I suppose if you want to have a trace, you do need a thing to trace that is also unique.

For your story, it's entirely possible that it's just in the infant stages and people aren't really sure *what* the soul is, but they can measure it somehow. E.g. like 21 grams.. I think the story there was that the body weighs 21 grams less when someone dies. Of course, this doesn't make it unique or traceable. So the science bit needs to be some kind of spectrum that is complex like DNA so it can be unique. Probably you want markers that show how often someone has reincarnated, stuff like that. Perhaps the soul will be marked by negative deeds and positive deeds, which shows that karma exists. Or you could opt that karma doesn't exist, which makes the universe more random and cruel. You also need to make a choice if the trace is active or a snapshot. Active means that e.g. you could trace when someone is being reborn after you've taken the trace once. This can be nifty, but it's also more vague... how can the trace be updated like that? Is it part of the soul still? There is science that shows something about two parts which are brought very far apart, will still respond instantaneously. On the other hand, the snap shot is closer to a blood sample/DNA kind of thing.

Anyway, I should probably stop now.