On the first experiment, you can create a random id and ask the respondent to memorize it for their future survey access. In the same first survey, create a contact list action, in which you will put this random id in the external data reference of the exprtiment.
In the follow up surveys, ask the respondent their assigned random id as the first question and using authenticator, authenticate this with the same contact list external data reference used in the first survey. If auhenticated, allow them to continue with the survey else terminate or take required actions.

I've written a help sheet for this. Go to https://researchguides.library.syr.edu/dataservices/Qualtrics
and look for the Conducting Repeated Measures Surveys document.

Paul

Thanks so much, Paul. This is very helpful.