  We propose a holographic dictionary which comes from reducing the bulk
theories in an asymptotically flat spacetime to its null infinity. A general
boundary theory is characterized by a fundamental field, an infinite tower of
descendant fields, constraints among the fundamental field and its descendants
as well as a symplectic form. For the Carrollian diffeomorphisms, we can
construct the corresponding Hamiltonians which are also the fluxes from the
bulk, and whose quantum operators realize this algebra with a divergent central
charge. This central charge reflects the propagating degrees of freedom and can
be regularized. For the spinning theory, we need a helicity flux operator to
close the algebra which relates to the duality transformation.
