Readout of electron spins in quantum point contacts
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This thesis is motivated by recent interest in the binding of single spins on quantum point contacts (QPCs). While this regime is inaccessible to experiments performed on single QPCs, this thesis provides strong evidence for the spin binding by studying the interaction between coupled QPCs, one of which is biased in the regime where the spin binding is expected to occur. Thus, this thesis describes a series of experiments aimed at understanding and manipulating single electron spins confined in QPCs, with the long term goal of the implementation of a QPC-based quantum computer. In these experimental studies, we demonstrate a resonant interaction between closely coupled QPCs due to bound-state formation, and show that this may give rise to robust confinement of single spins, which show clear Zeeman splitting in a magnetic field. Our experimental investigations of the resonance exhibited by coupled QPCs clearly show that this feature is consistent with the Fano form, providing strong independent support for the idea of bound-state formation in QPCs near pinch-off. Although the Fano mechanism is typically the dominant effect responsible for the coupled-QPC interaction, we also demonstrate evidence of a crossover from Fano to Coulomb coupling between the QPCs as their wavefunction overlap, and so the Fano mechanism, is suppressed. We show also that the coupling strength characterizing the Fano effect grows as the two QPCs are brought closer to one another. Finally, we discuss the observation of a novel nonlinear Fano effect, by using source-drain bias spectroscopy of coupled QPCs. The key result here is that the coupling strength between the resonant and non-resonant paths can be tuned in situ, starting from weak coupling in the linear (small source-drain bias) regime but becoming stronger at large bias.