Quantum entanglement — when two particles become linked so that measurements performed on one affect the other — has now been observed on a scale visible to the naked human eye.

Physicist Nicolas Gisin of the University of Geneva entangled two photons, then amplified one of them by having it trigger the release of thousands of other photons, each with the same polarization as the original. When the un-amplified partner photon was observed, the triggered photons behaved similarly to what was predicted for the single entangled photon.  Because thousands of photons were affected, the result was visible to unaided human observers.

Subsequent calculations showed that the triggered photons were not strictly entangled with the partner photon, but did nevertheless provide a way for humans to observe the effects of entanglement.

As with all other quantum effects, entanglement had previously only been observed at the microscopic scale.

Ref.: Enrico Pomarico, Bruno Sanguinetti, Pavel Sekatski, Hugo Zbinden, Nicolas Gisin, “Experimental amplification of an entangled photon: what if the detection loophole is ignored?,” arXiv:1104.2212