Editor's note (6/1/2011): We are making the text of this July 1985 article freely available for 30 days to coincide with the publication of a paper on entropy and quantum systems by Vlatko Vedral. He authored our June 2011 cover story and blogs about his latest work, which discusses the research featured in this 1985 article.The Vedral paper was just submitted to HN here: http://news.ycombinator.com/item?id=2610734