Quantum entropy and ...?? 1. The problem statement, all variables and given/known data My problem is that I have to write about ten pages about SUPERDENSE CODING and QUANTUM CRYPTOGRAPHY, and my professor has taken for granted that these are strongly linked to quantum entropy. He never told us why! Indeed he talked about that as applications of the property quantum entropy. However, on the two books I am using (Nielsen&Chuang and Preskill's Lectures at CalTech), these two subjects are treated with no link to entropy. Could someone please tell me where is the link? And when can I found material to get a reasonable understanding of that? Thank you very much! 2. Relevant equations 3. The attempt at a solution I have studied the properties of entropy and what there is on my books on superdense coding and quantum cryptography, but I can't find any link.