Quantum physics for dummies! The best experiments. Quantum physics for kids

In 1803, Thomas Young directed a beam of light onto an opaque screen with two slits. Instead of the expected two stripes of light on the projection screen, he saw several stripes, as if there was interference (superposition) of two waves of light from each slot. In fact, it was at this moment that quantum physics was born, or rather the questions at its core. In XX and XXI centuries it was shown that not only light, but any single elementary particle and even some molecules behave like a wave, like quanta, as if passing through both slits at the same time. However, if you place a sensor at the slits that determines what exactly happens to the particle in this place and through which particular slit it still passes, then only two stripes appear on the projection screen, as if the fact of observation (indirect influence) destroys the wave function and the object behaves like matter. ( video)

Heisenberg's uncertainty principle is the foundation of quantum physics!

Thanks to the 1927 discovery, thousands of scientists and students repeat the same simple experiment by shining a laser beam through a narrowing slit. Logically, the visible trace from the laser on the projection screen becomes narrower and narrower as the gap decreases. But at a certain moment, when the slit becomes narrow enough, the spot from the laser suddenly begins to become wider and wider, stretching across the screen and dimming until the slit disappears. This is the most obvious proof of the quintessence of quantum physics - the uncertainty principle of Werner Heisenberg, an outstanding theoretical physicist. Its essence is that the more accurately we determine one of the paired characteristics of a quantum system, the more uncertain the second characteristic becomes. In this case, the more accurately we determine the coordinates of the laser photons with a narrowing slit, the more uncertain the momentum of these photons becomes. In the macrocosm, we can also accurately measure either the exact location of a flying sword by picking it up, or its direction, but not at the same time, since this contradicts and interferes with each other. ( , video)

Quantum superconductivity and the Meissner effect

In 1933, Walter Meissner discovered an interesting phenomenon in quantum physics: when cooled to minimum temperatures In a superconductor, the magnetic field is displaced beyond its boundaries. This phenomenon is called the Meissner effect. If an ordinary magnet is placed on aluminum (or another superconductor), and then cooled with liquid nitrogen, the magnet will fly up and hang in the air, since it will “see” its own magnetic field of the same polarity displaced from the cooled aluminum, and the same sides of the magnets repel . ( , video)

Quantum superfluidity

In 1938, Pyotr Kapitsa cooled liquid helium to a temperature close to zero and discovered that the substance lost its viscosity. This phenomenon in quantum physics is called superfluidity. If cooled liquid helium is poured onto the bottom of a glass, it will still flow out of it along the walls. In fact, as long as helium is sufficiently cooled, there is no limit for it to spill, regardless of the shape or size of the container. At the end of XX and beginning of XXI centuries, superfluidity under certain conditions was also discovered in hydrogen and various gases. ( , video)

Quantum tunneling

In 1960, Ivor Jayever conducted electrical experiments with superconductors separated by a microscopic film of non-conducting aluminum oxide. It turned out that, contrary to physics and logic, some electrons still pass through the insulation. This confirmed the theory about the possibility of a quantum tunnel effect. It applies not only to electricity, but also to any elementary particles, they are also waves according to quantum physics. They can pass through obstacles if the width of these obstacles is less than the wavelength of the particle. The narrower the obstacle, the more often particles pass through it. ( , video)

Quantum entanglement and teleportation

In 1982, physicist Alain Aspe, future laureate Nobel Prize, sent two simultaneously created photons to multidirectional sensors for determining their spin (polarization). It turned out that measuring the spin of one photon instantly affects the position of the spin of the second photon, which becomes opposite. Thus, the possibility of quantum entanglement of elementary particles and quantum teleportation was proven. In 2008, scientists were able to measure the state of quantum entangled photons at a distance of 144 kilometers and the interaction between them was still instantaneous, as if they were in the same place or there was no space. It is believed that if such quantum entangled photons end up in opposite parts of the universe, the interaction between them will still be instantaneous, although light takes tens of billions of years to travel the same distance. It’s curious, but according to Einstein, there is also no time for photons traveling at the speed of light. Is this a coincidence? Physicists of the future don’t think so! ( , video)

Quantum Zeno effect and time stopping

In 1989, a group of scientists led by David Wineland observed the rate of transition of beryllium ions between atomic levels. It turned out that the very fact of measuring the state of ions slowed down their transition between states. At the beginning of the 21st century, in a similar experiment with rubidium atoms, a 30-fold slowdown was achieved. All this is confirmation of the quantum Zeno effect. Its meaning is that the very fact of measuring the state of an unstable particle in quantum physics slows down the rate of its decay and, in theory, can completely stop it. ( , video english)

Quantum eraser with delayed choice

In 1999, a team of scientists led by Marlan Scali directed photons through two slits, behind which stood a prism that converted each emerging photon into a pair of quantum entangled photons and separated them into two directions. The first sent photons to the main detector. The second direction sent photons to a system of 50% reflectors and detectors. It turned out that if a photon from the second direction reached the detectors that determined the slit from which it emitted, then the main detector recorded its paired photon as a particle. If a photon from the second direction reached detectors that did not detect the slit from which it emitted, then the main detector recorded its paired photon as a wave. Not only did the measurement of one photon reflect on its quantum entangled pair, but this also happened beyond distance and time, because the secondary detector system recorded photons later than the main one, as if the future determined the past. It is believed that this is the most incredible experiment not only in the history of quantum physics, but also in the history of all science, since it undermines many of the usual foundations of the worldview. ( , video English)

