Bose Einstein condensate. States of matter - Bose-Einstein condensate. An experiment with a Bose–Einstein condensate is underway

In general, particles can be divided into fermions and bosons (with half-integer and integer spin). When you cool bosons to temperatures close to absolute zero, they can condense into a collective state of matter known as a Bose-Einstein condensate, where a fairly large number of atoms end up in an identical quantum state, which allows you to observe various unusual phenomena, such as superconductivity.

The first experiment in producing condensate dealt with rubidium atoms cooled almost to absolute zero. On the left - data on the distribution of atomic speeds before the appearance of condensation, in the center - immediately after, on the right - after some time. (Illustration by R. Zhang.)

60 years have passed from the theoretical postulation of condensate in 1925 to its first discovery in the laboratory, but it is still very far from conquering all the peaks associated with this phenomenon. In particular, the condensate was obtained based on rubidium atoms in the gaseous state, although it would be much better to deal with photons. In addition to purely theoretical significance, such a result could also find application - in lasers with unusual properties or even new types of solar cells.

But can photons “condense”? Particles of light have no mass, but its presence seems to be a key requirement for obtaining a Bose-Einstein condensate. To overcome this difficulty, physicists have tried trapping light in an optical cavity, between two parallel light-reflecting plates, which would make photons behave as if they had mass. To prevent light from escaping from such a trap, its walls should be slightly curved.

In 2010, it was experimentally shown that the creation of such a trap is quite possible, but serious problems remained with the interpretation of the results of such experiments. To be confident in them, several specific requirements had to be met. Firstly, the entire system must be two-dimensional, absolutely flat, which is very difficult to implement in a three-dimensional world. Secondly, you need to be sure that the medium between the photons (and this is not air) does not affect their “condensation” during cooling.

In addition to the three states of matter known to every seventh grader (solid, liquid and gas), there are other states of matter. One of them is a Bose condensate, a state of matter that is achieved at temperatures close to absolute zero. In this state, the substance begins to exhibit various interesting properties, for example, a group of particles behaves like a single particle. The possibility of such a state was predicted in 1925 by Albert Einstein. In 1995, American physicists Eric Cornell and Carl Wieman conducted an experiment in which they obtained the Bose-Einstein condensate (for this discovery, they, together with the German Wolfgang Ketterle, received the Nobel Prize in 2001).

In their experiment, scientists used metal atoms (rubidium). But the idea of ​​creating a Bose-Einstein condensate from other particles, in particular photons, so that the system behaves like a single “superphoton” ran into a fundamental problem. The fact is that photons, although they have the properties of particles, were absorbed by surrounding materials during cooling, thereby revealing their wave nature.

Physicists from the University of Bonn, led by Martin Weitz, managed to solve this problem.

Moreover, they created a Bose-Eystein condensate at room temperature.

In one of the descriptions of this work there is, for example, a phrase such as “little sensation.” Zoran Hadzibabić from the University of Cambridge told New Scientist, that the work of German scientists, which published in Nature, “closes the circle that Bose and Einstein theoretically began to draw 85 years ago.”

Volker Lannert, University of Bonn

The simplicity of the experimental setup of the German physicists also deserves admiration. In their experiment, they used two highly reflective concave mirrors spaced 1 micron (10 -6 meters) apart. The mirrors were placed in a “dye” - a red liquid organic medium. The experimenters pulsed green laser beams into this environment. The light, repeatedly reflected from the mirrors, passed through the “dye”. At the same time, the “dye” molecules absorbed laser photons and re-emitted them with lower energy, in the yellow region of visible color. That is, scientists have achieved an equilibrium energy state of photons at room temperature in their trap.

“During this process, the photons were cooled to room temperature without being lost,” explained Martin Weitz.

By increasing the number of photons in the installation (for this it was necessary to make the laser brighter), scientists achieved a density of about a trillion photons per cubic centimeter. At such a density, photons appeared that could not participate in energy equilibrium. These excess photons simultaneously became a Bose-Einstein condensate and condensed into one large “superphoton.” “All the photons started walking side by side,” Weitz commented on this phenomenon.

Compared to the formation of a Bose-Einstein condensate from cooled rubidium atoms, the current experiment seems ridiculously simple,” told Nature News Matthias Weidemuller from the University of Freiberg. He believes that the light condensation technique proposed by German scientists may be especially effective for collecting and focusing sunlight in solar panels in cloudy weather, when direct light cannot be collected.

