One of the things that I always have to explain toward the end of my laser cooling spiel is that the technique only works well for particular atoms. Somewhere on the high side of twenty different elements have been laser cooled and trapped, but the standard techniques don’t generalize well to even simple diatomic molecules, let alone solids.
Given that, you might be surprised to see the headline “Laser Cooling of Coin-sized Objects,” referring to this paper in Physical Review Letters. The description sounds really cool:
Now, a collaboration of scientists from the LIGO Laboratory at MIT and Caltech and from the Max Planck Institutes in Potsdam and Hannover has used laser beams to cool a coin-sized mirror with a mass of 1 gram down to a temperature of 0.8 K. The goal of chilling such a comparatively large object (with more than 10^20 atoms) is to investigate the quantum properties of large ensembles of matter.
There is, alas, a catch:
An important caveat here is the fact that in all these experiments the “cooling” takes place in one dimension only. A temperature of 1 K applies to the motion of atoms along the direction of the laser beams, while the mirror is free to move (although not much) in other directions. Consequently, if you touched the sample it would not feel cryogenically cold.
Alas.
The title of the actual paper, “An All-Optical Trap for a Gram-Scale Mirror,” isn’t as catchy, but does a better job of explaining what they actually did. Basically, they set up a cavity consisting of two mirrors facing one another, with one of the mirrors hanging suspended from a couple of thin optical fibers. They sent two laser beams with very slightly different frequencies into the cavity, and by carefully choosing the frequencies of the two beams, they were able to create a force that kept the hanging mirror from moving.
When they analyze the motion of the mirror, they find that it behaves as if it were attached to an amazingly stiff spring– 20% stiffer than diamond– and that the residual motion along that direction corresponds to a temperature of less than one degree above absolute zero. Which is pretty damn impressive for an experiment using nothing but light forces to hold something with a mass of a gram.
The article mentions LIGO, and the interest they have in this sort of thing is obvious. LIGO is the Laser Interferometer Gravity Wave Observatory, and they’re looking for graviational waves using the world’s largest Michelson interferometer. What they hope to see is a tiny shift in the interference pattern due to a passing graviational wave causing the mirrors of their interferometer to move infinitesimally farther apart and then back together, as the entire fabric of space distorts slightly.
Obviously, this sort of signal could easily be swamped by anything else that might cause the mirrors to move– vibrations in the mounts, air currents in the lab, trucks rumbling by on nearby highways. They’ve worked very hard to suppress vibrations in their system, and they’ve done amazing things. Being able to cool the residual vibrations to millikelvin temperature using lasers would be a really dramatic improvement over what they’ve already done.
This is a long, long way from being useful for LIGO or anything else, but it’s cool to see. Pardon the pun.