Browsing by Author "Burgarth, Daniel"

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    Non-Markovian noise that cannot be dynamically decoupled by periodic spin echo pulses
    Burgarth, Daniel; Facchi, Paolo; Fraas, Martin; Hillier, Robin (2021-08)
    Dynamical decoupling is the leading technique to remove unwanted interactions in a vast range of quantum systems through fast rotations. But what determines the timescale of such rotations in order to achieve good decoupling? By providing an explicit counterexample of a qubit coupled to a charged particle and magnetic monopole, we show that such time-scales cannot be decided by the decay profile induced by the noise: even though the system shows a quadratic decay (a Zeno region revealing non-Markovian noise), it cannot be decoupled by periodic spin echo pulses, no matter how fast the rotations.
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    Quantum distance to uncontrollability and quantum speed limits
    Burgarth, Daniel; Borggaard, Jeffrey T.; Zimboras, Zoltan (American Physical Society, 2022-04-04)
    Distance to uncontrollability is a crucial concept in classical control theory. Here, we introduce quantum distance to uncontrollability as a measure of how close a universal quantum system is to a nonuniversal one. This allows us to provide a quantitative version of the quantum speed limit, decomposing the bound into geometric and dynamical components. We consider several physical examples including globally controlled solid state qubits, scrambling of quantum information, and a cross-Kerr system, showing that the quantum distance to uncontrollability provides a precise meaning to spectral crowding, weak interactions, and other bottlenecks to universality. We suggest that this measure should be taken into consideration in the design of quantum technology.