Quantum Gas Microscopy
I gave a 1-hour talk about Quantum Gas Microscopy at a CM/AMO journal club last month, and it was filled with stupid jokes and memes like all of my presentations. I went through the history of single-atom+single-lattice-site observation in cold atom systems, and then described in detail some 2018 results on antiferromagnetism from the Bloch group. Below I’ve linked the presentation and a latex document with all the citations I looked at to make the talk.
This talk was aimed at physicists with some background/interest in condensed matter (CM) and atomic, molecular, and optical (AMO) physics, but to exercise my writing a little, I’ll try to give some background for the talk here, hopefully aimed at the level of an undergrad physics major (?).
Background
This will probably go into an easily accessible page somewhere on this site because it’s background for my entire field.
The general field is called quantum simulation with cold atoms in optical lattices.
The point is to put atoms in periodic lattices, and use that to mimic, for example, electrons in solids (also periodic). It’s called “quantum simulation” because it’s using a simple quantum system to simulate a more complicated quantum system. This is in contrast to “normal” simulation by writing some code on a computer, which takes much longer for large quantum systems (infinitely long).
The atoms usually need to be cold (for “quantum” to make sense, etc.) and basically means Bose-Einstein condensates and their fermion counterparts, degenerate Fermi gases. These are always gases, so they’re also called “quantum gases.”
The periodic lattices are made by shining a laser on a mirror, which creates a standing wave. If the laser wavelength is chosen correctly (with respect to the atom’s energy levels), this makes a periodic potential for the atoms. You can do this in two perpendicular directions to make a 2D optical lattice, etc.
Quantum Gas Microscopes
Quantum gas microscopes refer to measuring cold atoms in optical lattices at single-atom and single-site resolution. This means that with QGMs, you can see individual atoms in individual lattice sites.
Implementation: Basically you just need a really big lens and put it really close to your atoms, so you get a huge magnification. One problem is that you can’t make something like this out of just one lens, so the “objectives” that people use are some complicated series of lenses in a big tube that you can just buy from optics wizards.
Another issue is the imaging process itself: to see a single atom, you need to be able to capture a bunch of photons from it. For fluorescence imaging, you pump the atom with some laser to excite it to some higher internal state, and then look at the photon that comes out via spontaneous/stimulated emission as it decays back down to the ground state. This cycle needs to be repeated many times, and the atom naturally gets kicked around due to momentum conservation as it emits photons. To prevent the atom from kicking around too much (heating), you need to cool the atoms while you image. Luckily, standard laser cooling techniques developed back in the 80s already emit light that you can capture (optical molasses/sisyphus cooling)… for bosons.
For fermions, these cooling techniques don’t work very well so it took a while for people to figure out and implement things like EIT and Raman sideband cooling.
History: While people were imaging single trapped atoms as early as 1994, the tight spacing of an optical lattice (about 500 microns) meant it wasn’t until 2009 that the first BEC microscope was constructed by Markus Greiner, followed closely by Immanuel Bloch. Fermion microscopes took longer due to the cooling issue, but five groups sort of simultaneously announced their results over the course of 2015.
Antiferromagnetism in the 1D and 2D Fermi-Hubbard model This was the specific topic I tried to discuss in my talk, and it’s based on a trio of papers by the group of Christian Gross and Immanuel Bloch:
- Revealing hidden antiferromagnetic correlations in doped Hubbard chains via string correlators
- Direct observation of incommensurate magnetism in Hubbard chains
- Imaging magnetic polarons in the doped Fermi-Hubbard model
These papers form a nice story about research using a fermionic QGM. Because it’s very difficult to describe three papers, I’ll just give a really, really basic overview and fill it in later if I have the time/effort (read: never).
In the model that this group studied, the spins of the fermionic atoms ordered themselves into an alternating pattern of spin up/spin down (antiferromagnetic order). In the first paper, this group found that if you “dope” the system by inserting an extra atom (“doublon”) or removing an atom (“hole”), the spin up/spin down pattern still stays around, but just skips over the hole or doublon.
In the second paper, the group varied the amount of holes or doublons they put in, and found that the period of the spin up/spin down pattern would vary linearly with the density of dopants. Then they took it in a really interesting direction by slowly making their 1D chains into a 2D lattice, and found that the dopants act drastically differently in 2D….
In the third paper, the group studied more about the weird 2D behavior, which showed that a dopant in 2D could completely destroy the AFM order in a small region around itself. (This is in stark contrast to 1D, where the order/pattern “skips” over the dopants like they don’t exist.) They called this dopant a “polaron” and showed that by pinning the polaron in place, the pattern/AFM order recovered a little… meaning that the movement of the dopant is what destroys the AFM order.
The bigger picture here is that by understanding the behavior of polarons and particles in this antiferromagnetic regime, we may understand parts of the more general model (Fermi-Hubbard model) which is predicted to show signs of superconductivity.