Timothy C. DuBois, Manolo C. Per, Salvy P. Russo, Jared H. Cole
Decoherence is currently a major limitation for superconducting qubits and Josephson junction based quantum devices in general. An important source of decoherence stems from environmental two-level systems. Recent experiments have even probed these defects directly and shown that they are stable, controllable and have relatively long decoherence times themselves. Little is known about the true microscopic nature of these defects, although many phenomenological theories exist. We take a novel approach to the problem: starting from atom positions and species, motivated by ab initio and molecular mechanics methods. Using this approach we compute experimentally observed parameters such as resonant frequency, defect-qubit coupling and response to strain, and find excellent agreement with experiments. We show that the quantum property of delocalisation of the oxygen atomic position in aluminium oxide naturally results in two-level defects without the need for additional impurities. Such defects are charge neutral but have non-zero response to both applied electric field and strain. This explains the observed long coherence time of TLSs in the presence of charge noise, while still coupling to the junction electric field and substrate phonons.
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http://arxiv.org/abs/1206.2441
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