Single photon experiment reveals the mystery of quantum world

　　As a single light particle, photons belong to the world of quantum mechanics, a place that looks completely different from the universe as we know it.

　　The physics professor told the students bluntly that an electron can appear in two places at the same time (quantum superposition), or the measurement of one photon can immediately affect another distant photon without any physical connection (quantum entanglement).

　　Perhaps the reason why we accept these incredible ideas so casually is because we usually don't have to integrate them into our daily life. Electrons can appear in two places at the same time, but football can't.

　　But in fact, photons are quantum particles that humans can directly perceive. Single photon experiment can make the quantum world clearly visible, and we can do many tests with today's technology without waiting.

　　The eye is a unique biometric device, which can open up exciting research fields. We really don't know what we will find.

　　Studying what we see when photons are in superposition state will help us understand the boundary between quantum world and classical world, and human observers can even participate in the test of the strangest consequences of quantum entanglement.

　　As a quantum detector, the working principle of human visual system is amazing. From the eyeball to the brain, it is a network of nerves and organs, which converts light into images we perceive.

　　Humans and our vertebrate relatives have two main types of living light detectors, rod cells and cone cells. These photoreceptor cells are located in the retina, the photoreceptor layer at the back of the eyeball.

　　Cone cells provide color vision, but they need bright light to work properly. Rod cells can only see black and white images, but they have night vision function and become most sensitive after about half an hour in the dark.

　　Rod cells are so sensitive that a photon can activate them. A photon of visible light carries only a few electron volts of energy. Even a flying mosquito has tens of billions of electron volts of kinetic energy.

　　The cascade chain reaction and feedback loop in rod-shaped cells amplify this tiny signal into measurable electrical response, that is, the language of neurons.

　　We know that rod cells are single photon detectors, because the electric response of rod cells to single photon has been measured in the laboratory.

　　Until recently, we still don't know whether these tiny signals can be transmitted to other parts of the visual system and make the observer see anything, or whether they will be filtered out as noise or lost in other ways.

　　This question is difficult to answer because there is no suitable tool. From the sun to neon lights, the light they emit is a random stream of photons, just like raindrops falling from the sky.

　　We can't accurately predict when the next photon will arrive, nor can we accurately predict how many photons will arrive in any time interval.

　　No matter how weak the light is, this fact makes it impossible for a human observer to be sure that she really only saw one photon. She may have seen two or three photons.

　　Now there are two possible experiments that can bring the strangeness of quantum into the field of human perception.

　　In the past 75 years or so, researchers have come up with many ingenious ways to try to bypass the random photon problem.

　　But in the late 1980s, a revolutionary tool, single photon source, was developed in a new field called quantum optics.

　　This is a new light source that has never been seen in the world. It enables researchers to generate one photon at a time. We now have a dropper instead of a rainstorm.

　　Nowadays, there are many ways to make single photons, including trapped atoms, quantum dots and defects in diamond crystals. Rebecca Holmes' favorite method was learned in graduate school, which is called spontaneous parametric down conversion.

　　Step 1: irradiate barium borate crystal with laser. Inside the crystal, laser photons sometimes spontaneously split into two sub-photons. A new pair of sub-photons emerge from the other end of the crystal to form a Y-shape.

　　Step 2: Take one of the sub-photons and send it to the single photon detector. Every time the detector detects a photon, it will make a "click" sound.

　　Because daughter photons are always produced in pairs, this "click" sound announces that there is just one photon at the other end of the Y-shape, which can be used in the experiment.

　　There is another important skill in studying single photon vision. Just send a single photon to the observer and ask, did you see it?

　　This experimental design is flawed because it is difficult for human beings to answer this question objectively. We don't like to say "yes" unless we are sure, but it is difficult to determine such a small signal.

　　Noise in the visual system, even in a completely dark environment, will produce illusions and increase confusion. A better strategy is to let the observer choose between two options.

　　In the experiment, a photon was randomly sent to the left or right side of the observer's eye, and in each experiment, the observer was asked whether it was left or right.

　　If observers can answer this question better than random guessing (only 50% accuracy at most), we will know what they see. This is the so-called forced choice experimental design, which is very common in psychological experiments.

　　In 2016, a team led by physicist Waziri used similar experiments to show that human observers can respond better to the forced selection of a single photon than random guessing, which provides the best evidence that humans can really see only one photon so far.

　　Using a single photon source based on spontaneous parametric down-conversion and forced selection experimental design, there are now two possible experiments that can bring quantum strangeness into the field of human perception.

　　One is to use superposition state to test, and the other is to use human observers to do so-called "Bell test" on nonlocality.

　　Superposition is a unique quantum concept. Quantum particles (such as photons) are described by the probability of finding them in a specific position through future measurements.

　　Before measurement, it was thought that they could be in two positions at the same time. This idea applies not only to the position of particles, but also to other characteristics, such as polarization, which refers to the direction of particles along the plane in the form of waves.

　　Measurement seems to make particles "collapse" in one way or another, but no one knows exactly how or why the collapse happened.

　　The human visual system provides an interesting new way to study this problem. A simple but bizarre test is to determine whether humans can perceive the difference between photons in a superposition state and photons in a certain position.

　　Physicists have been interested in this problem for many years and have proposed several methods-but now let's consider the single photon source described above, which transmits a photon to the left or right side of the observer's eye.

　　First, we can emit a photon at the overlapping position of the left and right positions (actually in two places at the same time), and then let the observers report on which side they think the photon appears.

　　In order to quantify the perceptual difference between the superposition state and the randomly guessed left and right state, the experiment will also include a set of control experiments, in which photons are really only sent to the left or right.

