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Quantum Point Contacts: a Tunable Single-Spin System with Electrical Readout


I discuss the results of recent experiments in which we have demonstrated that the semiconductor nanoconstrictions known as quantum point contacts (QPCs) may be used as a convenient, electrically-addressable, single-spin system, with the potential of application to future spintronic devices. The QPCs are realized by applying a negative voltage to metal Schottky gates on the top surface of a GaAs/AlGaAs heterojunction. The negative bias depletes electrons underneath the gates, forming a short nanoconstriction whose width and electron density may then be tuned by further variation of the gate bias. Close to pinch-off, where the electron density in the constriction becomes vanishingly small, the self-consistent interaction among electrons has been argued to lead to the formation of a localized state in the QPC that binds a single spin. Recently, we have shown that a (“detector”) QPC exhibits a resonance when it is coupled to another (“swept”) QPC that is pinching-off. We have also developed a theoretical model, which relates this resonance to the formation of a bound spin in the swept QPC. In my presentation, I will discuss how such experiments can be used to obtain detailed information on the microscopic structure of the localized spins in QPCs. Our analysis reveals the spins to be strongly bound, and to be associated with quantized states whose spin degeneracy is broken even at zero magnetic field. Furthermore, I present evidence that spins localized on separate QPCs can be coherently coupled to each other. These latter results suggest a possible route for quantum computing in which the qubit is represented by the localized spin on a QPC, whose state can then be read out directly in an electrical measurement

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