After the metal is heated to a very hot 450°C, fast-moving atoms enter a one meter long tube from which they exit at 1/1000th of a degree above absolute zero. To cool the atoms this dramatically, Zwierlein's team exploits the quantum mechanical property of atoms to absorb and give off energy only at certain frequencies. Within an atom, electrons occupy quantized energy levels. This means an electron can only have certain amounts of energy dictated by the rules of quantum mechanics. Electrons can leap between these energy levels by absorbing or emitting photons. If the atom absorbs a photon with an energy equal to the difference between two energy levels, the electron can move from the lower level to the higher level. Likewise, an electron can jump down to a lower level by emitting a photon. In the process of absorbing a photon, the atom receives a small push, a push in the direction away from the source of light, which is the key to laser cooling.