Quantum vision: can a human see a single photon?

“I spent a lot of time in the dark in graduate school. Not only because I studied field of quantum optics, where we usually deal with a single particle of light, or photon, at the same time. But because of my studies instrument of measurement was the eye. I studied how people perceive the tiniest amount of light, and she was the first test every time,” says Rebecca Holmes, a physicist at National laboratory Los Alamos. Her work, which you now read, was published by Physics World and Applied Optics, among other places. Hereinafter in the first person.

To see a photon

I conducted these experiments in a room the size of a toilet on the eighth floor of the Department of psychology at the University of Illinois, working with my graduate Advisor Paul Cuatom and psychologist, Ransaw Francis Wong. The space was equipped with a special black-out curtains and a closed door for complete darkness. During the six years I spent countless hours in this room, sitting in an uncomfortable chair with my head on his chin for emphasis, concentrating on dull, tiny flares and waiting for the tiny flashes from the accurate light source ever created for the study of human vision. My goal was to calculate how I perceive flashes of light from several hundreds of photons to just one.

Being individual particles of light, photons belong to the world of quantum mechanics — the place which may seem quite unlike the known Universe. Physics professors say to students very seriously that the electron can be in two places at once (quantum superposition), or that the measurement of one photon can instantly affect another photon, which is far away and has no physical connection (quantum entanglement). May we take these incredible ideas just because they are in no way part of our daily existence. The electron can be in two places at once, and a soccer ball — no.

But photons are quantum particles, which humans can perceive directly. Experiments with individual photons can lead to the fact that the quantum world becomes visible, and we don’t have to wait — some experiments already possible to carry out with existing technologies. The eye is a unique biological measurement device, and its use opens up an amazing field of study where we don’t even know that could find. The study of what we see when photons are in a state of superposition, can change our understanding of the boundary between quantum and classical worlds, while people-watcher can even participate in the test of the strange consequences of quantum entanglement.

The human visual system works surprisingly well as a quantum detector. This network of nerves and organs, from eyeballs to the brain, which converts light into images that we perceive. People and relatives among the vertebrates have two main types of living light detectors: rods and cones. These photoreceptor cells reside in the retina, the light sensitive layer at the back of the eyeball. Cones give color vision, but they need bright light to work. Sticks can only see in black and white, but are configured for night vision and are most sensitive after half an hour spent in the dark.

The sticks are so sensitive that they can be activated by one photon. One photon of visible light carries only a few electron volts of energy. (Even a flying mosquito tens of billions of electron volts of kinetic energy). Cascading chain reactions and reverse loop in the wand amplifies this tiny signal to a measurable electrical response in the language of neurons.

We know that sticks are able to detect even a single photon, because the electrical response of the sticks to a single photon was measured in the laboratory. What remained unknown until recently is the question: these tiny signals pass through the rest of the visual system and allow the observer to see something or are filtered out in the form of noise and confusion. The issue is complex, because we need tools to check simply did not exist. The light that is emitted from everywhere, from the Sun to neon lights, this is just a random stream of photons like rain, falling from the sky. There is no way to accurately predict when there will be another photon, or how many photons will appear at a specified time. No matter how dim is the light, this fact does not allow to ensure that the person-the observer in fact sees only one photon — he can see two or three.

The problem of the randomness of photons

Over the past 75 years or so scientists have come up with cunning ways to get around the problem of accidental photons. But in the late 1980s a new field called quantum optics has given rise to an amazing tool: a source of single photons. It was a totally new type of light, which the world has never seen before, and it gave researchers the ability to produce exactly one photon simultaneously. Instead of the rain we received the dropper.

Today there are many recipes create single photons, including the trapped atoms, quantum dots and defects in diamond crystals. My favorite recipe is the spontaneous parametric scattering with decreasing frequency. You need to take a laser and aim it at a crystal of the borate is beta barium. Inside crystal laser photons spontaneously split into two sister photons. The newborn child of a pair of photons appears at the other end of the crystal, forming a Y-shape. Second step: take one of the daughter photons and send it to a detector of single photons, which will “beep” when it detects a photon. As a subsidiary of photons always come in pairs, the beeping will report that there is exactly one photon at the other end of the Y shape, ready for use in the experiment.

There is another important trick to study single-photon vision. Just send one photon to the observer and ask “did you see?” is incorrectly constructed experiment, because people will not be able to answer this question objectively. We don’t like to say “Yes” if you are not sure, but in such a tiny signal is difficult to be sure. The noise in the visual system — which can produce phantom flash even in complete darkness — also adds noise. It would be best to ask the observer which of the twohe would prefer. In our experiments, we randomly choose where to send the photon to the left or right eye part of the observer, and each trial was asked: “Left or right?”. If the observer can answer this question better than just trying to guess (that would give at best 50% accuracy), we know he sees something. This is called the experimental design with forced choice and it is often used in psychology.