In addition, this scheme may make it possible to create new sources of short-wave laser radiation, in particular x-rays.

Waitz himself believes that the work of him and his colleagues can help further reduce the size of electronic devices, in particular computer microchips. This, in turn, could make it possible to create a new generation of computers with greater performance than the current ones.

Well, Wolfgang Ketterle, one of the Nobel Prize winners for producing the Bose-Einstein condensate from rubidium atoms, said: “When I lecture, I tell students why the Bose-Einstein condensate cannot be obtained using photons, in order to show the fundamental difference between photons and atoms. But now this difference has disappeared."

The theory of the existence of superfluid matter was developed in the first third of the 20th century, but scientists managed to obtain it only 70 years later.

Relatively recently, scientists managed to obtain a hypothetical Bose-Einstein condensate based on photons. It is unlikely that this news would mean anything to an ordinary person, but in the world of science this discovery is considered simply unique. What's the point?

The Bose-Einstein condensate was predicted by Albert Einstein in 1925 based on the work of the Indian physicist Bose. Condensate is a specific form of matter, its new fifth state. It is not a liquid, gas, solid or plasma. When a substance takes this form, it exhibits quantum effects. The substance becomes superfluid. All its atoms move in concert. Essentially, the condensate becomes one large quantum particle.

The theory of the existence of superfluid matter was developed in the first third of the 20th century, but scientists managed to obtain it only 70 years later. The reason was that the particles of matter had to behave as a single quantum system to produce the supposed condensate. To do this, they had to be cooled to temperatures below absolute zero (-273.15 degrees Celsius) by a few millionths of a degree. Such temperatures are called nanokelvins. They are more than a million times lower than the temperature of interstellar space.

In those years, physicists simply did not know how to achieve such low temperatures. In addition, most substances cooled to absolute zero begin to behave like liquids. To obtain a Bose-Einstein condensate, the substance must remain a “gas,” that is, not lose mobility.

In the mid-1990s, it became known that the alkali metals sodium and rubidium, when cooled, retain the necessary properties in order to turn into condensate. To lower the temperature of rubidium atoms to the required ultra-low values, the researchers used laser cooling together with evaporative cooling.

But in 2010, German scientists from the University of Bonn obtained a Bose-Einstein condensate from photons already at room temperature. How did they do it? For the experiment, a camera with two curved mirrors was used. The empty space between them was gradually filled with photons. At one point, the launched photons lost their stable state, unlike the particles that were previously there. Such photons began to condense and move into the fifth state of matter. This means that the scientists obtained the Einstein–Bose condensate at room temperature, without cooling.

Superfluid matter can be used in a wide range of problems. For example, in an atomic laser. Photons in a conventional laser have the same energy, phase and wavelength. If they assume the condensate state, then it is possible to obtain radiation for more efficient laser operation. In addition, the method of producing condensate from photons can find application in solar energy. This will make it possible to increase the efficiency of solar cells in cloudy weather in the future.

Bose-Einstein condensate - the fifth state of matter

The Bose-Einstein condensate is a specific state of aggregation, a state of aggregation of matter, which is represented mostly by bosons under ultra-low temperature conditions.

It is a condensed state of a Bose gas - a gas consisting of bosons and subject to quantum mechanical effects.

In 1924, Indian physicist Satyendra Nath Bose proposed quantum statistics to describe bosons, particles with integer spin, which were also named after him. In 1925, Albert Einstein generalized Bose's work by applying his statistics to systems consisting of atoms with integer spin. Such atoms, for example, include Helium-4 atoms. Unlike fermions, bosons do not obey the Pauli exclusion principle, meaning multiple bosons can exist in the same quantum state.

Bose-Einstein statistics can describe the distribution of particles with integer or zero spin. In addition, these particles should not interact and should be identical, that is, indistinguishable.

Bose-Einstein condensate

A Bose-Einstein condensate is a gas consisting of particles or atoms with integer spin. As is known, particles are capable of taking on several quantum states at once - the so-called quantum effects. According to Einstein's work, as the temperature decreases, the number of quantum states available to the particle will decrease. The reason for this is that particles will increasingly prefer the lowest energy states as the temperature decreases. Considering that bosons are capable of being in the same state at the same time, as the temperature decreases they will go into the same state.

