The interaction properties of atoms are, at low
temperatures, fully determined by the s-wave scattering length of
the interatomic interaction potential. The magnitude and sign of
this quantity strongly depend on the presence of bound states in
this potential and, more precisely, on the energy of the bound
state that is closest to the continuum threshold. In the
multichannel case of a Feshbach resonance, the energy of the two
colliding atoms in the incoming open channel is close to the
energy of a bound state, i.e., a molecular state, in a coupled
closed channel. Due to the different spin arrangements of the
atoms in the open channel and the atoms in the molecular state,
the energy difference between the bound state and the continuum
threshold is experimentally accessible by means of the Zeeman
coupling of the atomic spins to a magnetic field. As a result, one
is able to vary the scattering length to any possible value by
tuning the magnetic field. This level of experimental control has
opened the road for many beautiful experiments which recently led
to the demonstration of coherence between atoms and molecules, by
observing coherent oscillations between atoms and molecules,
analogous to coherent oscillations that are observed in ordinary
two-level systems. We review the theory that describes coherence
between atoms and molecules in terms of an effective quantum field
theory for Feshbach-resonant interactions. The theoretical
predictions resulting from this theory are in excellent agreement
with experimental results