  Because the valleys in the band structure of graphene are related by
time-reversal symmetry, electrons from one valley are reflected as holes from
the other valley at the junction with a superconductor. We show how this
Andreev reflection can be used to detect the valley polarization of edge states
produced by a magnetic field. In the absence of intervalley relaxation, the
conductance G_NS=2(e^2/h)(1-cos(Theta)) of the junction on the lowest quantum
Hall plateau is entirely determined by the angle Theta between the valley
isospins of the edge states approaching and leaving the superconductor. If the
superconductor covers a single edge, Theta=0 and no current can enter the
superconductor. A measurement of G_NS then determines the intervalley
relaxation time.
