Quantum vision: can a person see a single photon?

“I spent a lot of time in the dark in graduate school. Not only because I studied the field of quantum optics - where we usually deal with one particle of light, or a photon, at the same time. But also because in my research the instrument of measurement was the eyes. I studied how people perceive the smallest amounts of light, and I myself became the first test subject every time, ”says Rebecca Holmes, a physicist at the Los Alamos National Laboratory. Her work, which you will now read about, was published by Physics World and Applied Optics, among other places. Next - from the first person.

See the photon

I conducted these experiments in a room the size oftoilet on the eighth floor of the psychology department of the University of Illinois, working together with my graduate consultant Pavel Kvyat and the psychologist Ranciao Francis Wong. The space was equipped with special thick curtains and a closed door to achieve total darkness. For six years, I spent countless hours in this room, sitting in an uncomfortable chair, with my head on my chin for a rest, concentrating on dim, tiny flashes and waiting for tiny flashes from the most accurate light source that was ever created to study human vision. . My goal was to calculate how I perceive flashes of light from a few hundred photons to just one.

Being individual particles of light, photonsbelong to the world of quantum mechanics - a place that may seem completely different from the known Universe. Physics professors tell students absolutely seriously that an electron can be in two places at the same time (quantum superposition) or that measuring one photon can instantly affect another photon that is far away and has no physical connection (quantum entanglement). Perhaps we take these incredible ideas so casually, because they in no way fit into our daily existence. An electron can be in two places at the same time, but a soccer ball is not.

But photons are quantum particles that humanscan perceive directly. Experiments with individual photons can lead to the fact that the quantum world will become visible, and we will not have to wait - some experiments can already be carried out with existing technologies. The eye is a unique biological measuring device, and its use opens up an amazing area of ​​research in which we don’t even know what we could find. The study of what we see when photons are in a superposition state can change our understanding of the boundary between the quantum and classical worlds, while the human observer can even take part in testing the strange effects of quantum entanglement.

The human visual system works surprisinglygood as a quantum detector. It is a network of nerves and organs, from the eyeballs to the brain, which transforms light into images that we perceive. People and other relatives among vertebrates have two main types of living light detectors: rods and cones. These photoreceptor cells are located in the retina, the photosensitive layer in the back of the eyeball. Cones give color vision, but they need a bright light to work. The rods can only be seen in black and white, but they tune in to night vision and become most sensitive after half an hour spent in the dark.

The rods are so sensitive that they canactivate one photon. One photon of visible light carries only a few electron-volts of energy. (Even a flying mosquito has tens of billions of electron-volt kinetic energy). The cascade chain of reactions and the loopback in the wand amplify this tiny signal to a measurable electrical response in the language of neurons.

We know that sticks are capable of catching evenone photon, because the electrical response of the wand to one photon was measured in the laboratory. What remained unknown until recently was the question: these tiny signals pass through the rest of the visual system and allow the observer to see something or are filtered out as noise and are lost. The question is difficult, because the necessary tools for verification simply did not exist. The light that is emitted from everywhere, from the Sun to neon lights, is just a random stream of photons, like rain falling from the sky. There is no way to accurately predict when the next photon will appear, or how many specific photons will appear at a given time interval. No matter how dim the light will be, this fact does not make sure that the observer actually sees only one photon - he can see two or three.

The problem of photon randomness

Over the past 75 years or so scientistscame up with clever ways to circumvent the problem of random photons. But in the late 1980s, a new field called quantum optics spawned an amazing tool: a source of single photons. It was a completely new type of light that the world had never seen before, and it gave scientists the opportunity to produce exactly one photon at a time. Instead of rain, we got a pipette.

Today there are many recipes for creatingindividual photons, including trapped atoms, quantum dots and defects in diamond crystals. My favorite recipe is spontaneous parametric scattering with decreasing frequency. To do this, take the laser and send it to the beta-barium borate crystal. Inside the crystal, laser photons are spontaneously split into two daughter photons. A newborn pair of daughter photons appears at the other end of the crystal, forming a Y-shape. The second step: take one of the daughter photons and send it to a single photon detector, which will "piknet" when a photon is detected. Since daughter photons are always formed in pairs, this squeak will indicate that there is exactly one photon at the other end of form Y, ready for use in the experiment.

There is another important trick to learn.single photon vision. Just sending one photon to an observer and asking "well, did you see?" - this is a wrong experiment, because a person will not be able to answer this question objectively. We don’t like to say yes if we’re not sure, but it’s hard to be sure about such a tiny signal. The noise in the visual system — which can produce phantom flashes even in complete darkness — also adds interference. It would be best to ask the observer which of the two alternatives would he prefer. In our experiments, we randomly choose where to send a photon — to the left or right side of the observer’s eye — and in each test they asked: “Left or right?”. If the observer can answer this question better than just trying to guess (which would give 50% accuracy at best), we know that he sees something. This is called experiment design with forced choice and is often used in psychology.

