Scientific Series

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.

Spin and hybrid spin-charge qubits have been a long-term goal of the fields of Spintronics and Quantum Information. Such a qubit in an array would ideally interact separately from other qubits and “tunably” with a Microwave field. In this way, a single qubit may be turned “on”, interact with its environment to acquire information (for example, flip a spin state), and be turned “off” so it may indefinitely store that information. The necessary tunability for such a task has been demonstrated in a single hole of a p-type GaAs/AlGaAs Double Quantum Dot (DQD) with strong Spin-Orbit Interaction (SOI), in which the interdot tunnel coupling is strong so that hybridized quantum molecular states form [1]. If the interdot coupling is reduced, this form of tunability is lost. However, a new phenomenon may permit tunability: the weak interdot coupling gives rise to distinct dot-specific quantum dot g-factors smoothly connected by the spin-orbit interaction. By sweeping gate voltages such that we choose which dot is being probed, a qubit may be turned “on” or “off” as necessary. During my work terms with the low-temperature Quantum Physics Group at the National Research Council of Canada, I investigated the appearance of dot-specific g-factors and partially characterized the double quantum dot system using a theoretical model.