At the heart of quantum mechanics

Richard Feynman, one of the greatest physicists of the XNUMXth century, argued that the key to understanding quantum mechanics is the "double slit experiment". This conceptually simple experiment, conducted today, continues to yield amazing discoveries. They show how incompatible with common sense is quantum mechanics, which eventually led to the most important inventions of the last fifty years.

For the first time he conducted a double-slit experiment. Thomas Young (1) in England in the early nineteenth century.

Young's experiment

The experiment was used to show that light is of a wave nature and not of a corpuscular nature, as previously stated. Isaac Newton. Young just demonstrated that light obeys вмешательство - a phenomenon that is the most characteristic feature (regardless of the type of wave and the medium in which it propagates). Today, quantum mechanics reconciles these two logically contradictory views.

Let us recall the essence of the double-slit experiment. As usual, I mean a wave on the surface of the water that spreads concentrically around the place where the pebble was thrown. 

A wave is formed by successive crests and troughs radiating from the point of disturbance, while maintaining a constant distance between the crests, which is called the wavelength. A barrier can be placed in the path of the wave, for example, in the form of a board with two narrow slots cut through which water can flow freely. Throwing a pebble into the water, the wave stops on the partition - but not quite. Two new concentric waves (2) now propagate to the other side of the partition from both slots. They are superimposed on each other, or, as we say, interfere with each other, creating a characteristic pattern on the surface. In places where the crest of one wave meets the crest of another, the water bulge intensifies, and where the hollow meets the valley, the depression deepens.

2. Interference of waves emerging from two slots.

In Young's experiment, single-color light emitted from a point source passes through an opaque diaphragm with two slits and hits the screen behind them (today we would prefer to use laser light and a CCD). An interference image of a light wave is observed on the screen in the form of a series of alternating light and dark stripes (3). This result reinforced the belief that light was a wave, before discoveries in the early XNUMXs showed that light was also a wave. photon flux are light particles that have no rest mass. Later it turned out that the mysterious wave-particle dualityfirst discovered for light also applies to other particles endowed with mass. It soon became the basis for a new quantum mechanical description of the world.

3. Vision of Young's experiment

The particles also interfere

In 1961, Klaus Jonsson from the University of Tübingen demonstrated the interference of massive particles - electrons using an electron microscope. Ten years later, three Italian physicists from the University of Bologna performed a similar experiment with single-electron interference (using a so-called biprism instead of a double slit). They reduced the intensity of the electron beam to such a low value that the electrons passed through the biprism one after the other, one after the other. These electrons were registered on a fluorescent screen.

Initially, the electron trails were randomly distributed over the screen, but over time they formed a distinct interference image of the interference fringes. It seems impossible that two electrons passing through the slits in succession at different times could interfere with each other. Therefore, we must acknowledge that one electron interferes with itself! But then the electron would have to pass through both slits at the same time.

It may be tempting to look at the hole through which the electron actually passed. Later we will see how to make such an observation without disturbing the motion of the electron. It turns out that if we get information about what the electron has received, then the interference ... will disappear! The “how” information destroys interference. Does this mean that the presence of a conscious observer influences the course of the physical process?

Before talking about the even more surprising results of double-slit experiments, I will make a small digression about the sizes of interfering objects. Quantum interference of mass objects was discovered first for electrons, then for particles with increasing mass: neutrons, protons, atoms, and finally for large chemical molecules.

In 2011, the record for the size of an object was broken, on which the phenomenon of quantum interference was demonstrated. The experiment was carried out at the University of Vienna by a doctoral student of the time. Sandra Eibenberger and her associates. A complex organic molecule containing about 5 protons, 5 thousand neutrons and 5 thousand electrons was chosen for the experiment with two breaks! In a very complex experiment, the quantum interference of this huge molecule was observed.

This confirmed the belief that The laws of quantum mechanics obey not only elementary particles, but also every material object. Only that the more complex the object, the more it interacts with the environment, which violates its subtle quantum properties and destroys interference effects..

Quantum entanglement and polarization of light

The most surprising results of the double-slit experiments came from using a special method of tracking the photon, which did not disturb its motion in any way. This method uses one of the strangest quantum phenomena, the so-called quantum entanglement. This phenomenon was noticed back in the 30s by one of the main creators of quantum mechanics, Erwin schrödinger.

The skeptical Einstein (see also 🙂 called them ghostly action at a distance. However, only half a century later the significance of this effect was realized, and today it has become a subject of special interest to physicists.

What is this effect about? If two particles that are close to each other at some point in time interact so strongly with each other that they form a kind of "twin relationship", then the relationship persists even when the particles are hundreds of kilometers apart. Then the particles behave as a single system. This means that when we perform an action on one particle, it immediately affects another particle. However, in this way we cannot timelessly transmit information over a distance.

A photon is a massless particle - an elementary part of light, which is an electromagnetic wave. After passing through a plate of the corresponding crystal (called a polarizer), the light becomes linearly polarized, i.e. the vector of the electric field of an electromagnetic wave oscillates in a certain plane. In turn, by passing linearly polarized light through a plate of a certain thickness from another particular crystal (the so-called quarter-wave plate), it can be converted into circularly polarized light, in which the electric field vector moves in a helical (clockwise or counterclockwise) motion along direction of wave propagation. Accordingly, one can speak of linearly or circularly polarized photons.

