From New York Magazine, scans on Torrilla's weibo:
"Hiddleston tends to wax philosophical about his work and dodge most lines of questioning about himself. (Of his rumored relationship with Elizabeth Olsen, he says only that 'some things have to remain sacred. You could write that I'm single. There's no ring on this finger.')"

Looks like we've finally got this relationship's version of "when there's something to write home about..."