Scientists could one day make direct measurements of individual atoms in an optical lattice using a new microscopy technique proposed by physicists in Switzerland and the UK. The technique, which has yet to be proven experimentally, involves using a single particle that hovers over the lattice before occasionally swooping down to interact with the ultracold atoms below. The researchers claim that the technique could be used to determine if an atom is occupying a specific lattice site -- and even the magnetic properties of the occupant (arXiv:0704.1283v1).
Optical lattices are regular arrays of identical energy wells created by criss-crossing laser beams. By injecting ultracold atoms into the energy wells, the lattices can be used to create and study a range of materials such as Bose Einstein condensates, in which all the atoms are in the same quantum state. The lattices can also be used to mimic the conditions normally found in solids. The advantage of an optical lattice is that the interactions between atoms can be tuned by tweaking the laser beams, whereas the interactions in solids are fixed by the structure and composition of the material.
While researchers have been very successful at measuring the collective properties of atoms in optical lattices, they cannot currently make measurements on individual lattice sites or on individual atoms. Such measurements would be particularly useful when studying lattices in which several different phases – such as magnetic and superconducting – coexist.
Now Corinna Kollath and Thierry Giamarchi of the University of Geneva, along with Michael Köhl of Cambridge University have come up with a way of using a single trapped ion to probe optical lattices with a spatial resolution of about 20 nm.
Under the proposal, the ion would be held above the optical lattice in a trap created by focussing radio waves using a configuration of four electrodes. To make a measurement, the ion would be dropped down into the lattice, where a laser pulse would cause the ion to bind momentarily with a resident atom to create a molecule.
The properties of this molecule could be detected either through a change in the oscillation frequency of the ion within the trap or by observing changes in light scattered from the ion when it binds with the atom. This information could then be used to determine the atomic density of the lattice site – and in some cases its spin density, which is related to the magnetization of the site.
According to Kollath, such magnetic measurements could be used to confirm the antiferromagnetic state of fermions in an optical lattice by observing that the lattice sites alternate between spin-up/spin-down configurations. The technique could also be used to cause a small local perturbation to the optical lattice, which Kollath says could help physicists gain a better understanding of energy gap that defines the superconducting phase of matter.
Meanwhile, Köhl told Physics Web that he and several other groups are currently working on ways to combine single-ion trapping with lattices of ultracold atoms. While both techniques are well-established on their own, Köhl said that they put very different demands on the nature of the experimental set up.
