  We demonstrate a two-dimensional atom interferometer in a harmonic magnetic
waveguide using a Bose-Einstein condensate. Such an interferometer could
measure rotation using the Sagnac effect. Compared to free space
interferometers, larger interactions times and enclosed areas can in principle
be achieved, since the atoms are not in free fall. In this implementation, we
induce the atoms to oscillate along one direction by displacing the trap
center. We then split and recombine the atoms along an orthogonal direction,
using an off-resonant optical standing wave. We enclose a maximum effective
area of 0.1 square mm, limited by fluctuations in the initial velocity and the
coherence time of the interferometer. We argue that this arrangement is
scalable to enclose larger areas by increasing the coherence time and then
making repeated loops.