Thus, the Bose-Einstein condensate will consist of many non-interacting particles that are in the same state. It is noteworthy that also with decreasing temperature the wave nature of particles will become more and more apparent. At the output we will have one quantum mechanical wave on a macroscale.

Velocity distribution data (3 types) for a gas of rubidium atoms, confirming the discovery of a new phase of the substance, the Bose-Einstein condensate. Left: before the appearance of the Bose-Einstein condensate. Center: immediately after condensation appears. Right: After further evaporation, leaving a sample of almost pure condensate.

How to obtain a Bose-Einstein condensate?

This state of aggregation was first achieved in 1995 by American physicists from the National Institute of Standards and Technology - Eric Cornell and Karl Wieman. The experiment used laser cooling technology, thanks to which it was possible to lower the temperature of the sample to 20 nanokelvins. Rubidium-87 was used as a material for the gas, 2 thousand atoms of which passed into the state of Bose-Einstein condensate. Four months later, German physicist Wolfgang Ketterle also achieved condensate in much larger volumes. Thus, scientists experimentally confirmed the possibility of achieving the “fifth state of aggregation” under ultra-low temperatures, for which they received the Nobel Prize in 2001.

In 2010, German scientists from the University of Bonn under the leadership of Martin Weitz obtained a Bose-Einstein condensate from photons at room temperature. For this, a camera with two curved mirrors was used, the space between which was gradually filled with photons. At some point, the photons “launched” inside could no longer reach an equilibrium energy state, unlike the photons previously located there. These “extra” photons began to condense, passing into the same lowest energy state and thereby forming the fifth state of aggregation. That is, scientists managed to obtain a condensate from photons at room temperature, without cooling.

Already by 2012, it was possible to achieve condensate from many other isotopes, including isotopes of sodium, lithium, potassium, etc. And in 2014, an installation for creating condensate was successfully tested, which in 2017 will be sent to the International Space station for conducting experiments in zero gravity conditions.

Application of condensate

Although this phenomenon is difficult to imagine, like any quantum effects, such a substance can find application in a wide range of problems. One example of the application of a Bose-Einstein condensate is an atomic laser. As is known, the radiation emitted by a laser is coherent. That is, photons of such radiation have the same energy, phase and wavelength. If the photons are in the same quantum mechanical state, as is the case with the Bose-Einstein condensate, then it is possible to synchronize this cooled substance in order to obtain radiation for a more efficient laser. Such an atomic laser was created back in 1997 under the leadership of Wolfgang Ketterle, one of the first scientists to create the condensate.

The method of producing condensate from photons, which was used by German scientists in 2010, can be used in solar energy. According to some physicists, this will improve the efficiency of solar cells in cloudy weather conditions.

Bose-Einstein condensate - graphical visualization

Since the Bose-Einstein condensate was obtained relatively recently, the scope of its application has not yet been precisely determined. However, according to various scientists, condensate could be useful in many areas, ranging from medical equipment to quantum computers.

As soon as we say “quantum mechanics,” we imagine elementary particles, atoms, or something similar. In fact, the formulas of quantum mechanics are quite applicable to macroscopic bodies. The main thing is that these bodies do not interact with the outside world, so that they are ideally isolated from it.

It is no coincidence that scientists have recently been particularly interested in macroscopic objects that behave according to the laws of the quantum world. An example of this is the Bose-Einstein condensate, a tiny cloud of many atoms cooled to an ultra-low temperature - up to billionths of a degree above absolute zero, when thermal motion practically stops. Such a cloud, being in a magnetic trap, literally behaves like one huge “atom”. The individual atoms that made it up lose their freedom; they cease to be independent of each other. “Atoms keep pace,” as it was aptly said in one of the articles devoted to this phenomenon. The resulting macroscopic quantum object reaches several micrometers in diameter; it is many times larger than an ordinary atom. Now this object as a whole reacts to any influence, although there are almost no forces connecting them between its individual atoms.

A cloud of atoms cooled to an incredible temperature begins to “keep pace” - a Bose-Einstein condensate appears.

The bizarre world of atoms. Left: Sodium and iodine atoms on the surface of a copper substrate. Right: “wall” built from iron atoms on a copper substrate

“Usually all the atoms are flickering, rushing in all directions, but if you cool them very much, they suddenly begin to march in formation, like an army. The difference is almost the same as between a light bulb and a laser: in a light bulb all the particles of light rush in different directions, but in a laser they march. So we managed to build a laser that emits not light, but matter. Actually, everything is very simple, isn’t it?” - German physicist Wolfgang Ketterle, who later received the Nobel Prize for his study of this condensate, which represented... a new state of matter, jokingly explained the essence of the discovery.

