Atomic, molecular, and optical (AMO) physics is the study of how small numbers of atoms (or molecules) interact with one another and with the electromagnetic field.  By using optical forces, a cloud of room-temperature atoms can be cooled to a temperature just above absolute zero. These ultracold atoms can then be used to create exceptionally accurate clocks and inertial sensors.  They also have many other applications in many-body physics, entanglement-enhanced measurement, and quantum computation.