Let there be light, and then matter

[Umbran]

Not quite. That's why a single photon can't produce pairs on its own. With two photons colliding, it's just the reverse process of an electron annihilating with a positron. The center of momentum moves slower than the speed of light, so momentum can be conserved.

I spoke quickly and sloppily.

The traditional way for experimenters to do pair production is not with a photon-photon interaction. As you mentioned, the cross section (meaning, the probability for any two photons to interact) is just too darn small to get any useful number of pairs produced. So, instead, what you do is create one big fat photon by some other means, and let that decay into a pair. This process (photon -> e+ e-) needs something nearby (like an atom) to be a momentum sink.

The problem is the very tiny cross-section for two photons to interact. In classical electromagnetism, the waves wouldn't interact at all.

Well, that's kind of irrelevant. In *classical* physics, you can't have electron-positron annihilation (or, in fact, *any* annihilation) either - it is entirely a quantum process.

 

[Mustrum_Ridcully]

The 3D printers would seem to be that, in their infancy.

Doesn't count, they don't create matter from energy.