Quantum superposition and Schrödinger's cat

In 2010, Aaron O'Connell placed a small metal plate in an opaque vacuum chamber, which was cooled almost to absolute zero. He then applied impulse to the plate so that it vibrated. However, the position sensor showed that the plate was vibrating and quiet at the same time, which exactly corresponded to theoretical quantum physics. This was the first time the principle of superposition on macro-objects was proven. In isolated conditions, when there is no interaction between quantum systems, an object can simultaneously be in an unlimited number of any possible provisions, as if he were no longer material. ( , video)

Quantum Cheshire Cat and Physics

In 2014, Tobias Denkmair and his colleagues split the neutron beam into two beams and carried out a series of complex measurements. It turned out that under certain circumstances, neutrons can be in one beam, and their magnetic moment in another beam. Thus, the quantum paradox of the Cheshire cat’s smile was confirmed, when particles and their properties can be, according to our perception, in different parts space, like a smile apart from the cat in the fairy tale “Alice in Wonderland”. IN Once again Quantum physics turned out to be more mysterious and amazing than any fairy tale! ( , video english.)

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To many people, physics seems so distant and confusing, and quantum physics even more so. But I want to open the veil of this for you great secret, because in reality everything turns out to be strange, but unraveling.

And also quantum physics is a great subject to talk to smart people about.

Quantum physics made easy

First, you need to draw one big line in your head between the microworld and the macroworld, because these worlds are completely different. Everything you know about the space you are familiar with and the objects in it is false and unacceptable in quantum physics.

In fact, microparticles have neither speed nor a specific position until scientists look at them. This statement seems simply absurd to us, and it seemed so to Albert Einstein, but even the great physicist backed down.

The fact is that research has proven that if you look once at a particle that occupied a certain position, and then turn away and look again, you will see that this particle has already taken a completely different position.

These naughty particles

Everything seems simple, but when we look at the same particle, it stands still. That is, these particles move only when we cannot see it.

The essence is that each particle (according to probability theory) has a scale of probabilities of being in one position or another. And when we turn away and then turn again, we can catch the particle in any of its possible positions precisely according to the probability scale.

According to the study, they looked for the particle in different places, then stopped observing it, and then again looked at how its position changed. The result was simply stunning. Summing up, scientists were really able to create a scale of probabilities where this or that particle could be located.

For example, a neutron has the ability to be in three positions. After conducting research, you may find that in the first position it will be with a probability of 15%, in the second - 60%, in the third - 25%.

No one has yet been able to refute this theory, so it is, oddly enough, the most correct.

Macroworld and microworld

If we take an object from the macrocosm, we will see that it also has a probability scale, but it is completely different. For example, the probability that you turn away and find your phone on the other side of the world is almost zero, but it still exists.

Then the question arises: how come such cases have not yet been recorded? This is explained by the fact that the probability is so small that humanity would have to wait as many years as our planet and the entire universe have not yet lived to see such an event. It turns out that your phone is almost 100% likely to end up exactly where you saw it.

Quantum tunneling

From here we can come to the concept of quantum tunneling. This is the concept of the gradual transition of one object (to put it very roughly) to a completely different place without any external influences.

That is, everything can start with one neutron, which at one point falls into that same almost zero probability of being in a completely different place, and the more neutrons are in a different place, the higher the probability becomes.

Of course, such a transition will take as many years as our planet has not yet lived, but, according to the theory of quantum physics, quantum tunneling takes place.

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In this article we will give useful tips on studying quantum physics for dummies. Let us answer what should be the approach in learning quantum physics for beginners.

The quantum physics - This is a rather complex discipline that not everyone can easily master. Nevertheless, physics as a subject is interesting and useful, which is why quantum physics (http://www.cyberforum.ru/quantum-physics/) finds its fans who are ready to study it and ultimately get practical benefits. To make it easier to learn the material, you need to start from the very beginning, that is, with the simplest quantum physics textbooks for beginners. This will allow you to get a good basis for knowledge, and at the same time well structure your knowledge in your head.

You need to start self-studying with good literature. It is literature that is the decisive factor in the process of acquiring knowledge and ensures its quality. Quantum mechanics is of particular interest, and many begin their studies with it. Everyone should know physics, because it is the science of life, which explains many processes and makes them understandable to others.

Please note that when you start studying quantum physics, you must have knowledge of mathematics and physics, as without them you simply will not cope. It will be good if you have the opportunity to contact your teacher to find answers to your questions. If this is not possible, you can try to clarify the situation on specialized forums. Forums can also be very useful in learning.

