What is the state of the art in online voting?
This question is probably too broad to be answerable here, because the answer depends on what kinds of security requirements you have, what the threat model is, and what assumptions we're willing to make -- there are many different versions of those.  In other words, the "secure voting" problem is not one problem, but a broad class of problems.  I suggest doing a literature search, reading some of the classic papers, then see if you have a more specific question.  You could start with papers published at EVT and WOTE use them to seed your literature search.

If you're specifically interested in coercion, I suggest looking at Juels, Catalano, and Jakobson (WPES 2005) and Civitas (IEEE S&amp;P 2008).  Those are two important early papers.  Also, by doing a search on papers that cite them, you'll be able to turn up a lot of more recent work on coercion.

All of these schemes have something in common; they do require some kind of trusted channel to the voter or some situation where the voter does register in person in a supervised environment.  They might not require supervision during the time of voting; that can be pushed to a setup phase.

Also, the solutions may depend on whether you are willing to assume that voters have a personal device (e.g., smartphone) and whether voters are willing to trust it.  Obviously the problem gets easier if you answer that in the affirmative.