  Coulomb interactions have a major role in one-dimensional electronic
transport. They modify the nature of the elementary excitations from Landau
quasiparticles in higher dimensions to collective excitations in one dimension.
Here we report the direct observation of the collective neutral and charge
modes of the two chiral co-propagating edge channels of opposite spins of the
quantum Hall effect at filling factor 2. Generating a charge density wave at
frequency f in the outer channel, we measure the current induced by
inter-channel Coulomb interaction in the inner channel after a 3-mm propagation
length. Varying the driving frequency from 0.7 to 11 GHz, we observe damped
oscillations in the induced current that result from the phase shift between
the fast charge and slow neutral eigenmodes. We measure the dispersion relation
and dissipation of the neutral mode from which we deduce quantitative
information on the interaction range and parameters.
