Defining the (atom) laser

An atom laser is an as-yet unrealized device that would produce an atomic field analogous to the electromagnetic field of a photon laser. Here I argue that for this analogy to be meaningful it is necessary to have a precise definition of a laser that applies equally to photon or atom (or any other boson) lasers. The definition I propose uses the fundamental principle that the output of a laser is well approximated by a classical wave of fixed intensity and phase. This principle yields four quantitative conditions that the output of a device must satisfy in order for the device to be considered a laser. While explaining these requirements, I analyze the similarities and differences between atom and photon lasers. I next show how these conditions are satisfied first by an idealized photon laser model, and then by a more general model that can apply to atom lasers also. Lastly, I briefly discuss the current proposals for atom lasers and whether they could be true lasers.