Human Eye Is Able to Detect a Single Photon

There was an interesting article published the other day in a scientific discipline that I had never even heard of before: psychophysics. Asides from being a physically weird word to look at it sounds like a module from the first semester of Super Villain School. The open access article, published in Nature Communications, purports to be the first example of the human eye being capable of detecting a single photon.

It is normally quite difficult to reliably produce a single photon but in this experimental set up the researchers used spontaneous parametric down-conversion. This is a quantum optical technique where a highly energetic photon is used to produce two photons of lesser energy. The two photons pass through a crystal which separates them out. One of the pair was sent to the eye of the test subject whilst the other was sent to a photon detector. This allowed the team to know if a pair of photons had been produced as intended or if more photons or perhaps even none had been produced.

Test subjects were put in a light-proof container in a darkened room for over half an hour to allow their eyes to become dark-adapted. Once ready to begin the trial they kept their head in the correct position with a bite bar and a head rest. Sounds like a lot of fun, doesn’t it?

When ready the participant would trigger the experiment. The subject was presented with two intervals each of 1ms duration and separated by ~800ms, with the initiation of each interval being heralded by a synchronous acoustic signal of ~10ms duration. During one of the two intervals a single photon would be fired into their eye, during the other interval nothing would happen. The subject then had to indicate during which interval they thought they saw the photon and how confident they were of their decision. Finally they would receive acoustic feedback to let them know if they were correct or not. The whole process took about 2.5 seconds after which they began again at their leisure.

Because one of of the photon pair was sent to a detector the researchers were able to weed out any trials where none, or more than one, photon was sent to the participant’s eye. From more than 30,000 trials they were left with 2,420 which, I guess, demonstrates how difficult it is to make single photons reliably.

Taken together the trial participants were able to detect the single photon 51.6% of the time. Given that there are only two possible outcomes this, on the face of it, is only fractionally better than chance. There are further factors to allow for, however.

The human eye is far from perfect; I’ve discussed in a previous post the shoddy architecture involved that no ‘designer’ would dare give their name to due to its shocking inefficiency. That single photon has to negotiate the cornea, all the vitreous humour inside the eyeball and, crucially, the network of blood vessels that overlay the retina. Then, if it happens to hit the right cell in the right place there is a chance for the human brain to detect it. Once you allow for this being slightly better than 50:50 chance is actually quite impressive.

Having said all this their P-values are not particularly impressive, not that P-values are the be all and end all. P=0.0545 means that there is a greater than 5% chance that the null hypothesis is valid.

One interesting effect they did find was that there was a greater chance of detecting a photon if the eye had already recently (last 10 seconds) seen one. It’s as if the first photon primes the eye for detection of the second. It is unclear why this would be the case.

In the final paragraph of the paper they speculate that if we can see single photons then perhaps we can do more. As we’re dealing with single particles then the whole world of quantum physics is potentially opened up to us. Photons are capable of being in two places at the same time, yes, I know, quantum physics makes my brain hurt. This is known as superposition and could allow humans to be used as observers for experimental tests of quantum non-locality. Personally, I think they’re getting a little ahead of themselves here. We would need to see the single photon detection experiment repeated a couple of times to even be completely sure that that effect is real. It is certainly an intriguing concept, though.



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