Why and how can we control decoherence – Niels Bohr Institute - University of Copenhagen

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Why and how can we control decoherence

A unified approach to quantum decoherence and its dynamic control will be presented. Applications of this approach to the control of quantum noise , relaxation and quantum thermodynamic processes will be discussed . In this lecture, we present the framework for universal dynamical control of two-level systems (TLS) or qubits experiencing amplitude or phase noise (AN or PN) due to coupling to a thermal bath.  Completely analogous formulae are obtained for the dynamical control of the AN and PN decoherence rates, thus underscoring the unified nature of this universal formalism [1,2]. We next apply this universal control to create disturbances of thermal equilibrium between TLSs and a bath, by frequent, brief quantum non-demolition measurements of the TLS energy states. By making the measurements increasingly frequent, we encounter first the anti-Zeno regime and then the Zeno regime (namely where the TLSs' relaxation respectively speeds up and slows down). The corresponding entropy and temperature of both the system and the bath are then found to either decrease or increase depending only on the rate of observation, contrary to the standard thermodynamical rules that hold for memory-less (Markov) baths. From a practical viewpoint, these anomalies may offer the possibility of very fast control of heat and entropy in quantum systems, allowing cooling and state purification over an interval much shorter than the time needed for thermal equilibration or for a feedback control loop [3].   [1] G. Gordon, N. Erez and G. Kurizki, J. Phys. B-AMO 40, S75(2007).[2] G. Gordon and G. Kurizki, Phys Rev Lett 97, 110503(2006).[3] Noam Erez, Goren Gordon, Mathias Nest and Gershon Kurizki, Nature 452, 724-727(2008).