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I was just reading this question and I didn't understand the question at first. But I think I understood it after a bit of thought. Here is my question inspired from that question:

Are the processes of pair production and matter-antimatter annihilation instantaneous? If not, what are the intermediate forms of matter-energy involved in the process?

NOTE: I don't know exactly how weird quantum mechanics is. But is the answer that there is no sense of instantaneous at the sub-atomic level?

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Although the question is about QED, I don't think the answer is really very different from the answer we would have for any quantum-mechanical process involving a decay or transition. You have a state vector that starts out as $|1\rangle$ and then then evolves into $c_1|1\rangle+c_2|2\rangle$. As time goes on, $|c_2|$ increases. The evolution is smooth and deterministic, and is described by the Schrödinger equation. There are no sudden jumps.

Although we draw Feynman diagrams as if the vertices happened at a definite point in spacetime and were instantaneous, Feynman diagrams shouldn't really be interpreted that literally.

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The atomic and subatomic level is quantum mechanical and the rule of thumb is the Heisenberg uncertainty principle which limits the measured values of the size of pairs of variables :

hup

So in any subatomic interaction from pair creation to decays, the better the energy is measured the larger the uncertainty in measuring the time.

This is not a statement about the inaccuracy of measurement instruments, nor a reflection on the quality of experimental methods; it arises from the wave properties inherent in the quantum mechanical description of nature. Even with perfect instruments and technique, the uncertainty is inherent in the nature of things.

So no interaction or decay or pair creation is instantaneous, it is limited by the HUP.

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