In 1964, John Bell suggested that if Bob saw one of a pair of photons and Alice saw the other, the duo would measure a shared quantum state only if the photons were entangled. But Bob and Alice needn't have carried man-made photon detectors, according to calculations by Nicolas Brunner and his co-workers at the University of Geneva in Switzerland: by adding a few more photons, their eyes would have been good enough.
A healthy human eye cannot spot a single photon, but can usually pick up fewer than ten. This threshold should not hinder a person's ability to detect entanglement in a similar set-up to Bell's experiment in which several photon pairs are emitted at once. Brunner suggests four observers for the task — two Alices and two Bobs — so that the observers can agree on whether they really saw such low light intensities.