Physicists have "cooled" a nanoparticle to the lowest temperature allowed by quantum mechanics. The particle's motion reached the particle's ground state, or lowest possible energy level. For a typical material, the amount that its atoms jostle around indicates temperature. But in the case of the nanoparticle, a glass bead about 140 nanometers wide, scientists can define temperature based on the motion of the entire particle, which is made up of about 100 million atoms. That temperature reached an equivalent of twelve-millionths of a kelvin, scientists report online January 30 in Science.
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