When you decide on the choice of a textbook, you must be prepared for the fact that it is quite complex and you will have to not just read it, but delve into everything that is written in it. So that at the end of the training you do not have the idea that this is all unnecessary knowledge for anyone, try to connect theory with practice each time. It is also important to determine in advance the purpose for which you began to learn quantum physics, in order to prevent the emergence of thoughts about the uselessness of the knowledge acquired. People fall into two categories: people who think quantum physics is an interesting and useful subject and those who don't. Choose for yourself which category you belong to and accordingly determine whether quantum physics has a place in your life or not. You can always remain at the beginner level in studying quantum physics, or you can achieve real success, everything is in your hands.

First of all, choose really interesting and quality materials in physics. You can find some of them using the links below.
And that's all for now! Study quantum physics in an interesting way and don’t be a dummie!

Here I had a conversation for days on the topic delayed choice quantum erasure, not so much a discussion as a patient explanation to me by my wonderful friend dr_tambowsky of the fundamentals of quantum physics. Since I didn’t study physics well at school, and in my old age, I absorb it like a sponge. I decided to collect the explanations in one place, maybe for someone else.

To begin with, I recommend watching a cartoon for children about interference and paying attention to the “eye”. Because that's actually the whole point.

Then you can start reading the text from dr_tambowsky, which I quote below in its entirety, or, if you are smart and savvy, you can read it right away. Or better yet, both.

What is interference?
There are really a lot of different terms and concepts here and they are very confused. Let's go in order. Firstly, interference as such. There are countless examples of interference and there are a lot of different interferometers. A particular experiment that is constantly suggested and often used in this erasure science (mostly because it is simple and convenient) is two slits cut side by side, parallel to each other, in an opaque screen. First, let's shine light on such a double slot. Light is a wave, right? And we observe the interference of light all the time. Take it on faith that if we shine light on these two slits, and put a screen (or just a wall) on the other side, then on this second screen we will also see an interference pattern - instead of two bright spots of light “passing through the slits” on the second screen (wall ) there will be a fence of alternating bright and dark stripes. Let us note once again that this is a purely wave property: if we throw pebbles, then those that fall into the slots will continue to fly straight and hit the wall, each behind its own slot, that is, we will see two independent piles of stones ( if they stick to the wall, of course 🙂), no interference.

Next, do you remember in school they taught about “wave-particle duality”? That when everything is very small and very quantum, then objects are both particles and waves? In one of the famous experiments (the Stern-Gerlach experiment) in the 20s of the last century, they used the same setup as described above, but instead of light they shone... with electrons. Well, that is, electrons are particles, right? That is, if you “throw” them onto the double slot, like pebbles, then what will we see on the wall behind the slots? The answer is not two separate spots, but again an interference picture!! That is, electrons can also interfere.

On the other hand, it turns out that light is not exactly a wave, but also a little bit a particle—a photon. That is, we are now so smart that we understand that the two experiments described above are the same thing. We throw (quantum) particles onto the slits, and the particles on these slits interfere - alternating stripes are visible on the wall (“visible” - in the sense of how we register photons or electrons there, actually eyes are not necessary for this :)).

Now, armed with this universal picture, let’s ask the following, more subtle question (attention, very important!!):
When we shine light on the slits with our photons/electrons/particles, we see an interference pattern on the other side. Wonderful. But what happens to an individual photon/electron/pi-meson? [and from now on, let’s talk—solely for convenience—only about photons]. After all, this option is possible: each photon flies like a pebble through its own slot, that is, it has a very definite trajectory. This photon flies through the left slot. And that one over there is on the right. When these pebble photons, following their specific trajectories, reach the wall behind the slits, they somehow interact with each other, and as a result of this interaction, an interference pattern appears on the wall itself. So far, nothing in our experiments contradicts this interpretation - after all, when we shine bright light onto the slit, we send many photons at once. Their dog knows what they are doing there.

We have an answer to this important question. We know how to throw one photon at a time. They left. We waited. They threw the next one. We look closely at the wall and notice where these photons arrive. A single photon, of course, cannot create an observable interference pattern in principle - it is alone, and when we register it, we can only see it in a certain place, and not everywhere at once. However, let's return to the analogy with pebbles. One pebble flew by. He hit the wall behind one of the slots (the one he flew through, of course). Here's another one - it hit behind the slot again. We are sitting. We count. After some time and throwing enough pebbles, we will get a distribution - we will see that many pebbles hit the wall behind one slot and many behind the other. And nowhere else. We do the same with photons - throw them one at a time and slowly count how many photons arrive at each place on the wall. We are slowly going crazy, because the resulting frequency distribution of photon impacts is not at all two spots under the corresponding slits. This distribution exactly repeats the interference pattern that we saw when we shone with bright light. But the photons were now arriving one at a time! One - today. The next one is tomorrow. They couldn't interact with each other on the wall. That is, in full accordance with quantum mechanics, one, separate photon is simultaneously a wave and nothing wavelike is alien to it. The photon in our experiment does not have a specific trajectory - each individual photon passes through both slits at once and, as it were, interferes with itself. We can repeat the experiment, leaving only one slit open - then the photons will, of course, cluster behind it. Let's close the first one, open the second one, still throwing photons one at a time. They cluster, of course, under the second, open, crack. Open both - the resulting distribution of places where photons like to cluster is not the sum of the distributions obtained when only one slit was open. They are now still huddled between the cracks. More precisely, their favorite places for grouping are now alternating stripes. In this one they are huddled together, in the next one - no, again - yes, dark, light. Ah, interference...

