Figure 25.2 — Laser cooling of a microparticle Figure 25.2 — Laser cooling of a microparticle. (a) Experimental configuration for feedback cooling of a levitated microparticle. A counter-propagating optical tweezers (solid beams) creates a trapping potential where the particle is held. Three additional pairs of counter-propagating beams (outlined beams) provide the active feedback cooling. (b) Equilibrium temperature of a (1.5 μm radius) silica bead laser-cooled at different background pressures, achieving temperatures down to the millikelvin range.Reprinted by permission from Macmillan Publishers Ltd: Li et al., Nature Phys. 7, 527–30, copyright 2011.