Mizuno refers to "we" in the report, so that means he is not working alone on this.
No, it is just him. Although another person in Japan replicated the experiment. He lives a good distance away.
I wrote the paper. I used "we" throughout because that is the correct style for a paper with two or more authors. (Okay, I guess I am more bound to academic traditions than I let on, in my jeremiad against Nature.)
Mizuno asked me to be the co-author. It is a great honor. My main contribution to the actual work has been to kibitz and ask dumb questions. Dumb questions can sometimes be helpful.