Modern Physics

# Bose-Einstein condensate

Bose – Einstein condensate (BEC) is a state of matter obtained when a gas of extremely rarefied bosonic atoms is cooled to temperatures close to absolute zero . When the gas temperature reaches a critical value, the atoms condense, giving rise to a single macroscopic superatom, in which all atoms have identical properties. In 1925, the possibility of the existence of the Bose-Einstein condensate was predicted by Albert Einstein , after having read an important article published by the physicist and mathematician Satyendra Nath Bose.

## Understanding the Bose-Einstein condensate

In order to fully understand the Bose-Einstein condensate, a vast mathematical knowledge of statistical physics and quantum physics is necessary , so in this article, we will present the theories behind the Bose-Einstein condensate in a qualitative and simplified way, within of the possible limits, let’s go?

First, you need to know what a boson is. Bosons are particles that have integer spin, and fermions have fractional spin. Spin , in turn, is a characteristic of matter, just like mass or electric charge . In addition to spin, bosons differ from fermions in that they are indistinguishable, since two bosons are capable of occupying the same energy state, starting to behave as a “single” particle.

Bosons are usually “virtual” particles, so called because they have no mass, however, some particles that have mass can behave like fermions — an example of this is atoms formed by an odd number of fermions. This characteristic of atoms is what made possible the emergence of the Bose-Einstein condensate.

According to Bose statistics, a cluster of bosons can occupy the same quantum state , this means that all the individual properties of atoms are reflected in the set as a single “superatom”. Starting from this theory, Einstein generalized this logic , imagining that Bose’s statistics could be applied to integer spin fermions, as is the case with 4 He (Helium-4) and several other isotopes of the Periodic Table .

Although he had predicted the possibility of the existence of the Bose-Einstein condensate, Einstein was never able to confirm it. Obtaining the first BEC was only possible in 1995, through an experiment that involved the condensation of a gas of 87 Rb (Rubidium-87) . The production of the first Bose-Einstein condensate won the Nobel Prize in Physics for physicists Eric Cornell, Carl Wieman and Wolfgang Ketterle.

## How is Bose-Einstein condensate made?

Bose-Einstein condensate has two key ingredients: a gas of bosonic atoms and an extremely low temperature . The gas is kept in a chamber, and its density should be close to 10 12 atoms per cm³. Low density is necessary so that the gas does not become liquid or solid during the cooling process.

Gas cooling is the most ingenious and complex part of the entire BEC production Since condensate cannot be done under any surface as that would make it liquid or solid, cooling cannot be done by conventional cryogenic methods . In this case, the gas is cooled by means of lasers. The lasers, at this first moment, act by “reflecting” the atoms back to the center of the container.

Since temperature is concerned with the kinetic energy of atoms, photons from lasers are emitted at a slightly lower frequency than the discrete energy levels of atoms in the gas. Atoms that move away from the center of the container, and move towards the laser sources, perceive the photons with frequencies slightly higher than their real frequency, due to the Doppler effect , and, therefore, absorb these photons, gaining speed in the opposite direction, thus returning to the center of the container.

The cooling process is done using six laser beams that point in the three directions of space, and the frequency of the photons is reduced as the gas cools and condenses into smaller and smaller spaces. By means of this technique, it is possible to obtain temperatures close to 10 -7 K.

The last cooling step was developed by Cornell and Wieman and allowed to obtain the state known as Bose-Einstein condensate. The method consisted of using a weak magnetic field to trap the atoms , then this magnetic field was turned off and the “hottest” atoms escaped, as happens in boiling water , causing the condensate to cool more and more, until the incredible 50 nK mark (5.10 -8 K).

## What is the Bose-Einstein condensate used for?

Through the Bose-Einstein condensate, physicists were able to observe quantum effects in the macroscopic world, a fact never before observed and thought to be impossible. Furthermore, as all the atoms in the Bose-Einstein condensate are at the same energy, by breaking it up, it is possible to produce high-coherence atomic laser beams. This characteristic of condensates can promote the advancement of an important emerging area of ​​technology, nanolithography, which concerns the manufacture of circuits, devices and even robots on the nanometer scale, that is, of a size comparable to that of atoms.