　　Creating a superposition state is the simplest part. We can use a polarized beam splitter to divide photons into equal superposition of left and right positions.

　　Many surfaces can do this, even ordinary window glass can transmit and reflect light at the same time, which is why you can see both the outdoor light and your own reflection.

　　The beam splitter is designed to do this reliably, and the probability of transmission and reflection is determined in advance.

　　Standard quantum mechanics predicts that, in the observer's view, the photons superimposed on the left and right are no different from those randomly sent to the left or right.

　　After reaching the eyes, the left-right superposition may collapse to one side or the other so quickly that it cannot be detected. But no one has tried such an experiment, so we can't be sure.

　　In the superposition experiment, any statistically significant difference in the proportion of people reporting things on the left or right will be unexpected, which may mean that what we know about quantum mechanics is missing.

　　We can also ask observers to record their subjective feelings when observing the superposition state and compare them with random mixtures.

　　Similarly, according to standard quantum mechanics, we don't want to see any differences, but if we do, it may point to new physics and let us better understand the problem of quantum measurement.

　　If human beings can see a single photon, then the observer can directly play a role in the local reality test, and human observers can also participate in the test of another decisive concept of quantum mechanics, entanglement. Entangled particles share a quantum state, and no matter how far apart they are, they behave as if they are connected.

　　Bell experiment, named after Northern Ireland physicist John Bell, is a kind of experiment that proves that quantum entanglement violates some of our natural views on reality.

　　In the Bell experiment, the measurement results of entangled particle pairs show results that can't be explained by any theory that obeys the principle of local reality. Local realism is a pair of seemingly obvious assumptions.

　　The first is locality. Things that are far away from each other will not interact faster than the speed at which signals propagate between them (relativity tells us that the speed limit is the speed of light).

　　Secondly, reality, things in the physical world have certain properties at any time, even if they are not measured, they have not interacted with anything else.

　　The concept of Bell experiment is that two particles interact and get entangled, and then we separate them and measure each particle.

　　We have to make at least two measurements, two polarization measurements in different directions-we randomly decide which measurement to make, so the two particles can't "agree" on the results in advance.

　　This may sound like a bizarre conspiracy theory, but when it comes to entangled strange experimental results, it is very important to exclude all other explanations.

　　The experiment will be repeated many times with new particle pairs to get statistical results. Local realism sets strict mathematical limits on the results between two particles. If they are not connected in some strange way, how much correlation should there be between them?

　　In dozens of Bell experiments, this limitation was broken, which proved that quantum mechanics neither abides by locality nor reality, or neither.

　　Entangled photons are usually the particles selected by Bell test, while the measurement that violates local reality is carried out by using electronic single photon detector.

　　But if humans can see a single photon, then the observer can replace one of the detectors and play a direct role in the local authenticity test.

　　Conveniently, spontaneous parametric down-conversion can also be used to generate entangled photons. A different experiment can use polarized entangled photon pairs.

　　The experiment will be designed so that when a certain kind of polarization measurement has a specific result, one photon will enter the observer, and all other measurements will be completed by a single photon detector, at least in the first experiment.

　　The job of observers is to record the frequency of such measurement results, and the number of times they observe will help to calculate the correlation to measure whether it violates local realism.

　　However, observers are probably lucky enough to notice their photons in a small number of experiments, so they can never measure the true frequency of the measurement results.

　　Just like the single photon vision test, the experiment will be carefully designed to eliminate the bias and help the observer to be as objective as possible.

　　Compulsory choice of design is the secret. We will randomly choose to send entangled photons to the left or right side of the observer's eyes, and at the same time send a second non-entangled control photon to the opposite side. The probability is equal to the maximum expected frequency of the measurement results, which will not violate the local reality.

　　In each experiment, observers will decide whether they see anything on the left and right sides, so they can react left, right or at the same time.

　　If the frequency of the side with entangled photons selected by the observer is the same as or higher than that of the control side, then the result violates local realism.

　　Why do these experiments? In addition to the "unreachable" factors, there are serious scientific reasons. Why and how the superposition state collapses into a definite result is still a big mystery in physics today.

　　Using a new, unique and readily available measuring instrument-human visual system to test quantum mechanics can exclude some theories or provide evidence for them.

　　In particular, a kind of theory called macro realism puts forward that there is an undiscovered physical process that can always make the superposition state of large objects (such as eyeballs and cats) collapse quickly.

　　This means that large-scale superposition is actually impossible, not just because of the interference caused by the interaction with the environment.

　　Leggett, winner of the Nobel Prize in Physics, has been pushing for the experimental test of this theory. In particular, if the superposition experiment with human visual system shows obvious differences from standard quantum mechanics, it may be evidence that macro realism is working.

　　Bell experiment is still an active research field. In 2015, all the major loopholes in the Bell experiment, no matter how small the possibility is, may make the experimental technical problems of local reality continue to exist, and finally blocked.

　　Now, researchers have proposed and carried out a variety of more exotic Bell tests, trying to break through the limits of entanglement and nonlocality.

　　In 2017, researchers led by David Kaiser of MIT and Anton Selinger of Vienna University conducted a "cosmic bell test".

　　They use light from distant stars to trigger measurement settings, trying to prove that the predetermined correlation between entangled particles (which may open a loophole for the existence of local realism) must extend to the past hundreds of years.

　　An international cooperative project named "Bell Test" used the random selection of more than 100,000 human participants to determine the measurement settings for the Bell Test in 2016. Bell test with human observers will be a wonderful supplement to these experiments.