In 2016, the research team from Vienna under the guidance of physics Alipasha Vaziri from the Rockefeller University in new York used a similar experiment to show that the person-the observer was able to answer forced choice with one photon at a time is better than trying to guess randomly, and thus convincingly demonstrated that people are actually able to see one photon. Using a source of individual photons based on spontaneous parametric scattering and the experimental design with forced choice, the scientists created two possible experiment, which can bring quantum weirdness to the human perception: a test using a state of superposition and the so-called “bell test” with nonlocality and the human observer.

Superposition is a unique quantum concept. Quantum particles — e.g., photons are described by a probability that a future measurement will find them in a certain place. Therefore, before the measurement, we believe they can be in two (or more) places at once. This idea applies not only to the location of the particles, but also other properties, such as polarization, which refers to the orientation of the plane along which the particles are distributed in the form of waves. The measurement leads to the fact that particles like “quanta collapse into the dark”, collapse into one state or another, but never knows exactly how or why the collapse happens.

The human visual system provides new and exciting ways to study this problem. One simple but scary test would be, whether people perceive the difference between a photon in a superposed state and a photon in a particular place. Physicists were interested in this question for many years and they offered a bunch of approaches — but for the moment let us consider the source of individual photons, as described above, which delivers a photon to the left or right eye part of the observer.

First, we can deliver a photon in a superposition of left and right positions — literally in two places at once — and ask the observer to tell which side, in his opinion, there was photon. To calculate any differences in the perception of the state of superposition and random guesses between “left” and “right”, the experiment will include a control group of trials in which the photon will actually be sent to just left or just right.

To create a state of superposition is the easy part. We can divide the photon in an equal superposition of left and right positions using a polarizing beam splitter, an optical component that transmits and reflects light depending on polarization. It can even ordinary window glass so you can see your reflection and what is behind the glass. The light divider just do it securely, with a pre-defined chance of transmission and reflection.

Standard quantum mechanics predicts that the superposition of left and right positions should not be no difference for the observer in comparison with the photon, which arrives randomly to the left or right. Before reaching eyes a superposition of left and right positions most likely will collapse on one side or the other so fast that nobody will notice. But yet no one will undertake such an experiment, we don’t know for sure. Any statistically significant differences in the proportion of people who report outbreaks to the left or right in a superposition will be unexpected and may mean that we don’t know something about quantum mechanics. The observer may also be asked to describe the subjective experience of the perception of photons in a superposition. And again according to standard quantum mechanics, any difference should not be — however, if it is, it can lead to new physics and improve our understanding of the quantum measurement problem.

Is it possible to see entangled particles?

People-observers could also take the test another interesting concept of quantum mechanics: entanglement. Entangled particles have the same quantum state and behave as if linked, regardless of how far away from each other.

Tests Bella, named after the Irish physicist John S. bell, is a category of experiments that show that quantum entanglement violates some of our natural ideas about reality. In the test of bell measurement of a pair of entangled particles results cannot be explained by any theory that obeys the principle of local realism. Local realism is a pair of seemingly obvious assumptions. The first is locality: things that are far away from each other, can influence each other faster than a signal travelling between them (and the theory of relativity tells us that this speed is the speed of light). The second is realism: things in the physical world always have specific properties, even if not measured and does not interact with anything else.

The essence of bell’s test is that the two particles interact and become entangled, and then we share them and hold each measure. We conduct several types of measurements — for example, measuring polarization in two different directions and agree, what they carry out “accidentally” so that two particles could not “reconcile” the results in advance. (Sounds strange, but when it comes to the quantum world, things are getting weird). The experiment is repeated many times and a new pair of particles allow to save the statistical result. Local realism imposes strict mathematical limit on how much the results between the two particles should be correlated if not linked in some bizarre way. In dozens of tests done Bella, this limit was violated, arguing that quantum mechanics does not obey locality, realism, or both of them.

Entangled photons usually prefer among the particles in the bell tests, and measurement of the violation of local realism made by means of electronic single-photon detectors. But if people can see individual photons, the observer could replace one of these detectors, playing a direct role in the verification of local realism.

Conveniently, the spontaneous parametric conversion can also be used for obtaining entangled photons.

Why are these experiments? In addition to the factor of the exception, there are serious scientific reasons. The reason why and how the state of superposition collapses with the generation of a certain result, it’s still one of the greatest mysteries of physics. Test of quantum mechanics using a new, unique, ready-to-measurement apparatus — the human visual system — could rule out certain theories. In particular, there are a number of theories about macrorealism, from which it follows that there is no open physical process that always leads to the fact that the superposition of large objects (like eyeballs and cats) collapses very quickly. This would mean that the superposition of large objects almost impossible and not unlikely. The Nobel laureate physicist Anthony Leggett of the University of Illinois has been active in developing tests of such theories. If the experiments with superposition with the participation of the human visual system have shown a clear deviation from standard quantum mechanics, it would prove that macrorealism is quite significant.