Just trying to wrap my head around this. Photon energies are distributed across quantised states. We can obtain the single photon energy for a given wave-length

Just trying to wrap my head around this. Photon energies are distributed across quantised states. We can obtain the single photon energy for a given wave-length by the Planck-Einstein relation. We then assume the photons become "entangled", and decrease their speed. Is this to conserve momentum/energy, assuming the frequency is unaltered. I assume this frequency stays the same on the premise of constructive interference, and hence not altering the phase or frequency.