One of the things I forgot to mention in yesterday’s post about why I like AMO physics is that AMO systems have proven to be outstanding tools for solving problems from other fields of physics. In particular, ultra-cold atoms have proven to be a fantastic venue for studying problems from condensed matter physics. There’s a comprehensive review of the subject in this Reviews of Modern Physics paper, which is also freely available on the arxiv. I say “comprehensive review,” but, of course, it’s almost certainly already out of date, given how much work is going on in this area.
To understand why this is cool, you need to know a little bit about how solid state physics works. The basic idea is that you consider some periodic arrangement of atoms bound into a solid– the exact arrangement depends on the specific solid– and look at what happens as you add electrons to the system. When you’re dealing with truly macroscopic numbers of atoms, the electrons are no longer bound to specific atoms, but are, roughly speaking, shared between all the atoms in the solid. The states available to the electrons form broad bands, and the way those bands fill up determines the properties of the material– whether it’s a conductor or an insulator, what the thermal properties are, etc.
The structure of the bands and the way they fill up depend on a lot of things– how the atoms are arranged, how the electrons interact with one another, how the atoms in the solid interact with each other, etc. In any real sample, there is also disorder– impurities in the material, boundaries between regions with slightly different orientations, etc.– which also has a large effect on how the electrons behave.
In a real sample, these properties are more or less fixed– you make a sample, and you’re stuck with whatever set of properties it happens to have. There can be some variation, depending on how many impurities you have, and that sort of thing, but for the most part, the crystal structure, internal interactions, and so on are what they are, and only vary by a small amount for most materials.
Cold atoms give you a way to change this.
The key technology is the “optical lattice,” which is made from a pattern of interfering laser beams. The interference pattern consists of bright and dark spots in some regular arrangement, and depending on the atoms you’re using and the laser you have, you can arrange for a sample of cold atoms to be trapped at either the bright spots or the dark spots of the pattern.
If the laser is very intense, the atoms are bound to specific bright or dark spots. If you make the lattice a little weaker, though, the proper description of the atoms in the lattice is just like that for electrons in a solid– the atoms occupy energy bands that are spread through the whole lattice. You can map the atoms-in-light-field system directly onto the electrons-in-solid-crystal one.
But the “electrons” in this case are atoms, and you can do a number of things to change the way that they interact with each other– by applying appropriate magnetic or light fields, you can “tune” the interaction between two colliding atoms so that they appear to repel one another, or you can tune it so that they are attracted to one another. This is the same as changing the strength of the electron-electron interaction in a solid state system, only much, much easier to do.
You can also look at different statistics– fermionic atoms behave like electrons, but bosonic atoms behave like pairs of electrons, which are the key to superconductivity. You can use different isotopes of the same element to look at radically different statistical behavior.
You can also add disorder to the system in a controlled way. Unlike a real crystal, an optical lattice is perfectly periodic, with no gaps or dislocations in the structure. You can change this, though, by adding additional light fields, either by turning on a weak lattice with a different period, or by superimposing a truly random “speckle” pattern on the lattice, to make some of the “wells” in the lattice trap the atoms more strongly than others. The amount of disorder can be varied continuously, by changing the intensity of the additional light field, without needing to make a completely different system.
All of these things are tricks that theorists in condensed matter physics have been able to do for years– you can easily change the interaction between simulated electrons by changing a line or two of code– but that are extremely challenging to do experimentally. Cold atoms in optical lattices give you a clean and readily controllable way to realize some of these systems, and as a result have led to an explosion of interesting work demonstrating effects related to superconductivity and superfluidity in recent years.
The linked review article gives a good summary of the state of the field as of a year or two ago, but there’s more stuff happening all the time. New atomic systems, new types of lattices, and new experimental techniques mean that this can be expected to continue to expand for a good while yet.
Bloch, I., & Zwerger, W. (2008). Many-body physics with ultracold gases Reviews of Modern Physics, 80 (3), 885-964 DOI: 10.1103/RevModPhys.80.885