Shulou(Shulou.com)11/24 Report--

Atoms or molecules in matter are constantly doing irregular thermal motion, and the intensity of this motion is described by the temperature of the system. The more intense the exercise, the higher the temperature, and vice versa.

This relationship can be specifically expressed as the average kinetic energy of the motion of the particle's center of mass on the left, and the Boltzmann constant on the right, but the temperature expressed by the absolute temperature scale.

This law tells us that if we want the temperature of an object to drop, all we have to do is to make the movement of the atoms in it as slow as possible.

This reminds people of a Chinese saying: the mind is calm and cool. It means that when you calm down, you naturally feel cool. And the word "calm" is portrayed in place-cold and quiet are made for each other.

The temperature itself is a statistical average, which makes sense only for a large number of particles. Therefore, only a large number of atoms have temperature, and a few atoms do not have temperature.

But since the average kinetic energy of the atomic center of mass is equivalent to the temperature, it is customary to describe the "slow motion" of the atom as "low temperature" and the "calm the atom" as "cooling the atom", even though there are few atoms at this time.

In general, atoms are always moving, like naughty boys bouncing around.

However, for many scientific studies, such as atomic clocks, spectral measurements, and observing new forms of matter, Bose-Einstein condensation (BEC), it is important for atoms to calm down as much as possible.

So, is there any way to calm the atom down?

Yes, there is! If you cool down, you can use all kinds of low temperature technology. As long as the temperature of the environment of the atom is low enough, the atom is frozen and does not have the strength to move around.

However, for the atom to really calm down, the temperature needs to be close to absolute zero, and the general cryogenic technology can not meet this requirement at all.

What are we going to do?

You know, if you want the river to stop, you just have to dam it.

But the movement of atoms is disorganized, with different directions and speeds. In order to slow them down, it is impossible for them to act collectively on the atoms, but on the individual atoms. Just as you want to destroy the chaotic enemy chariots on the battlefield, all you can do is aim at them one by one.

But the atom is very small, what fine weapon can accurately apply the action to the individual atom?

Yes, you can do it alone!

Although the light shines on the atoms collectively, each photon hits each atom with the same precision as a projectile, ensuring that the action is in place.

For example, an atom is like a car on a road, while a photon is like a stone flying toward a car. According to the law of conservation of momentum, just as a car is constantly hit head-on and slows down, atoms gradually slow down because they are constantly hit by photons.

The problem, however, is that the incident light may slow down the oncoming atoms, but it will also accelerate those atoms that travel in the same direction as the photons. And it is conceivable that there should be as many opportunities for atoms to be accelerated and decelerated by photons. So you can't slow down the atoms.

It seems that in order to achieve the goal of slowing down the atom, the key question that must be solved is: how to let only the atom facing the light be hit by the photon, and avoid the atom moving in the direction of the light from being pushed by the photon?

To answer this question, you have to see what happens when a photon touches an atom.

We have to start with the concept of light quantum.

When electromagnetic waves encounter other particles, they look like an elastic ball, and their energy and momentum are not unique to ordinary light sources. So for ordinary light, the energy of photons is different.

Laser is a kind of coherent light, which has good directivity and monochromaticity, but it can be approximately considered to have only one frequency, and each photon has the same energy.

When a photon touches an atom, it may be absorbed by the atom-it is possible because this absorption does not necessarily occur, depending on whether the atom can jump to other energy levels after absorbing the energy of the photon.

So, what is the energy level?

Atoms have energy, which includes the kinetic energy of the atom as a whole (mainly the nucleus), the kinetic energy of electrons, and the energy of the interaction between electrons and nuclei.

The overall energy of the atom is very large, except for high-energy photons, the energy of an ordinary laser is not enough to reach this level, so it is difficult to exchange energy with it. Just like you throw a ping-pong ball on an iron ball, the ping-pong ball is bounced back directly at the original speed, and the iron ball still moves at its original speed.

The electron is different, because it is much smaller than the nucleus, not so cold, photon energy can be favored by it. In particular, because the outer electrons are less constrained by the nucleus, they are more likely to interact with foreign photons.

In other words, electrons take over almost all the external communication activities of atoms, and it is up to electrons to determine how foreign particles are absorbed or rejected. This is why we always talk only about electrons when we talk about atomic states or atomic energy levels.

So, is it possible that electrons can absorb photons of different energies at will?

No, the electronics can't help it! Due to the limitation of quantum mechanics, the electron can only move on some discrete orbits, which makes it unable to accept the energy that does not meet the requirements.

Electrons move around the nucleus, and for some atoms, the possible orbitals of electrons are determined. Electrons can only move in these orbits, just like the solar system, those planets can only move in those definite orbits, and if they deviate a little, the solar system may collapse.

The electrons in the orbit have energy, and the farther away from the nucleus, the higher the energy, and vice versa. Because the orbitals of electrons are discontinuous, this energy is also discontinuous, and this energy is the energy level of the atom.

When an electron jumps from a low orbit to a high orbit, it absorbs the corresponding energy.

If a group of atoms are in the ground state, they need at least the energy absorbed to excite the electron to the second orbital. If the energy of a photon does not reach this value, the atom will not absorb the photon, the energy of the atom will remain the same, and of course the speed will not change.

Only when the incident photon provides enough energy for the atom to make an energy level transition will the atom absorb the photon, which will lead to a change in the velocity of the atom according to the law of conservation of momentum.

Well, there is a way to solve the previous key problem: just find a way to increase the energy of the photons facing the moving atom and make the photons chasing the moving atom have less energy!

So how can this be done?

The Doppler effect is coming to the rescue!

We have the experience that when the train comes from a distance, its whistle is higher, and when it is away from us, the voice becomes lower. This is the Doppler effect of sound.

Light has a similar effect. When the receiver moves relative to the light source, the receiving frequency caused by Doppler is

Close to away from which is the relative rate between the receiver and the light source.

It can be seen that for atoms moving towards light, the frequency of light is higher, so the energy is higher, while for those atoms moving in the same direction with light, the frequency of light is lower, so the energy is lower.

In this way, we have a way:

As long as the frequency of the laser is lower than the frequency of photons absorbed by atomic energy by a certain value!

Specifically, the laser frequency is adjusted to an appropriate value for the atom to be cooled, so that the frequency of the photon received by the atom moving toward the light meets the requirements of the atomic energy level transition, the photon is absorbed, and the speed of the atom is reduced.

For atoms moving along the direction of light, due to the Doppler effect, the energy of photons is not enough to excite electrons to reach higher orbits, so the atoms do not absorb photons and avoid being accelerated.

If a laser is incident from multiple directions, no matter which direction the atom moves, it always absorbs oncoming photons. So in each scattering, the atom loses momentum and slows down. These atoms seem to be in "sticky syrup", and their movement is constantly blocked until they are almost at a standstill.

There are many different technologies to realize laser cooling, of which the most important principle is Doppler effect, so laser cooling is also called Doppler cooling. Through Doppler cooling, rubidium-85 atoms were successfully cooled to a low temperature of 150 micrometers.

In modern physics research, laser cooling atoms is a very important technology. The 1997 Nobel Prize in Physics was awarded to Cohen-Tannoudji, Steven Chu and Daniel Phillips for their development of "laser cooling and trapping of atoms".

This article is from the official account of Wechat: University Physics (ID:wuliboke), by Xue Debao.

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