We study how phase information is developed and transferred between Bose-Ei
nstein condensates. First, we investigate how the phase of an output pulse
of an idealized atom laser can be controlled. We then use quantum simulatio
ns to show how a relative phase between coupled systems is established by d
issipation and atomic interactions. Our Simulations show that each realizat
ion settles to the same relative phase between modes: for uncoupled systems
the phase varies from realization to realization. We use this to demonstra
te how condensates may 'naturally' possess a relative phase, and to explain
the phase-locking mechanism in a recent experimental demonstration of a ma
de-locked atom laser.