Close your eyes. What do you see?

The answer isn’t “nothing.” Your eyelids are thin enough to let in light from your surroundings. Perfect darkness is elusive. Even on a dark night – no moon, no light pollution – a million photons are falling into your eye every second, just from the dim light of the night sky.

Even in such low-light conditions, once we get accommodated to it, we still manage to see. This is thanks to rod cells specialized for night vision. They have a pigment called rhodopsin which is activated by photons and sends a “light detected” signal to the brain. But even the dark night air is brimming with photons. The Löffler Lab at Leiden University’s Institute of Physics is asking a question at the very limits of perception: Can humans consciously see a single photon?

Wolfgang Löffler and his team are part of the section Quantum Matter and Optics. This research group explores how light interacts with the physical substances of the world, from single atoms to the complex systems of the human eye. Löffler has worked  in quantum physics for over fifteen years, but remains perpetually excited about what discoveries are coming next.

“Quantum physics started around one hundred years ago, but around twenty-five years ago it became clear that we are really at the advent of a new quantum revolution. This is how I started my career: I always need to work on the next ‘next thing.’ We need to go towards new realms and new challenges in physics.” Together with colleague Tom van der Reep, Löffler’s latest fascination is how the brain interacts with the quantum world.

Einstein

Single photons are the smallest quantum, or energy unit, of light. “What does small mean? Well, people have done all kinds of experiments. You can lower and lower the intensity of light, and at a certain point you see that the light is either absorbed at multiples of a particular energy – or not at all.” In other words, there’s no such thing as half of a photon.

This fact has been known for over a century. “That is what Einstein did in 1905. If you shine light on a metal, electrons can be emitted sometimes, and this only depends on the wavelength of the light, not the intensity.” This is called the photoelectric effect, and it earned Einstein the Nobel Prize. “It then became quickly clear that we need a completely new description of the world.” This was the birth of quantum mechanics, in a nutshell.

At the microscopic level, particles behave strangely. For instance, photons can be in two places at the same time. But Löffler explains that even large molecules can exhibit this quantum weirdness. “1500 atoms, that’s the record.” The limit? According to Löffler, it probably doesn’t exist – theoretically, everything exhibits quantum behaviour.

The suggestion that everything is quantum doesn’t bother him. “I’m just happy with it. I like the idea that our world is more exciting than what I see. But I don’t claim to understand it completely. You just have to make up your mind to sleep at night.”

On a small cart in Löffler’s office is a very expensive machine wrapped in a dark blanket. “Just a minute,” he says, switching off the lights and connecting various wires into a circuit. A speaker, a power source, and the €5,000 main feature: a single photon detector. He turns it on, and the speaker emits a rapid clicking noise, like a Geiger counter in a radioactive area. This machine allows you to hear single photons.

clicking noises

“It’s detecting around 100 to 200 photons every second.” Each photon which enters the machine triggers a click. This is with the detector still wrapped tightly in the blanket, a thousand times darker than starlight.

Löffler lifts a flap of the blanket, just slightly. Even without exposing the device, the clicking noise increases drastically. The detector saturates at one million photons per second, so this is about as loud as it will get, even though the device is still in darkness.

Photon detectors like this have many applications. If your phone has face identification, it fires a weak laser at your face every time you use it to unlock your phone. The phone measures how long it takes for the photons to return, “to check if you have just a picture of somebody in front of it or a three-dimensional face.”

“With these single-photon detectors, we can characterize the darkness of a room. This is important because some materials might emit unwanted photons – even the human body.”

Cannabis tents

The Löffler lab has also solved the problem of achieving pitch darkness. They have an unexpected hack to obtain a truly dark darkroom: a grow tent for cannabis. Usually, the reflective interior of these tents keeps heat and light trapped inside – great for nurturing a certain therapeutic plant. When turned inside out, they keep the light outside instead. Placing this in an already dark optical lab makes unwanted photons infrequent.

“Now why is this a complicated problem?” Löffler asks. If they can successfully shoot single photons into the eye in a sufficiently dark room, “it all should be easy.”

Surprisingly, when it comes to testing the human perception of individual photons, the challenge that Löffler faces isn’t the quantum unpredictability of photons – it’s actually the complexity of human biology.

“We have to look at the rhodopsin molecule, the thing absorbing the light in the rods,” Löffler explains. It has the unfortunate trait of signalling spontaneously, even without absorbing a photon. This is called dark activation. The good news: “the dark activation rate of one rhodopsin molecule is only once in a thousand years.” The bad news: “We have millions and billions of rhodopsin molecules.” After doing the math “we get, ah, a few tens of dark activations per second.”

But that’s not all. “Evolution of course has done something with our brain to ignore the dark activations, otherwise we would go crazy. The fact we can sleep and don’t have a light show in our eyes, this says, yeah, evolution did something. There are several suppression mechanisms that suppress dark activations.”

Biased

This raises a new issue. If the brain suppresses spontaneous dark activations in this way, it might also suppress the legitimate signal from a single photon sent into the eye. “And therefore, I honestly must say, I do not know what the outcome of our experiment might be.”

To test whether humans can perceive a single photon will require careful control of conditions. Before the experiment, test subjects will have to stay in absolute darkness for about half an hour.

“The experiment works like this: as a guinea pig, you have to answer if you have seen a photon before or after a sound. You don’t answer ‘Have I seen a photon or not?’ That would be a very bad question, because you will be biased.”

Imagine showing a test subject an extremely dim light, twenty times, each time asking the yes/no question: “Did you see the light?” The subject may truthfully answer “no” every time, because they honestly believe that they haven’t seen anything.

However, when repeating the experiment with the before/after question, the subject can reliably determine the timing of the light. How can this be? By forcing the subject to make a decision, it taps into their unconscious perception of the light – they noticed it, even if they don’t realize it.

Happy, stressed or sick

The second part of this photon perception project is something that you could take part in yourself. “We got this grant from the National Science Agenda that we’re very happy about, because we can make the public interested in our experiments.”

“We will design an experiment in parallel with slightly stronger light pulses and a much shorter dark adaptation time.” This will be a citizen science project where anyone is welcome to contribute. “Our aim is to correlate light perception with all kinds of things: from population demographics to what neuroscientists would call it brain states. I would call it how happy you are, are you stressed, are you sick …  because there’s very little known about the difference in light detection levels depending on properties of people.”

Löffler also hopes to take advantage of this citizen science project to teach people more about physics. “For instance, the quantum behaviour of photons.” He hopes to place the experiment set-up in an accessible place, like a museum, library, or somewhere in the university. “Or even at Leiden Centraal,” he jokes. “There’s a piano in Utrecht Centraal, so why not a little box for testing light perception?”