Matthias Christandl, Institute for Theoretical Physics ETH Zurich
Abstract
In quantum mechanics, the state of a particle is described by a complex vector and, if noise is present, by a matrix. Particular attention in the field of quantum information theory is given to "entangled" quantum states. These are quantum states of two or more particles that give rise to correlations that are stronger than any classical correlation. Entangled states are responsible for the security of quantum cryptography, the speed-up in quantum computation and properties of many physical systems. If an experimenter has determined the quantum state of his system, how can he find out whether or not the state is in fact entangled? Answering this question has kept the field of quantum information theory busy since its beginning. After an introduction to the subject of quantum information theory, I will explain the fastest way of determining when a state is entangled. The result is based on new insights into the behavior of quantum entropy which we have recently obtained using information-theoretic methods.