Single Photon Detection: Limits of the Human Eye

Fields covered: Biology, Quantum Physics, Quantum Biology

Related Articles: 
§ Nature Communications – Logos Download Direct detection of a single photon by humans  § The Journal of General Physiology | Publons Energy, Quanta, and Vision

Light at the tiniest scales appears as photons bouncing off objects. Can our eyes detect these individual particles?

Eyes are remarkable organs, capable of sensing light. For most of us, they are our main conduit for interpreting the world around us. We know that when moving from a bright location to a dark area, our eyes need time to adapt to be able to see in low light conditions. However, what is the absolute limit of darkness that our eyes can operate under?

For a while now, it is known that astronauts working outside the earth’s protective magnetic field report seeing flashes of light even with their eyes closed. Current hypotheses involve high energy particles slamming through the spacecraft and into the spacefarer’s eyes, where they release light upon entering the vitreous medium. This has led researchers to wonder about the capability of our eyes in detecting such low-light events.

Light in our everyday world behaves as a ray that travels in straight lines. Going down to the quantum scale, light can behave as individual particles, known as photons. These photons are the fundamental limit of low-light levels – a torch can produce an uncountable number of photons per second. 

In 2016, Jonathan Tinsley looked into whether the human eye can detect single photons. Test subjects were kept in a dark room to acclimatise their vision, and single photons were sent to their eyes. There were two time-intervals when the photons could be sent – the subjects had to determine which interval the single photon was emitted, and feedback their confidence in their answer.

Figure 1: Experimental Protocol, Taken via Nature

Tinsley found that the participants managed to obtain the correct answer at a rate slightly above random chance (51.6%). Moreover, this result was statistically significant, showing that it is just barely possible for humans to directly perceive single photons.

This was very remarkable, given that current technologies to create single photon detectors (and counters) often require elaborate devices, cooled down to extremely low temperatures. Admittedly, these devices have higher accuracy (quantum efficiency) than the human eye. Nevertheless, the capability of biological apparatuses to even approach the quantum limit is an exciting discovery. 

Humanity’s ability to detect light at the quantum scale, and our capacity to prepare light in various quantum states (polarisations, entanglement, etc.), invites the possibility of experimenting with humans to detect quantum effects (such as non-locality). Any quantum experiments on the human scale would be a new landmark in integrating quantum technologies in our everyday lives.

Further Readings:

Tinsley, J., Molodtsov, M., Prevedel, R. et al. Direct detection of a single photon by humans. Nat Commun 7, 12172 (2016). doi:10.1038/ncomms12172.

Hecht, Selig; Shlaer, Simon; Pirenne, Maurice Henri (July 1942). “Energy, Quanta, and Vision”. Journal of General Physiology. 25 (6): 819–840. doi:10.1085/jgp.25.6.819. PMC 2142545. PMID 19873316.

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