Laser Cooling of Coin-sized Objects

Laser cooling of coin-sized objects down to one-kelvin temperatures is now possible. In a set of experiments performed last year, a variation on the laser-cooling technique used in chilling vapors of gases down to sub-kelvin temperatures had been used in macroscopic (but still tiny) samples in the nano- and micro-gram range.

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.

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. Beyond the record low temperature achieved for an object as large as 1 gram, another interesting feature of the experiment pertains to the strength of the force exerted by the laser beams.

In the chosen dimension, the beams fix the mirror so steadfastly that it's as if it were being held in place by a spring that's stiffer than a diamond with the same dimensions as the laser beam (long and thin). According to MIT researcher Nergis Mavalvala (nergis@ligo.mit.edu) the sample is held by a rigidity (if the laser beam were solid) characterized by a Young's modulus (the parameter specifying stiffness) of 1.2 tera-pascals, some 20% stiffer than diamond. (Corbitt et al., Physical Review Letters, upcoming article; lab wiki at http://baikal.mit.edu/sqwiki/moin.cgi/Pictures