The eye is a remarkable detector. Neuroscientists have long known that a single rod taken from a retina can respond to single photons. That’s comparable to the best artificial detectors.

But the performance of living and breathing humans is significantly worse. When researchers fire photons into the eyes of volunteers, it takes a hundred or so photons to reliably induce a response. Much of this difference is down to the fact that only about 10 per cent of the photons that enter the cornea actually reach the retina. 

But that still leaves an order of magnitude between the detection efficiency of individual rods and the behaviour of humans. Why the difference?

Today, Gibran Manasseh and pals at the University of Geneva provide an answer. 

These guys fired varying numbers of photons into the eyes of 12 healthy volunteers and who they asked to press a button when they perceived a flash. At the same time, the researchers measured each volunteer’s brain activity using EEG recordings.

The results make for interesting reading. These guys say that half the subjects regularly reported false positives, saying they saw a flash when no photons had been sent. 

They say the retina itself introduces some noise but their new result is that the brain itself introduces even more noise. “The brain adds a large contribution to the decrease in sensitivity,” they conclude.

Why is this important? This group at the University of Geneva led by Nic Gisin has an other interesting project on the go that we’ve looked at before.

Back in 2008, these guys calculated that it ought to be possible for humans to experience entanglement, the strange quantum phenomenon in which two objects share the same existence even though they are spatially separated. 

Physicists routinely measure entanglement in the lab. They create a pair of entangled photons and send them to detectors in different places. A measurement on one photon immediately influences the other, regardless of the distance between them. That’s what Einstein called “spooky action at a distance”.

Gisin and co’s idea is that you could replace one or both of the detectors with human eyes.  The process of seeing a single photon would be equivalent to measuring it which would immediately influence the other photon.

That should work when detectors can spot single photons, as single rods can do. 

Since then they’ve discovered that the experiment is significantly more difficult because humans need to receive more photons to see them.  In 2010, they produced a paper examining the challenge of entangling the required number of photons.

This latest work is important because it attempts to quantify the kind of noise that such an experiment would introduce. This will be an important factor if and when they ever attempt to use a human in such an entanglement experiment. 

What’s clear is that human entanglement experiments are looking increasingly difficult. We’ll keep our eyes open for the next instalment.  

Ref: arxiv.org/abs/1208.1652: Retinal And Post-Retinal Contributions To The Quantum Efficiency Of The Human Eye