Natural sunlight can be used directly to produce entangled photon pairs, without a laser to power the setup. This experiment challenges the idea that a highly coherent light source is essential for this type of quantum preparation.
Entanglement connects the properties of several particles in a way that classical physics cannot reproduce. Measuring one then provides correlations with the other, even when their individual outcomes remain unpredictable. In quantum photonics, these photon pairs are usually produced with carefully controlled lasers.
Here, the starting point is instead a freely available and naturally incoherent source: the Sun. Its light brings together many wavelengths and waves whose phases vary continuously. The researchers concentrated and filtered it before sending it into a nonlinear optical crystal.
In this crystal, a phenomenon called spontaneous parametric down-conversion can turn an incoming photon into two lower-energy photons. The properties of the two new photons can then become entangled. The setup was adjusted to produce this entanglement in their polarization, that is, the orientation of their electric field.
The result does not rely only on a resemblance to an expected quantum state. The team also measures a concurrence of 0.905 ± 0.053, an indicator that quantifies entanglement between two systems. They also obtain an S value of 2.5408 ± 0.2171 in a Bell test, above the classical limit set at 2.
In other words, the observed correlations do not behave like those of two classical objects that simply received common properties at the start. This point is important because sunlight introduces a lot of noise and does not have the stability of a laboratory laser.
The authors also indicate that the generation rate remains comparable to that of laser-driven setups.
The potential interest concerns above all environments where available energy and equipment mass are limited. The researchers cite in particular interplanetary missions, where a natural source could reduce certain material needs. The next question will be to determine how reliable this approach remains when the light varies and when the setup leaves the controlled conditions of the laboratory.