Can Humans Observe Single Particle Of Light?

Hoping to learn something about the human brain, Leiden researchers are creating a setup to shoot single photons, particles of light, into someone's eye. 'The eye is a passageway to the brain.'

A neuroscientist who wants to understand the brain and a physicist with a lot of knowledge about light. Not the most obvious collaboration, and the lab where the research is being conducted is equally unexpected. The completely dark room is virtually empty except for two cannabis growing tents. Such tents keep heat and light in - or out in this case. Every crack and keyhole where light could get through is taped off.

'I mostly enjoy simply understanding how things work.'

So how weak is the light from a single photon? 'Imagine a dark night with only one star,' begins Tom van der Reep, postdoc in Wolfgang Löffler's research group (Leiden Institute of Physics). 'From that one star, millions of photons reach your eye every second. So a single photon is really incredibly dim. I spent weeks blocking all ambient light so that we were left with only the photons from our light source. Only after some 100 meters of aluminum tape and 600 hours of measurements could I conclude that it is truly dark in here.'

Blankers and Van der Reep next to the growing tent and taped off windows.

Experimenting in the dark

Trainee Steffi Blankers is in the third year of the university of applied science progamme Applied Physics and is tasked with developing the experiment further. 'In our setup, we are currently shooting pulses of between zero and 1000 photons into someone's eye. Ultimately, these should become pulses of between zero and ten photons. We use a simple LED light that we attenuate considerably. We do this mainly by coupling the cables through which the light passes poorly,' the student laughs. 'That's not how you're supposed to use them.'

'My challenge was to get the light pulse to exactly the right place in someone's eye. Near the rods to be precise, which enable us to see in the dark,' Blankers explains. 'That's a very precise job.' The test subject sits in one growing tent with their head in a headrest, looking at a red light while the pulses come in. This ensures that the eye stays in the same position. The researcher oversees the experiment in the other growing tent. 'I enjoy working in the lab on a question we don't know the answer to yet.'

A single photon sheds light on the brain

This research, according to Van der Reep, is interesting for several reasons. 'We understand so much about the world around us and yet in many ways our own brain remains a mystery. We don't know exactly what happens there. And the eye is a passageway to the brain. With weak signals you can best follow the pathways in your brain,' he explains. Therefore, the project was set up together with neuroscientist Yaïr Pinto of the University of Amsterdam.

'If we can pull this off, it raises many more interesting questions,' Van der Reep continues. 'What happens when someone is distracted and what does that say about what we observe unconsciously? Could you also see the signal from a single photon on an EEG scan of someone's brain? And could you tell where a signal breaks down in people with visual impairment? I have no idea if that's ever going to be possible. I mostly enjoy simply understanding how things work.'

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