Quantum mechanics redefines information and its fundamental properties. Researchers at Perimeter Institute work to understand the properties of quantum information and study which information processing tasks are feasible, and which are infeasible or impossible. This includes research in quantum cryptography, which studies the trade-off between information extraction and disturbance, and its applications. It also includes research in quantum error correction, which involves the study of methods for protecting information against decoherence. Another important side of the field is studying the application of quantum information techniques and insights to other areas of physics, including quantum foundations and condensed matter.
Quantum mechanics, information theory, and relativity theory are the basic foundations of theoretical physics. The acquisition of information from a quantum system is the interface of classical and quantum physics. Special relativity imposes severe restrictions on the transfer of information between distant systems. Various applications will be presented.
Superselection rules are limitations on the physically realizable quantum operations that can be carried out by a local agent. For example, it is impossible to create or destroy an isolated particle that carries locally conserved charges, such as an electrically charged particle, a fermion, or (in a two dimensional medium) an anyon. Recently, Popescu has suggested that superselection rules might have interesting implications for the security of quantum cryptographic protocols. The intuitive idea behind this suggestion is that superselection rules could place inescapable limits on the cheating strategies available to the dishonest parties, thus enhancing security. Might, say, unconditionally secure bit commitment be possible in worlds (perhaps including the physical world that we inhabit) governed by suitable superselection rules? An affirmative answer could shake the foundations of cryptography. The purpose of this paper is to answer Popescu's intriguing question. Sadly, our conclusion is that superselection rules can never foil a cheater who has unlimited quantum computational power.