The chemistry of the ultracold

All things are made of atoms. That is the key hypothesis

It was Richard Feynman who contended that the most valuable scientific idea was that everything was made of atoms [1].   Knowing that there are different kinds of atoms that can be put together in different ways forms the basis of chemistry, materials science and our understanding of the processes of life itself.  In the other direction, probing the internal structure of atoms has revolutionised our view of the physical world (through the discovery of quantum mechanics)  and has enabled the technology that underpins our civilization – from lasers and electronics to atomic clocks and nuclear energy [2].

But for all the different kinds of atoms that populate the period table, when it comes to the ultracold world [3], there are only two types of atom that matter: bosons and fermions.  You won’t find these listed on the periodic table; but they form the fundamental dichotomy into which all particles can be classified.

(The dichotomy has to do with what happens to the many-body wavefunction when you swap two particles.  If nothings changes, you have bosons.  If the wavefunction changes sign, you have fermions.)

Normal chemistry (at room temperature and one atmosphere of pressure) doesn’t really care whether its particles are fermions or bosons.  But at absolute zero, this is the key distinction that sets the rules of the game, and the periodic table becomes largely irrelevant [4].  Bosons, for example, are gregarious by nature and tend to collect in the same state, forming the matter equivalent of coherent laser light.  Fermions, on the other hand, are rather protective of their personal space, with no two occupying the same state.  What fermions lack in easy-going coherence, they make up for in the propensity to form stable, highly correlated configurations [5].

What determines whether an atom is a boson vs fermion?  It comes down to a simple counting game.  If the number of protons + electrons + neutrons that make up the atom is even, you have a boson, if it is odd, you have a fermion.  Since, for neutral atoms, the number of electrons equals the number of protons, the determining factor is the number of neutrons.

Now here is a thing that I find very strange.  Despite the fact that one extra neutron will radically alter the personality of the atom, the neutrons themselves are locked away within the nucleus, quite inaccessible at ultracold energy scales.  Thus a collection of Lithium-6 atoms behaves entirely differently at ultracold temperatures to a collection of Lithium-7 atoms, despite the two isotopes having almost identical physical and chemical properties [6].  It underlines the point, I suppose, that the ultracold dichotomy of bosons-fermions, and the “chemistry” that derives from it, has nothing to do with interactions between atoms, as in the case of ordinary chemistry, but is a purely quantum statistical effect.

Now on this blog I will try to avoid gratuitously provoking my colleagues by making overreaching claims for the field of ultracold atoms.  But I will say this: The field of optics (or at least the quantum version of it) is preoccupied with photons, which are bosons; the vast field of condensed matter physics arises primarily from the rich physics of electrons, which are fermions.  With ultracold atoms, you can choose which personality you want to deal with, which makes for an interesting playground.

[1] R. P. Feynman, “Six easy pieces”

[2] Less controversial forms of power generation – eg  solar – also depend on knowledge of the internal structure of an atom

[3] where the temperature is measured in nanokelvin, i.e. in thousandths of a degree above absolute zero.

[4] OK, there are some technical reasons to do with cooling and trapping that mean only some candidates from the periodic table are feasible for getting to nanoKelvin temperatures.  Furthermore, the choice of atomic species has consequences for the strength and nature of the interatomic interaction, but these are things that can be controlled by other means (eg external magnetic or electric fields) and are thus not intrinsic to particular kinds of atom.

[5] I hope you’ll forgive the silly anthropomorphism.

[6] Their mass will be slightly different, naturally, but this is just another incidental property.