  The bulk-to-boundary dictionary for 4D celestial holography is given a new
entry defining 2D boundary states living on oriented circles on the celestial
sphere. The states are constructed using the 2D CFT state-operator
correspondence from operator insertions corresponding to either incoming or
outgoing particles which cross the celestial sphere inside the circle. The BPZ
construction is applied to give an inner product on such states whose
associated bulk adjoints are shown to involve a shadow transform. Scattering
amplitudes are then given by BPZ inner products between states living on the
same circle but with opposite orientations. 2D boundary states are found to
encode the same information as their 4D bulk counterparts, but organized in a
radically different manner.
