Bose-Einstein Condensation

Most of us have learned at some stage in our lives that there are three states of matter; solids, liquids and gases, however these are simply the most abundant states of matter found on Earth. Many more interesting states of matter exist but only under extreme conditions. Here I will be discussing one such state of matter, namely Bose-Einstein Condensation.

Bose-Einstein Condensation (BEC) was first postulated in the 1920s by Albert Einstein upon receiving a paper from Satyendra Nath Bose on the quantum statistics of light. Einstein recognised that the same statistics could apply to certain atoms and predicted the possibility of this new state of matter. It wasn’t until 1995, however, that a Bose-Einstein Condensate was actually created in a lab.

BEC is a phenomenon which is specific to bosonic particles. These are particle whose spin takes integer values and can share quantum states with any number of other bosons. As low density bosonic gases are cooled they will eventually reach a critical temperature at which a substantial fraction of the particles will occupy the lowest energy state. What this means is that all of the particles will be described by the same wave function and quantum effects can now be seen on a macroscopic level.

So why exactly did it take so long to form such a substance? The main difficulty lies in achieving the ‘low temperatures’ needed for BEC to occur. Temperature is a measure of the average kinetic energy of the particles in a substance. This means that there is a point of zero temperature where particles will have zero kinetic energy. This is known as absolute zero and has a value of zero when measured on the Kelvin temperature scale. So when we talk about low temperatures here we mean temperatures near zero Kelvin  (–273.15C). These kinds of temperatures could not be created in a lab environment at the time when BEC was first theorised.  In 1995 researchers at MIT successfully cooled a gas of sodium atoms to around 1 billionth of a kelvin and observed the predicted condensate for the first time. This was achieved by using lasers and magnets. The lasers act to slow down the particles, while the magnets act to trap them in a confined region. Atoms are then further cooled by evaporative cooling – where more energetic atoms are allowed to leave the trap. This process results in temperatures low enough for a condensate to form. At this point the wavelengths of the atoms stretch and overlap, resulting in a single ‘particle’ which obeys quantum rules.

The importance of BEC does not just lie within scientific curiosity. BEC also has many practical applications. By giving us a window into pure quantum behaviour, BECs help us build better atomic clocks, create coherent atom lasers, simulate materials we can’t directly observe, and even explore ideas for quantum computing.

The behaviour of superfluids can be described by BEC. A superfluid is a fluid with zero viscosity (flows without friction so does not lose kinetic energy or momentum). Superfluids can be described by a two-fluid model. In this model there is both a condensate and a normal fluid (containing atoms in higher quantum states). Superfluids exhibit some very interesting properties, among them is the occurrence of quantum vortices. These vortices are like little whirlpools that arise when a superfluid is rotated and are direct evidence of the macroscopic manifestation of quantum principles. Such superfluids can be used to model the behaviour of light near a black hole. This is achieved by introducing a small wave to a superfluid with a single giant vortex.

I hope that I have given some insight into the wonderful properties of BEC. By studying this unusual state of matter we can view the quantum world which exists alongside our own. As cooling technologies continue to improve, BEC may be achieved at higher temperatures and for different systems of particles, yielding exciting possibilities for future technologies.

References

[1] Davis, K. B., Mewes, M.-O., Andrews, M. R., van Druten, N. J., Durfee, D. S., Kurn, D. M., & Ketterle, W. (1995). Bose-Einstein condensation in a gas of sodium atoms. Physical Review Letters, 75(22), 3969–3973.