We’ve discussed in a couple of the past BMC posts the difference between Bosons and Fermions, and the degenerate states that fermions can form.  Well, today I’m going to talk about something that you may have heard of, but probably have never had explained to you: Bose-Einstein Condensation.  I’ll tell you a bit about what it is, and tell you about a cool application of them: atomtronics.


Bose-Einstein Condensation

The Bose-Einstein Condensate is similar to the case of Fermi degeneracy, but with Bosons rather than fermions.  When Fermions are cooled, they pile up as low energy levels get “filled up”.  Bosons don’t have this problem, and they can all be shoved into the ground state.  That is really all there is to the condensation, cooling of bosons and removal of high-energy particles gets you a cluster of them crammed into the ground state.  The trick is getting particles to behave as bosons.  Neither neutrons, protons, or electrons are bosons.  The trick is making sure that you have an even number of fermions bound together.  Their half-integer spins then combine to form integer spins, allowing the bound state to behave as a boson.  A number of atoms are excellent bosons for this process, the primary being helium-4.  With heavy-duty cooling (you need to get to the nanokelvin levels), you can get a bunch of these atoms to all fall into the ground state, forming a Bose-Einstein condensate.

Then What?

Well, the thing with Bosonic condensates is that they behave… strangely.  As they all fall into the ground state, the cluster begins behaving as if it is a single particle.  Inside the gas, you get superfluid vortices, superconductivity, and all kinds of other weirdness.  All of this is a wonderful way to test the implications of various quantum theories of matter, as the gas is at a much larger scale than a photon or other subatomic particles.  One of the cooler applications is atomtronics: the use of atoms in the condensate as the working particles for diodes and transistors, by shifting their state from the ground to ground+1 state.  This is a really nifty use, as it allows logic gates and the like to be created using a totally different substrate than the usual semiconductor + electron basis that we are used to, and may ultimately serve as the interface technology between the classical parts and the qubits in a quantum computer!  So, while esoteric, Bosonic condensates are far from useless.