In 2016, a research team from Vienna underThe guidance of physicist Alipasha Vaziri of Rockefeller University in New York used a similar experiment to show that a human observer was able to respond to a forced choice with one photon better than trying to guess by chance, and thus convincingly showed that a person is really able to see one photon. Using a source of individual photons based on spontaneous parametric scattering and the design of a forced-choice experiment, scientists created two possible experiments that can bring quantum strangeness to the human perception area: a test using the state of superposition and the so-called Bell test with nonlocality and human observer .

Superposition is a unique quantum concept. Quantum particles - for example, photons - are described by the probability that the future dimension will find them in a certain place. Therefore, even before the measurement, we believe that they can be in two (or more) places at the same time. This idea applies not only to the location of the particles, but also to other properties such as polarization, which refers to the orientation of the plane along which the particles propagate in the form of waves. The measurement leads to the fact that the particles seem to "collapse", collapse into one state or another, but never know exactly how or why collapse occurs.

The human visual system provides newinteresting ways to research this problem. One simple, but eerie test would be whether people perceive the difference between a photon in a state of superposition and a photon in a particular place. Physicists have been interested in this question for many years and they have offered a bunch of approaches - but for now let's consider the source of individual photons, described above, which delivers the photon to the left or right side of the observer's eye.

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

Creating a superposition state is simple.part. We can divide the photon into an equal superposition of the left and right positions using a polarizing beam splitter, an optical component that transmits and reflects light depending on polarization. Even ordinary window glass is capable of this - so you can see both your reflection and what is behind the glass. The beam splitters simply do this reliably, with a predetermined chance of transmission and reflection.

Standard quantum mechanics predicts thatthe superposition of the left and right positions should not carry any difference for the observer compared to a photon that randomly flies to the left or right. Before reaching the eye, the superposition of the left and right positions is likely to collapse on one side or the other so quickly that no one will notice. But while no one will conduct such an experiment, we will not know for sure. Any statistically significant differences in the ratio of people who report flares on the left or right in a superposition will be unexpected - and may mean that we do not know anything about quantum mechanics. The observer can also be asked to describe the subjective experience of the perception of photons in superposition. And again, according to standard quantum mechanics, there should be no difference — however, if it does, it can lead to new physics and an improved understanding of the problem of quantum measurements.

Can you see intricate particles?

Observers could also take the test.Another interesting concept of quantum mechanics: entanglement. The entangled particles have one quantum state and behave as if they are interconnected, no matter how far they are from each other.

Bell tests, named after the Irish physicistJohn S. Bell, this is a category of experiments proving that quantum entanglement violates some of our natural notions of reality. In Bell's test, measurements of a pair of entangled particles show results that 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 from each other cannot influence each other faster than the signal travels 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 they are not measured and do not interact with anything else.

The essence of the Bell test is that two are givenparticles that interact with each other and get confused, after which we separate them and take measurements of each. We carry out several types of measurements — say, the measurement of polarization in two different directions — and agree on which one to take “randomly,” so that the two particles cannot “coordinate” the results in advance. (It sounds weird, but when it comes to the quantum world, everything becomes strange). The experiment is repeated many times and new pairs of particles make it possible to accumulate a statistical result. Local realism imposes a strict mathematical limit on how strongly the results between two particles must correlate, if not connected in some bizarre way. In dozens of tests performed by Bell, this limit was violated, proving that quantum mechanics does not obey locality, realism, or both of them.

Entangled photons are usually preferred amongparticles in Bell's tests, and measurements of violations of local realism are made using electronic single-photon detectors. But if people can see individual photons, the observer could replace one of these detectors, playing a direct role in testing local realism.

Conveniently, spontaneous parametric transformation can also be used to produce entangled photons.

Why do we need such experiments? In addition to the exclusion factor, there are serious scientific reasons. The reason why and how the state of superposition collapses with the generation of a certain result is still one of the greatest mysteries of physics. Testing quantum mechanics with the help of a new, unique, measurement-ready 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 not yet an open physical process, which always results in a superposition of large objects (like eyeballs and cats) collapsing very quickly. This would mean that superposition of large objects is almost impossible - and not unlikely. Nobel laureate, physicist Anthony Leggett from the University of Illinois was actively developing tests of such theories. If experiments with superposition with the participation of the human visual system showed a clear deviation from standard quantum mechanics, this would prove that macrorealism is quite significant.

To think only how much interesting follows from each strange consequence of quantum mechanics — and how many we still have to discover. You can read about all this in our place in Zen.