What is superposition and spin?
So. Let us assume that we understand everything about interference as such. Let's do superposition. I don’t know how you are with quantum mechanics, sorry. If it’s bad, then you’ll have to take a lot on faith; it’s difficult to explain in a nutshell.

But in principle, we were already somewhere close - when we saw that a single photon was flying through two slits at once. We can say simply: a photon has no trajectory, a wave and a wave. And we can say that the photon simultaneously flies along two trajectories (strictly speaking, not even two, of course, but all at once). This is an equivalent statement. In principle, if we follow this path to the end, we will arrive at the “path integral” - Feynman’s formulation of quantum mechanics. This formulation is incredibly elegant and just as complex, it is difficult to use in practice, much less use it to explain the basics. Therefore, let’s not go all the way, but rather meditate on a photon flying “along two trajectories at once.” In the sense of classical concepts (and the trajectory is quite a well-defined classical concept, either the stone flies head-on or past), the photon is in different states at the same time. Once again, the trajectory is not even exactly what we need, our goals are simpler, I just urge you to realize and feel the fact.

Quantum mechanics tells us that this situation is the rule, not the exception. Any quantum particle can be (and usually is) in “several states” at once. In fact, you don't need to take this statement too seriously. These “multiple states” are actually our classical intuitions. We define different “states” based on some of our own (external and classical) considerations. And a quantum particle lives according to its own laws. She has a fortune. Dot. All that the statement about “superposition” means is that this state may be very different from our classical ideas. We introduce the classical concept of trajectory and apply it to a photon in the state it likes to be in. And the photon says - “sorry, my favorite state is that in relation to these trajectories of yours, I am on both at once!” This does not mean that the photon cannot at all be in a state in which the trajectory is (more or less) determined. Let's close one of the slits - and we can, to some extent, say that the photon flies through the second along a certain trajectory, which we understand well. That is, such a state exists in principle. Let's open both - the photon prefers to be in superposition.

The same applies to other parameters. For example, its own angular momentum, or spin. Remember about two electrons that can sit together in the same s orbital - if they have opposite spins? This is exactly it. And the photon also has spin. The good thing about photon spin is that in the classics it actually corresponds to the polarization of a light wave. That is, using all sorts of polarizers and other crystals that we have, we can manipulate the spin (polarization) of individual photons if we have them (and they will appear).

So, spin. The electron has a spin (in the hope that orbitals and electrons are more familiar to you than photons, so everything is the same), but the electron is absolutely indifferent to what “spin state” it is in. Spin is a vector and we can try to say “spin points up.” Or “the spin is looking down” (relative to some direction we have chosen). And the electron tells us: “I don’t care about you, I can be on both trajectories in both spin states at once.” Here again, it is very important that not many electrons are in different spin states, in an ensemble, one looks up, the other down, and each individual electron is in both states at once. Just like not different electrons pass through different slits, but one electron (or photon) passes through both slits at once. An electron can be in a state with a certain direction of spin if you ask it very much, but it itself will not do this. The situation can be described semi-qualitatively as follows: 1) there are two states, |+1> (spin up) and |-1> (spin down); 2) in principle, these are kosher states in which the electron can exist; 3) however, if you do not make special efforts, the electron will be “smeared” across both states and its state will be something like |+1> + |-1>, a state in which the electron does not have a specific spin direction (just like the 1+ trajectory trajectory 2, right?). This is a “superposition of states.”

About the collapse of the wave function.
There is very little left for us to understand what measurement and “collapse of the wave function” are. The wave function is what we wrote above, |+1> + |-1>. Just a description of the condition. For simplicity, we can talk about the state itself, as such, and its “collapse,” it doesn’t matter. This is what happens: the electron flies to itself in such an uncertain state of mind, either it is up, or down, or both at once. Then we run up with some scary-looking device and let’s measure the direction of the spin. In this particular case, it is enough to insert an electron into a magnetic field: those electrons whose spin points along the direction of the field should deviate in one direction, those whose spin points against the field - in the other. We sit on the other side and rub our hands - we see in which direction the electron has deviated and we immediately know whether its spin is facing up or down. Photons can be put into a polarizing filter - if the polarization (spin) is +1, the photon passes through, if -1, then not.

