.
Geometric quantum discord is a way to measure quantum correlations even in states where entanglement is zero. That is, particles can be quantumly 'connected' without being entangled in the usual sense.