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If you can make the photons of a laser light that hits atoms have more energy than those that leave the atoms, then the atoms get colder. The trick is to tune the energy of a laser photon to a value that is slightly below that of the energy of an electronic transition in the atom. Due to the Doppler red-shift of the photons, those atoms moving toward the beam absorb more photons than those moving in the opposite direction. Emitted photons leave the atom in random directions, and the result is a general loss of momentum and kinetic energy. The atoms get colder because temperature is proportional to the kinetic energy。