The axion coupling to photon is highly relevant experimentally. This coupling is topological in nature, quantized, and can transmit information from deep in the UV to observations, unpolluted by intervening physics. I will present simple arguments showing the quantization of the axion-photon coupling. I will show that this has far-reaching consequences, e.g. ongoing axion experiments can be thought of as probes of grand unification. I will present the general form of the quantized coupling including effects of mixing, using the well-motivated QCD axion as an example.