But excuse me - after all, the electron did not have a certain spin direction before the measurement? That's the whole point. There was no definite one, but it was, as it were, “mixed” from two states at once, and in each of these states there was very much a direction. In the process of measurement, we force the electron to decide who it should be and where to look - up or down. In the situation described above, we, of course, in principle cannot predict in advance what decision this particular electron will make when it flies into the magnetic field. With a probability of 50% he can decide “up”, with the same probability he can decide “down”. But as soon as he decides this, he is in a state with a certain direction of spin. As a result of our “measurement”! This is “collapse” - before the measurement, the wave function (sorry, state) was |+1> + |-1>. After we “measured” and saw that the electron deviated in a certain direction, its spin direction was determined and its wave function became simply |+1> (or |-1>, if it deviated in another direction). That is, the state has “collapsed” into one of its components; There is no longer any trace of “mixing” the second component!

To a large extent, this was the focus of empty philosophizing in the original entry, and this is why I don’t like the end of the cartoon. An eye is simply drawn there and an inexperienced viewer may have, firstly, the illusion of a certain anthropocentricity of the process (they say, an observer is needed to carry out the “measurement”), and secondly, of its non-invasiveness (well, we’re just looking!). My views on this topic were outlined above. Firstly, an “observer” as such is not needed, of course. It is enough to bring a quantum system into contact with a large one, classical system and everything will happen by itself (electrons will fly into the magnetic field and decide who they will be, regardless of whether we are sitting on the other side and watching or not). Secondly, non-invasive classical measurement of a quantum particle is impossible in principle. It’s easy to draw an eye, but what does it mean to “look at a photon and find out where it went”? To look, you need photons to hit your eye, preferably a lot. How can we arrange it so that many photons arrive and tell us everything about the state of one unfortunate photon, the state of which we are interested in? Shine a flashlight on it? And what will be left of him after this? It is clear that we will greatly influence his condition, perhaps to such an extent that he will no longer want to climb into one of the slots. It's not all that interesting. But we’ve finally gotten to the interesting part.

About the Einstein-Podolsky-Rosen paradox and coherent (entangled) photon pairs
We now know about superposition of states, but so far we have only talked about one particle. Purely for simplicity. But still, what if we have two particles? You can prepare a pair of particles in a completely quantum state, so that their overall state is described by a single, common wave function. This, of course, is not simple - two arbitrary photons in neighboring rooms or electrons in neighboring test tubes do not know about each other, so they can and should be described completely independently. Therefore, it is just possible to calculate the binding energy of, say, one electron on one proton in a hydrogen atom, without being at all interested in other electrons on Mars or even on neighboring atoms. But if you make a special effort, you can create a quantum state that encompasses two particles at once. This will be called a “coherent state”; in relation to pairs of particles and all sorts of quantum erasures and computers, this is also called an entangled state.

Let's move on. We can know (due to the constraints imposed by the process of preparing this coherent state) that, say, the total spin of our two-particle system is zero. It’s okay, we know that the spins of two electrons in the s-orbital must be antiparallel, that is, the total spin is zero, and this does not scare us at all, right? What we don't know is where the spin of a particular particle is pointing. We only know that no matter where he looks, the second spin must look in the other direction. That is, if we designate our two particles (A) and (B), then the state can, in principle, be like this: |+1(A), -1(B)> (A looks up, B looks down). This is a permitted state and does not violate any imposed restrictions. Another possibility is |-1(A), +1(B)> (vice versa, A down, B up). Also a possible condition. Doesn’t it still remind you of the states that we wrote down a little earlier for the spin of one single electron? Because our system of two particles, while it is quantum and coherent, can (and will) also be in a superposition of states |+1(A); -1(B)> + |-1(A); +1(B)>. That is, both possibilities are implemented simultaneously. Like both trajectories of a photon or both directions of the spin of one electron.

Measuring such a system is much more exciting than measuring a single photon. Indeed, let’s assume that we measure the spin of only one particle, A. We have already understood that measurement is severe stress for a quantum particle, its state will change greatly during the measurement process, collapse will occur... That’s all true, but in this case there is also the second particle, B, which is tightly connected with A, they have a common wave function! Suppose we measured the direction of spin A and saw that it was +1. But A does not have its own wave function (or in other words, its own independent state) for it to collapse to |+1>. All that A has is the state “entangled” with B, written out above. If measurement A gives +1 and we know that the spins of A and B are antiparallel, we know that B's spin is facing down (-1). The wave function of the pair collapses to whatever it can, or it can only to |+1(A); -1(B)>. The written wave function does not provide us with any other possibilities.

Nothing yet? Just think, the full spin is preserved? Now let's imagine that we created such a pair A, B and let these two particles fly apart into different sides, remaining coherent. One (A) flew to Mercury. And the other (B), say, to Jupiter. At this very moment we happened on Mercury and measured the direction of spin A. What happened? At that very moment we learned the direction of spin B and changed the wave function of B! Please note that this is not at all the same as in the classics. Let two flying stones rotate around their axis and let us know for sure that they rotate in opposite directions. If we measure the direction of rotation of one when it reaches Mercury, we will also know the direction of rotation of the second, wherever it ends up by that time, even on Jupiter. But these stones always rotated in a certain direction, before any of our measurements. And if someone measures a rock flying towards Jupiter, then he (s) will receive the same and quite definite answer, regardless of whether we measured something on Mercury or not. With our photons the situation is completely different. None of them had any specific spin direction at all before measurement. If someone, without our participation, decided to measure the direction of spin B somewhere in the Mars region, what would they get? That's right, with a 50% chance he would see +1, with a 50% chance -1. This is B’s state, superposition. If this someone decides to measure spin B immediately after we have already measured spin A, saw +1 and caused the collapse of the *entire* wave function,
then he will receive only -1 as a result of the measurement, with a probability of 100%! Only at the moment of our measurement, A finally decided who he should be and “chose” the direction of the spin - and this choice instantly affected the *entire* wave function and the state of B, who at this moment is already God knows where.