The substances around us are in liquid, solid or gaseous form. However, the theory allows for other states of aggregation. For example, all the atoms of a substance could condense at the lowest energy level. Such an object should have reacted to any influence as a single whole, although nothing connects its particles. Its behavior could be described by a single wave function. This strange phenomenon was predicted by Albert Einstein in the mid-1920s, analyzing calculations carried out by the Indian physicist Shatyendranath Bose. This metamorphosis must occur in close proximity to absolute zero on the Kelvin scale.

An experiment is being prepared to cool a substance almost to absolute zero and obtain a Bose–Einstein condensate

In fact, a similar state was subsequently observed, but it was not possible to obtain it in its pure form. Thus, in superconductors, some electrons exist in the form of a Bose-Einstein condensate. In superfluid helium, some of the atoms also behave as a single whole.

In the early nineties, several scientific laboratories were “hunting” for the Bose–Einstein condensate. The path to it ran through the region of superconducting materials. The next mark on the scientists’ path: 4.2 kelvin (about -269 °C). At this temperature, helium becomes a liquid. At a temperature of 2 Kelvin, it becomes superfluid, that is, without experiencing friction, it penetrates into the thinnest capillaries.

The actual field of ultra-low temperature physics begins at temperatures below 2 Kelvin. By the mid-1990s, physicists had improved cooling technology so much that the discovery of a new state of matter seemed inevitable.

Here is one of the methods - so-called laser cooling. The gas is held in a magnetic trap and a laser beam is directed at it. It absorbs some of the kinetic energy of the atoms, and this reduces the temperature of the gas. In the flow of light quanta, gas atoms are slowed down as if in an “optical syrup”. In a similar way, at the beginning of 1995, it was possible to cool a gas of cesium atoms to a temperature of 700 nanokelvins, that is, 0.0000007 kelvins.

Everything is ready to obtain the Bose–Einstein condensate

But the record did not last long. In the same year, American physicists Eric Cornell and Carl Wyman from the National Institute of Standards and Technology (Colorado) first cooled a gas formed from rubidium atoms to 200 nanokelvins, and a little later they broke this temperature record. The choice of gas played an important role. Due to their size, rubidium atoms are easier to cool than, for example, hydrogen. In addition, when working with them, condensation is easier to detect. In the case of hydrogen, the gas can condense, and no one will notice anything.

The rubidium gas was pre-cooled by lasers, and then, using directed radio waves, the hottest atoms were removed from a magnetic trap. “What happened was about the same as with a cup of coffee that is cooled, allowing the hottest parts of the drink to evaporate,” explains Eric Cornell.

Finally, at a temperature of 170 nanokelvins, the long-awaited moment arrived: rubidium gas began to condense, its density sharply increasing. More and more atoms occupied the most favorable energy positions instead of being distributed among different levels, which is typical for ordinary gas. Two thousand atoms accumulated in the center of the trap. Their speed and direction of movement were the same. This state lasted about fifteen seconds.

“When the explorers realized what kind of prey they had caught, everyone was filled with amazing excitement. After all, this bunch of atoms was not an ordinary gas at all! It was about a new form of substance, to which strange properties were attributed.” In the summer of 1995, the pages of many newspapers were full of similar messages.

Early comments on this experiment suggested that a Bose-Einstein condensate could set a new standard for measuring time. That it could conduct heat better than metal. That if you focus it, you get a beam that resembles a laser. Such a beam could become a powerful weapon for nanotechnologists. Using it, it would be possible to produce much smaller microcircuits than now.

“We have entered a completely new area of ​​research,” admitted future Nobel laureate Eric Cornell in one of his first interviews. – Very interesting phenomena are opening up before us. I think that in the coming years, ultra-low temperature physics will experience a renaissance.”

Since 1995, physicists have been able to produce an Einstein-Bose condensate from rubidium, sodium, hydrogen and helium atoms. In all cases, it consisted of bosons - quasiparticles with integer spin (intrinsic angular momentum), tending to be as close to each other as possible.

In 1999, a condensate of fermions—particles with half-integer spin that try to stay away from each other—was also obtained for the first time. In this case, the condensate contained potassium atoms. They connected in pairs, forming a kind of diatomic molecules with a whole spin.

