If in between transmitter and receiver an electric field would still have continuous spectra of energy (i.e., would propagate as a continious wave which can therefore be chopped at any size of "chunks", then we would be able to do the following experiment.
Take a red He-Ne laser (wavelength = 0.63 um, corresponding to the energy of "photon" E=hf to be about 2 electronvolt). Such laser wich sends 10-100 cm long photons (length of laser photon is l=L/(1-R1R2))depending on cavity size L and reflectivity of cavity mirrors R1 and R2). Get a fast shutter (say, Pokkels or Kerr cell) which is capable to cut a few mm "chunks" out of e/m wave by simply changing transparency of media very fast) and place it between the laser and detector (spectrometer is a proper detector if we want in to measure "color" (=energy E=hf) of e/m wave. Turn the laser on and start "chopping" propagating wave by the shutter.
If there would be no photons (i.e., e/m wave was continuous) then you would be able to cut out the continuous wave various length "chunks" of waves with different energy each. Long chunks (by having shutter open longer) with more energy, and short chunks with less energy. Therefore, detector would detect waves with DIFFERENT colors depending on shutter speed.
But it turns out that you can NOT do that - all waves are still the same color - red waves (w=0.63 microns), no infrared (smaller energy chopped chunks) or blue (bigger energy) or any other new colors emerge. Thus, energy of the propagating e/m wave (=color) is indivisible into smaller (than it was born) chunks.
The LENGTH of a wave trains indeed gets shorter - instead 10 cm, it can be chopped into few mm long trains, but each "chunk" still have the same ENERGY (=2 eV). Thus, because the energy of propagating e/m wave is indivisible into smaller chunks, we have to conclude that it is still quantizied in free space even during propagation between laser and spectrometer. Thus, photon (quantizied states of energy of electric field of moving with acceretaion charge) exists in free space too.
Mathematical analysis of what should the electric field of accelerated charge look like gives the same result (quantization of energy of field).