This trouble is called “nonlocality of quantum mechanics.” Also known as the Einstein-Podolsky-Rosen paradox (EPR paradox) and, in general, what happens in erasure is related to this. Maybe I’m misunderstanding something, of course, but for my taste erasure is interesting because it is precisely an experimental demonstration of nonlocality.

Simplified, an experiment with erasure could look like this: we create coherent (entangled) pairs of photons. One at a time: a couple, then the next one, etc. In each pair, one photon (A) flies in one direction, the other (B) in the other. Everything is as we already discussed a little higher. On the path of photon B, we place a double slit and see what appears behind this slit on the wall. An interference pattern emerges, because each photon B, as we know, flies along both trajectories, through both slits at once (we still remember about interference with which we started this story, right?). The fact that B is still coherently connected with A and has a common wave function with A is quite purple for him. Let’s complicate the experiment: cover one slot with a filter that allows only photons with spin +1 to pass through. We cover the second with a filter that transmits only photons with spin (polarization) -1. We continue to enjoy the interference pattern, because in the general state pairs A, B(|+1(A); -1(B)> + |-1(A);+1(B)>, as we remember), there are states B with both spins. That is, “part” B can pass through one filter/slot, and part through another. Just as before, one “part” flew along one trajectory, the other along another (this, of course, is a figure of speech, but the fact remains a fact).

Finally, the culmination: somewhere on Mercury, or a little closer, at the other end of the optical table, we place a polarizing filter in the path of photons A, and a detector behind the filter. Let's be clear that this new filter only allows photons with spin +1 to pass through. Every time the detector is triggered, we know that photon A with spin +1 has passed through (spin -1 will not pass through). But this means that the wave function of the entire pair collapsed and the “brother” of our photon, photon B, at this moment had only one possible state -1. All. Photon B now has “nothing” to get through, a slot covered with a filter that allows only +1 polarization to pass through. He simply doesn't have that component left. “Recognizing” this photon B is very simple. We create pairs one at a time. When we detect photon A passing through a filter, we record the time at which it arrived. Half past one, for example. This means that his “brother” B will fly to the wall at half past one too. Well, or at 1:36, if he flies a little further and, therefore, longer. There we also record times, that is, we can compare who is who and who is related to whom.

So, if we now look at what picture is emerging on the wall, we will not detect any interference. Photon B from each pair passes through either one slot or the other. There are two spots on the wall. Now, we remove the filter from the path of photons A. The interference pattern is restored.

...and finally about delayed choice
The situation becomes completely miserable when it takes longer for photon A to get to its filter/detector than for photon B to get to the slits. We make the measurement (and force A to solve and the wave function to collapse) after B should have already reached the wall and created an interference pattern. However, while we measure A, even “later than it should,” the interference pattern for photons B still disappears. We remove the filter for A - it is restored. This is already a delayed erasure. I can’t say that I understand well what they eat it with.

Amendments and clarifications.
Everything was correct, subject to inevitable simplifications, until we built a device with two entangled photons. First, photon B experiences interference. It doesn't seem to work with filters. You need to cover it with plates that change the polarization from linear to circular. This is already more difficult to explain 😦 But this is not the main thing. The main thing is that when we cover the slots with different filters, the interference disappears. Not at the moment when we measure photon A, but immediately. The tricky trick is that by installing the plate filters, we “labeled” photons B. In other words, photons B carry additional information that allows us to find out exactly which trajectory they flew. *If* we measure photon A, then we will be able to find out exactly which trajectory B flew, which means that B will not experience interference. The subtlety is that it is not necessary to physically “measure” A! This is where I was grossly mistaken last time. There is no need to measure A for the interference to disappear. If it *is* possible to measure and find out which of the trajectories photon B took, then in this case there will be no interference.

In fact, this can still be experienced. There, at the link below, people somehow shrug their hands somewhat helplessly, but in my opinion (maybe I’m wrong again? 😉) the explanation is this: by putting filters in the slots, we have already greatly changed the system. It doesn’t matter whether we actually registered the polarization or the trajectory along which the photon passed or waved our hand at the last moment. It is important that we have “prepared” everything for measurement and have already influenced the states. Therefore, there is no need to actually “measure” (in the sense of a conscious humanoid observer who brought a thermometer and recorded the result in a journal). Everything in some sense (in the sense of impact on the system) has already been “measured”. The statement is usually formulated as follows: “*if* we measure the polarization of photon A, then we will know the polarization of photon B, and therefore its trajectory, and since photon B flies along a certain trajectory, then there will be no interference; we don’t even have to measure photon A—it’s enough that this measurement is possible; photon B knows that it can be measured and refuses to interfere.” There is some mystification in this. Well, yes, he refuses. Simply because the system was prepared that way. If the system has Additional Information(there is a way) to determine which of the two trajectories the photon flew along, then there will be no interference.