This was reminiscent of the appearance of so-called Cooper pairs in superconductors, that is, pairs of electrons that can overcome mutual repulsion. Experts’ comments emphasized: “If it were possible to transform the fermion condensate into a solid state, the resulting substance could have the properties of a high-temperature superconductor.”

“The study of fermion condensates can significantly advance research in the field of high-temperature superconductivity, since the mechanism of formation of atomic pairs is of the same nature as the formation of Cooper pairs, but the atoms are much more resistant to the influence of high temperatures,” wrote Izvestia journalist Pyotr Obraztsov .

An experiment with a Bose–Einstein condensate is underway

Finally, in April 2001, reports appeared that employees of Rice University (Houston, Texas) had obtained a special state of matter: it simultaneously contained both bosonic and fermionic condensates.

A group of scientists - led by Randall Hulet - conducted experiments with a mixture containing isotopes of lithium-6 and lithium-7. The latter's atoms behave like bosons because they consist of an even number of elements: four neutrons, three protons and three electrons. Lithium-6 atoms belong to fermions. They consist of an odd number of particles: three neutrons, three protons and three electrons. Two identical fermions cannot be in the same place, move at the same speed, or in the same direction.

Mountains made of atoms are visible on the monitor of a scanning tunneling microscope

When the atomic cloud was cooled to a millionth of a degree Kelvin, lithium-7 atoms were located at the very center of the magnetic trap; they formed a compact cloud about half a millimeter in diameter. With further cooling it quickly decreased. The fermion cloud was diffuse and its size varied little. It was subject to the so-called Fermi pressure, which prevented atoms from accumulating in the middle of the trap even at such a low temperature. American scientists suggest that even at lower temperatures, fermionic and bosonic clouds avoid each other and tend to move away. A similar phenomenon was also observed in a mixture of liquid helium-3 and helium-4.

Other studies of the Bose–Einstein condensate are also interesting.

Thus, Eric Cornell and Carl Wyman, in an experiment with a condensate of rubidium isotope atoms, achieved rapid alternation of the forces of attraction and repulsion of atoms. This led to an almost explosive expansion of the condensate, reminiscent of a supernova explosion. Scientists have dubbed this process “Bose‑Nova.”

German physicists Josef Fortag and Theodor Hensch, who received the Nobel Prize in Physics in 2005, independently produced a microcircuit that can be controlled using a drop of Bose-Einstein condensate. Using it, you can accumulate and transmit information.

Wolfgang Ketterle showed that pieces can be “plucked off” from the Bose–Einstein condensate. This will make it possible to build an atomic laser that will generate radiation from matter rather than light. The condensate is an ideal matter wave, just as laser light is an ideal electromagnetic wave. Its individual atoms can be described by a wave function, just like coherent light. However, the wavelength of atoms is much shorter than the wavelength of light. Using an atomic laser, you can create the tiniest structures by moving atoms with nanometer precision. This discovery will bring tangible progress in nanotechnology. The advantage of atomic lasers over traditional light optics is their extremely high accuracy. “The use of an atomic laser,” says Theodor Hensch, “is, as far as I know, the most precise method by which atoms can be manipulated, moving them in a targeted manner.”

“The use of an atomic laser,” says Theodor Hensch, “is... the most precise method by which atoms can be manipulated, moving them in a targeted manner.”

“The Bose-Einstein condensate,” Ketterle notes, “opens the way to the creation and research of completely new materials.” Thus, flat strips or ribbons of condensate “have completely different properties than three-dimensional objects. This is completely different physics."

The condensate is ideal for experimental studies of the properties of quantum systems. In addition, it can be considered as a model of macroscopic systems in which many particles are forced to interact with each other. So, you can create an “optical lattice” of light waves and place a Bose–Einstein condensate inside it. The result will be a kind of object in which cooled gas atoms will be located strictly at certain points in space - almost like atoms in a crystal lattice. This extremely cooled gas can be used in laboratory experiments as a simplified model of a solid. Perhaps experiments with Bose-Einstein condensates will finally help to accurately describe the mechanism of high-temperature superconductivity.

It remains to add that, according to the Izvestia newspaper, “the largest Russian specialists in Bose-Einstein condensates work abroad: academician Vladimir Zakharov in the USA, academician Lev Pitaevsky in Italy. There are no experiments in this area in Russia.”