In wikipedia, I read about BEC. so I just wonder in that articles, so I write in this page. Adopt that article, " They did this by cooling a dilute vapor consisting of approximately two thousand rubidium-87 atoms to below 170nK using a combination of laser cooling and magnetic evaporative cooling." So I just wonder How they check the "two thousand rubidium-87 atoms"? It's my first time for writing in this forum, so maybe I choose wrong category. but please, if you understand what I mean, answer under this writing.