Experiments with entangled photons

4a. A non-linear BBO crystal converts a photon emitted by an argon laser into two entangled photons with half the energy and mutually perpendicular polarization. These photons scatter in different directions and are recorded by detectors D1 and D2, connected by a coincidence counter LK. A diaphragm with two slits is placed in the path of one of the photons. When both detectors register the almost simultaneous arrival of both photons, the signal is stored in the memory of the device, and the detector D2 steps parallel to the slits. The number of photons depending on the position of the detector D2, thus recorded, is shown in the box, showing the maxima and minima, indicating interference.

In 2001, a group of Brazilian physicists in Belo Horizonte performed under the guidance of Stephen Walborn unusual experiment. Its authors used the properties of a special crystal (abbreviated as BBO), which converts a certain part of the photons emitted by an argon laser into two photons with half the energy. These two photons are entangled with each other; when one of them has, for example, horizontal polarization, the other has vertical polarization. These photons move in two different directions and play different roles in the described experiment.

One of the photons we are going to name control, goes directly to the photon detector D1 (4a). The detector registers its arrival by sending an electrical signal to a device called a hit counter. LK An interference experiment will be carried out on the second photon; we'll call him signal photon. There is a double slit in its path, followed by a second photon detector, D2, slightly further from the photon source than detector D1. This detector can hop around the dual slot each time it receives the appropriate signal from the hit counter. When detector D1 registers a photon, it sends a signal to the coincidence counter. If in a moment the detector D2 also registers a photon and sends a signal to the meter, then it will recognize that it comes from entangled photons, and this fact will be stored in the memory of the device. This procedure excludes the registration of random photons entering the detector.

Entangled photons persist for 400 seconds. After this time, the D2 detector is displaced by 1 mm with respect to the position of the slits, and the counting of entangled photons takes another 400 seconds. Then the detector is again moved by 1 mm and the procedure is repeated many times. It turns out that the distribution of the number of photons recorded in this way depending on the position of the detector D2 has characteristic maxima and minima corresponding to light and dark and interference fringes in Young's experiment (4a).

We find out again that single photons passing through the double slit interfere with each other.

How so?

The next step in the experiment was to determine the hole through which a particular photon passed without disturbing its movement. Properties used here quarter wave plate. A quarter-wave plate was placed in front of each slit, one of which changed the linear polarization of the incident photon to circular clockwise, and the other to left-hand circular polarization (4b). It was verified that the type of photon polarization did not affect the number of photons counted. Now, by determining the rotation of the polarization of a photon after it has passed through the slits, it is possible to indicate through which of them the photon has passed. Knowing "in which direction" destroys interference.

4b. By placing quarter-wave plates (shaded rectangles) in front of the slits, "which way" information can be obtained and the interference image will disappear.

4c. Placing an appropriately oriented polarizer P in front of detector D1 erases the "which way" information and restores the interference.

In fact, after correct placement of the quarter-wave plates in front of the slits, the previously observed distribution of counts, indicative of interference, disappears. The strangest thing is that this happens without the participation of a conscious observer who can make the appropriate measurements! The mere placement of quarter-wave plates produces an interference cancellation effect.. So how does the photon know that after inserting the plates, we can determine the gap through which it passed?

However, this is not the end of the weirdness. Now we can restore signal photon interference without affecting it directly. To do this, in the path of the control photon reaching detector D1, place a polarizer in such a way that it transmits light with a polarization that is a combination of the polarizations of both entangled photons (4c). This immediately changes the polarity of the signal photon accordingly. Now it is no longer possible to determine with certainty what is the polarization of a photon incident on the slits, and through which slit the photon passed. In this case, interference is restored!

Erase delayed selection information

The experiments described above were carried out in such a way that the control photon was registered by the detector D1 before the signal photon reached the detector D2. The erasing of the "which path" information was performed by changing the polarization of the control photon before the signal photon reached detector D2. Then one can imagine that the controlling photon has already told its “twin” what to do next: to intervene or not.

Now we modify the experiment in such a way that the control photon hits detector D1 after the signal photon is registered at detector D2. To do this, move detector D1 away from the photon source. The interference pattern looks the same as before. Now let's place quarter-wave plates in front of the slits to determine which path the photon has taken. The interference pattern disappears. Next, let's erase the "which way" information by placing an appropriately oriented polarizer in front of detector D1. The interference pattern appears again! Yet the erasure was done after the signal photon had been registered by detector D2. How is this possible? The photon had to be aware of the polarity change before any information about it could reach it.

5. Experiments with a laser beam.

The natural sequence of events is reversed here; effect precedes cause! This result undermines the principle of causality in the reality around us. Or maybe time doesn't matter when it comes to entangled particles? Quantum entanglement violates the principle of locality in classical physics, according to which an object can only be affected by its immediate environment.

Since the Brazilian experiment, many similar experiments have been carried out, which fully confirm the results presented here. In the end, the reader would like to clearly explain the mystery of these unexpected phenomena. Unfortunately, this cannot be done. The logic of quantum mechanics is different from the logic of the world we see every day. We must humbly accept this and rejoice in the fact that the laws of quantum mechanics accurately describe the phenomena occurring in the microcosm, which are usefully used in ever more advanced technical devices.