In introductions to string theory it is common to start with the quantum theory of a single worldline by quantization of the Polyakov action for some $0$-brane.

What does this theory correspond to, exactly? I know it's not the theory of a single point particle, since those are not well defined for a variety of reasons, and from some arguments of ACuriousMind$^{[1]}$ it may not have a position operator defined. I also know that the full theory of worldlines can be considered as equivalent to QFT, with every worldline diagram corresponding to a Feynman diagram.

Would the theory of a single worldline correspond to the theory of a single QFT particle state because of this?

$ [1]$ private correspondance

This post imported from StackExchange Physics at 2017-10-11 16:29 (UTC), posted by SE-user Slereah