If I tell you that I arranged everything so that the photon flies through only one slot, you will immediately understand that there will be no interference? You can run to check (“measure”) and make sure that I’m telling the truth, or you can believe it that way. If I didn’t lie, then there won’t be interference regardless of whether you rush to check me or not :) Accordingly, the phrase “can be measured” actually means “the system is prepared in such a special way that...”. It is prepared and prepared, that is, there is no collapse in this place yet. There are “tagged” photons and no interference.

Next - why, in fact, erasure is all this - they tell us: let’s act on the system in such a way as to “erase” these marks from photons B - then they will begin to interfere again. An interesting point, which we have already approached, albeit in an erroneous model, is that photons B can be left untouched and the plates left in the slots. You can tug on photon A and, just as during collapse, a change in its state will cause (nonlocally) a change in the total wave function of the system so that we no longer have information sufficient to determine which slit photon B passed through. That is, we insert a polarizer in the path of photon A - the interference of photons B is restored. With delayed, everything is the same - we make it so that photon A takes longer to fly to the polarizer than B to get to the slits. And still, if A has a polarizer on its way, then B interferes (albeit, as it were, “before” A reaches the polarizer)!

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Hello dear readers. If you don't want to lag behind life, be truly happy and healthy person, you should know about the secrets of quantum modern physics, at least have a little idea to what depths of the universe scientists have dug today. You don’t have time to go into deep scientific details, but want to comprehend only the essence, but see the beauty of the unknown world, then this article: quantum physics for regular teapots or you can say for housewives just for you. I will try to explain what quantum physics is, but in simple words, show clearly.

“What is the connection between happiness, health and quantum physics?” you ask.

The fact is that it helps answer many unclear questions related to human consciousness and the influence of consciousness on the body. Unfortunately, medicine, based on classical physics, does not always help us to be healthy. But psychology cannot properly say how to find happiness.

Only a deeper knowledge of the world will help us understand how to truly cope with illness and where happiness lives. This knowledge is found in the deep layers of the Universe. Quantum physics comes to our aid. Soon you will know everything.

What quantum physics studies in simple words

Yes, quantum physics is indeed very difficult to understand because it studies the laws of the microworld. That is, the world is in its deeper layers, at very short distances, where it is very difficult for a person to see.

And the world, it turns out, behaves there very strangely, mysteriously and incomprehensibly, not as we are used to.

Hence all the complexity and misunderstanding of quantum physics.

But after reading this article, you will expand the horizons of your knowledge and look at the world in a completely different way.

Brief history of quantum physics

It all started at the beginning of the 20th century, when Newtonian physics could not explain many things and scientists reached a dead end. Then Max Planck introduced the concept of quantum. Albert Einstein picked up this idea and proved that light does not travel continuously, but in portions - quanta (photons). Before this, it was believed that light had a wave nature.

But as it turned out later, any elementary particle is not only a quantum, that is, a solid particle, but also a wave. This is how wave-particle dualism appeared in quantum physics, the first paradox and the beginning of discoveries of mysterious phenomena of the microworld.

The most interesting paradoxes began when the famous double-slit experiment was carried out, after which there were many more mysteries. We can say that quantum physics began with him. Let's look at it.

Double slit experiment in quantum physics

Imagine a plate with two slits in the form of vertical stripes. We will place a screen behind this plate. If we shine light on the plate, we will see an interference pattern on the screen. That is, alternating dark and bright vertical stripes. Interference is the result of the wave behavior of something, in our case light.

If you pass a wave of water through two holes located next to each other, you will understand what interference is. That is, the light turns out to be of a wave nature. But as physics, or rather Einstein, has proven, it is propagated by photon particles. Already a paradox. But that’s okay, wave-particle duality will no longer surprise us. Quantum physics tells us that light behaves like a wave but is made up of photons. But miracles are just beginning.

Let's put a gun in front of the plate with two slits that will emit electrons rather than light. Let's start shooting electrons. What will we see on the screen behind the plate?

Electrons are particles, which means that a flow of electrons passing through two slits should leave only two stripes on the screen, two traces opposite the slits. Imagine pebbles flying through two slits and hitting the screen?

But what do we actually see? The same interference pattern. What is the conclusion: electrons travel in waves. So electrons are waves. But this is an elementary particle. Again, wave-particle dualism in physics.

But we can assume that at a deeper level, the electron is a particle, and when these particles come together, they begin to behave like waves. For example, a sea wave is a wave, but it consists of drops of water, and at a smaller level of molecules, and then of atoms. Okay, the logic is solid.

Then let's shoot from a gun not with a stream of electrons, but release electrons separately, after a certain period of time. As if we were not passing through the cracks sea ​​wave, and would spit out individual drops from a child’s water pistol.

