As a thermal atom at room temperature has many thousands of times the momentum of a single photon, the cooling of an atom must involve many absorption-spontaneous emission cycles, with the atom losing up to ħk of momenta each cycle. Because of this, if an atom is to be laser cooled, it must possess a specific energy level structure known as a closed optical loop, where following an excitation-spontaneous emission event, the atom is always returned to its original state. Rubidium, for example, has a closed optical loop between the 5 S 1 / 2 F = 3 state and the 5 P 3 / 2 F = 4 state. Once in the excited state, the atom is forbidden from decaying to any of the 5 P 1 / 2 states, which would not conserve parity, and is also forbidden from decaying to the 5 S 1 / 2 F = 2 state, which would require an angular momentum change of −2, which cannot be supplied by a single photon.