Entropy 2010, 12, 1194-1245; doi:10.3390/e12051194 OPEN ACCESS entropy ISSN 1099-4300 www.mdpi.com/journal/entropy Review Quantum Entropy and Its Applications to Quantum Communication and Statistical Physics Masanori Ohya ? and Noboru Watanabe ? Department of Information Sciences, Tokyo University of Science, Noda City, Chiba 278-8510, Japan ? Author to whom correspondence should be addressed; E-Mails:
[email protected] (M.O.);
[email protected] (N.W.). Received: 10 February 2010 / Accepted: 30 April 2010 / Published: 7 May 2010 Abstract: Quantum entropy is a fundamental concept for quantum information recently developed in various directions. We will review the mathematical aspects of quantum entropy (entropies) and discuss some applications to quantum communication, statistical physics. All topics taken here are somehow related to the quantum entropy that the present authors have been studied. Many other fields recently developed in quantum information theory, such as quantum algorithm, quantum teleportation, quantum cryptography, etc., are totally discussed in the book (reference number 60). Keywords: quantum entropy; quantum information 1. Introduction Theoretical foundation supporting today’s information-oriented society is Information Theory founded by Shannon [1] about 60 years ago. Generally, this theory can treat the efficiency of the information transmission by using measures of complexity, that is, the entropy, in the commutative system of signal space. The information theory is based on the entropy theory that is formulated mathematically. Before Shannon’s work, the entropy was first introduced in thermodynamics by Clausius and in statistical mechanics by Boltzmann. These entropies are the criteria to characterize a property of the physical systems.