It is quite logical that in this case different drops of water would fall into different cracks. On the screen behind the plate one would see not an interference pattern from the wave, but two clear stripes from the impact opposite each slit. We will see the same thing: if you throw small stones, they, flying through two slits, would leave a mark, like a shadow from two holes. Let's now shoot individual electrons to see these two streaks on the screen from the electron impacts. They released one, waited, the second, waited, and so on. Quantum physics scientists were able to do such an experiment.

But horror. Instead of these two bands, the same interference alternations of several bands are obtained. How so? This could happen if an electron were flying through two slits at the same time, and behind the plate, like a wave, it would collide with itself and interfere. But this cannot happen, because a particle cannot be in two places at the same time. It either flies through the first gap or through the second.

This is where the truly fantastic things of quantum physics begin.

Superposition in quantum physics

With a deeper analysis, scientists find out that any elementary quantum particle or the same light (photon) can actually be in several places at the same time. And these are not miracles, but real facts microworld. Quantum physics says so. That's why, when we shoot a single particle from a cannon, we see the result of interference. Behind the plate, the electron collides with itself and creates an interference pattern.

The objects of the macrocosm that are common to us are always in one place and have one state. For example, you are now sitting on a chair, weigh, say, 50 kg, and have a heart rate of 60 beats per minute. Of course, these readings will change, but they will change after some time. After all, you cannot be at home and at work at the same time, weigh 50 and 100 kg. All this is understandable, it is common sense.

In the physics of the microworld, everything is different.

Quantum mechanics states, and this has already been confirmed experimentally, that any elementary particle can simultaneously be not only in several points in space, but also have several states at the same time, for example, spin.

All this boggles the mind, undermines the usual understanding of the world, the old laws of physics, turns thinking upside down, one can safely say drives you crazy.

This is how we come to understand the term “superposition” in quantum mechanics.

Superposition means that an object of the microworld can simultaneously be in different points of space, and also have several states at the same time. And this is normal for elementary particles. This is the law of the microworld, no matter how strange and fantastic it may seem.

You are surprised, but these are just the beginnings, the most inexplicable miracles, mysteries and paradoxes of quantum physics are yet to come.

Wave function collapse in physics in simple words

Then the scientists decided to find out and see more precisely whether the electron actually passes through both slits. All of a sudden it passes through one slit and then somehow splits and creates an interference pattern as it passes through it. Well, you never know. That is, you need to place some kind of device near the slit that would accurately record the passage of an electron through it. No sooner said than done. Of course, this is difficult to do; you need not a device, but something else to see the passage of an electron. But scientists did it.

But in the end, the result stunned everyone.

As soon as we begin to look through which slit the electron passes, it begins to behave not like a wave, not like a strange substance that is simultaneously located in different points of space, but like an ordinary particle. That is, the quantum begins to exhibit specific properties: it is located in only one place, passes through one slit, and has one spin value. It is not an interference pattern that appears on the screen, but a simple trace opposite the slit.

But how is this possible? It’s as if the electron is joking, playing with us. At first it behaves like a wave, and then, after we decided to watch it pass through a slit, it exhibits the properties of a solid particle and passes through only one slit. But this is how it is in the microcosm. These are the laws of quantum physics.

Scientists have seen another mysterious property elementary particles. This is how the concepts of uncertainty and wave function collapse appeared in quantum physics.

When an electron flies to the slit, it is in an indeterminate state or, as we said above, in a superposition. That is, it behaves like a wave, is simultaneously in different points of space, and has two spin values ​​at once (spin has only two values). If we didn’t touch it, didn’t try to look at it, didn’t find out where exactly it was, didn’t measure the value of its spin, it would have flown like a wave through two slits at the same time, which means it would have created an interference pattern. Quantum physics describes its trajectory and parameters using the wave function.

After we have made a measurement (and you can measure a particle of the microworld only by interacting with it, for example, by colliding another particle with it), then the collapse of the wave function occurs.

That is, now the electron is located exactly in one place in space and has one spin value.

You can say an elementary particle is like a ghost, it seems to exist, but at the same time it is not in one place, and can, with a certain probability, end up in any place within the description of the wave function. But as soon as we begin to contact it, it turns from a ghostly object into a real tangible substance that behaves like ordinary objects of the classical world that are familiar to us.

“This is fantastic,” you say. Of course, but the wonders of quantum physics are just beginning. The most incredible is yet to come. But let's take a little break from the abundance of information and return to quantum adventures another time, in another article. In the meantime, reflect on what you learned today. What can such miracles lead to? After all, they surround us, this is a property of our world, albeit on a deeper level. Do we still think that we live in a boring world? But we will draw conclusions later.

I tried to talk about the basics of quantum physics briefly and clearly.

But if you don’t understand something, then watch this cartoon about quantum physics, about the double-slit experiment, everything is also explained there in clear, simple language.

Cartoon about quantum physics:

Or you can watch this video, everything will fall into place, quantum physics is very interesting.

Video about quantum physics:

And how did you not know about this before?

Modern discoveries in quantum physics are changing our familiar material world.