Scientific Series

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.