I think that the frequency of the outgoing down-converted photons will remain the same whether the incoming high frequency photon is absorbed by one atom or collectively by N-atoms. A coherent multi-atom absorption will create a Schroedinger-Cat-like state of one excited atom and (N-1) ground state atoms, which should still radiate at the same lower frequencies. However, multi-atom absorption could result in strong variation in emitted intensity bursts (superradiance).
But, maybe there's more to it than that. Some anomalous down-conversion of gamma-rays were reported in the 1930s. I do not know whether they have been explained since then. If interested, the papers are at: "The Nature of the Interaction between Gamma-Radiation and the Atomic Nucleus" http://rspa.royalsocietypublishing.org/content/136/830/662.full.pdf+html "Phenomena Associated with the Anomalous Absorption of High Energy Gamma Radiation. II" http://rspa.royalsocietypublishing.org/content/143/850/681.full.pdf+html "Phenomena Associated with the Anomalous Absorption of High Energy Gamma Radiation. III" http://rspa.royalsocietypublishing.org/content/143/850/706.full.pdf+html > Some insights from quantum mechanicsÂ… > > Spontaneous parametric down-conversion > > Reference: > > http://en.wikipedia.org/wiki/Spontaneous_parametric_down-conversion > > The rule that comes out of this quantum mechanical process is that energy > is shared approximately equally between N entangled particles with each > entangled particle getting 1/N amount of the energy. > > The originating frequency of the nuclear radiation is also shared between > the N particles and is therefore divided approximately equally between the > N particles and is therefore also divided in its calculation by 1/N. > > Spontaneous parametric down-conversion (SPDC) is an important process in > quantum optics, used especially as a source of entangled photon pairs, and > of single photons. > [...]
