Quanta are specifically the quantization of the EM field, so if you will, a single photon at least theoretically is the smallest amplitude of an EM wave you can

Quanta are specifically the quantization of the EM field, so if you will, a single photon at least theoretically is the smallest amplitude of an EM wave you can have. You can see this mostly by noting the expectation of the number operator gives essentially the radiant intensity.