por We explain what the Bose-Einstein condensate is, its origin, characteristics, how it is obtained and its applications
What is Bose Einstein Condensate?
The Bose Einstein Condensate (BEC) is a state of aggregation of matter, just like the usual states: gaseous, liquid and solid, but which takes place at extremely low temperatures, very close to absolute zero.
It consists of particles called bosons, which at these temperatures are located in the lowest energy quantum state, called ground state. Albert Einstein predicted this circumstance in 1924, after reading the papers sent to him by the Indian physicist Satyendra Bose on the statistics of photons.
It is not easy to obtain the necessary temperatures for the formation of the Bose-Einstein condensate in the laboratory, which is why we had to wait until 1995 to have the necessary technology.
That year the American physicists Eric Cornell and Carl Wieman (University of Colorado) and then the German physicist Wolfgang Ketterle (MIT), managed to observe the first Bose-Einstein condensates. The Colorado scientists used rubidium-87, while Ketterle got it through a highly dilute gas of sodium atoms.
Thanks to these experiments, which opened the doors to new fields of research in the nature of matter, Ketterle, Cornell and Wieman received the Nobel Prize in 2001.
And it is that the very low temperatures make it possible for the atoms of a gas with certain characteristics to form such an ordered state, that they all manage to acquire the same reduced energy and amount of movement, something that does not happen in ordinary matter.
Characteristics of the Bose-Einstein condensate
Let’s see the main characteristics of the Bose-Einstein condensate:
The Bose-Einstein condensate occurs in gases made up of very dilute bosonic atoms.
The atoms in the condensate remain in the same quantum state: the ground or lowest energy state.
Extremely low temperatures are required, just a few nano-kelvins above absolute zero. The lower the temperature, the wave behavior of the particles is more and more evident.
In principle, matter in the Bose Einstein condensate state does not exist in nature, since no temperatures below 3 K have been detected in the universe to date.
Some CBE present superconductivity and super-fluidity, that is to say, lack of opposition to the passage of the current, as well as viscosity.
The atoms in the condensate, being all in the same quantum state, present uniformity in their properties.
Origin of the Bose-Einstein condensate
When you have a gas enclosed in a container, normally the particles that compose it keep enough distance from each other, interacting very little, except for occasional collisions between them and with the walls of the container. From there derives the well-known ideal gas model.
However, the particles are in permanent thermal agitation, and temperature is the decisive parameter that defines speed: the higher the temperature, the faster they move.
And while the speed of each particle can vary, the average speed of the system remains constant at a given temperature.
fermions and bosons
The next important fact is that matter is composed of two types of particles: fermions and bosons, differentiated by spin (intrinsic angular momentum), an entirely quantum quality.
The electron, for example, is a fermion with half-integer spin, while bosons have integer spin, making their statistical behavior different.
Fermions like to be different and therefore obey the Pauli exclusion principle, according to which no two fermions in the atom can have the same quantum state. This is why the electrons are located in different atomic orbitals and thus do not occupy the same quantum state.
On the other hand, bosons do not adhere to the exclusion principle, so they have no objection to occupying the same quantum state.
dual nature of matter
Another key fact in understanding the CBE is the dual nature of matter: wave and particle at the same time.
Both fermions and bosons can be described as a wave with a certain extension in space. wavelength λ of this wave is related to its momentum or amount of movement pthrough the De Broglie equation:
Where h is Planck’s constant, whose value is 6.62607015 × 10-34 Js
At elevated temperatures thermal agitation predominates, which means that the momentum p is large and the wavelength λ is small. Atoms thus display their properties as particles.
But when the temperature drops, the thermal agitation decreases and with it the momentum, causing the wavelength to increase and the wave characteristics to prevail. Thus, the particles are no longer localized, because the respective waves increase in size and overlap each other.
There is a certain critical temperature below which the bosons end up being in the ground state, which is the state with the lowest energy (it is not 0). That is when condensation occurs.
The result is that the bosonic atoms are no longer distinguishable and the system becomes a kind of super atom, described by a single wave function. It is equivalent to seeing it through a powerful magnifying lens with which its details can be appreciated.
How is the condensate obtained?
The difficulty of the experiment lies in keeping the system at sufficiently low temperatures so that the de Broglie wavelength remains high.
The Colorado scientists achieved this by using a laser cooling system, which consists of hitting the sample of atoms head-on with six beams of laser light to abruptly slow them down and thus drastically reduce their thermal agitation.
Then the cooler, slower atoms were trapped by a magnetic field, letting the faster ones escape to further cool the system.
Atoms confined in this way managed to form, for brief moments, a tiny droplet of CBE, which lasted long enough to be recorded in an image.
Applications and examples
The CBE applications are currently in full development and it will still be some time before they come to fruition.
quantum computing
Maintaining coherence in quantum computers is not an easy task, so CBEs have been proposed as a means of maintaining information exchange between individual quantum computers.
Reduction of the speed of light
The speed of light in a vacuum is a constant of nature, although its value in other media, such as water, may be different.
Thanks to CBEs it is possible to greatly reduce the speed of light, up to 17 m/s, according to some experiments. This is something that will allow not only to go even deeper into the study of the nature of light, but also its use in quantum computing to store information.
High precision atomic clocks
Cold atoms allow the creation of highly accurate atomic clocks, which experience minimal delays over long periods, on the order of millions of years, very useful qualities when synchronizing GPS systems.
Simulation of cosmological processes
The atomic forces that are generated in the condensate can help simulate the conditions under which physical processes take place inside some notable objects in the universe, such as neutron stars and black holes.
References
Bauer, W. 2011. Physics for Engineering and Science. Volume 1. Mc Graw Hill.
Chang, R. 2013. Chemistry. Eleventh edition. McGraw Hill Education.
LandSil. The five states of matter. Retrieved from: landsil.com.
The Qubit Report. Bose-Einstein Condensate Formation Speed Increased, Method of Formation Simplified. Recovered from: qubitreport.com.
Tipler, P. 2008. Modern Physics. 5th. Edit. W. H. Freeman & Company.