Universität Stuttgart

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Author: search.filters.author.Brandner, KayAuthor: search.filters.author.Seifert, Udo

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    Thermodynamics of micro- and nano-systems driven by periodic temperature variations
    (2015) Brandner, Kay; Saito, Keiji; Seifert, Udo
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    Coherence-enhanced efficiency of feedback-driven quantum engines
    (2015) Brandner, Kay; Bauer, Michael; Schmid, Michael T.; Seifert, Udo
    A genuine feature of projective quantum measurements is that they inevitably alter the mean energy of the observed system if the measured quantity does not commute with the Hamiltonian. Compared to the classical case, Jacobs proved that this additional energetic cost leads to a stronger bound on the work extractable after a single measurement from a system initially in thermal equilibrium (2009 Phys. Rev. A 80 012322). Here, we extend this bound to a large class of feedback-driven quantum engines operating periodically and in finite time. The bound thus implies a natural definition for the efficiency of information to work conversion in such devices. For a simple model consisting of a laser-driven two-level system, we maximize the efficiency with respect to the observable whose measurement is used to control the feedback operations. We find that the optimal observable typically does not commute with the Hamiltonian and hence would not be available in a classical two level system. This result reveals that periodic feedback engines operating in the quantum realm can exploit quantum coherences to enhance efficiency.
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    Multi-terminal thermoelectric transport in a magnetic field : bounds on Onsager coefficients and efficiency
    (2013) Brandner, Kay; Seifert, Udo
    Thermoelectric transport involving an arbitrary number of terminals is discussed in the presence of a magnetic field breaking time-reversal symmetry within the linear response regime using the Landauer-Büttiker formalism. We derive a universal bound on the Onsager coefficients that depends only on the number of terminals. This bound implies bounds on the efficiency and on efficiency at maximum power for heat engines and refrigerators. For isothermal engines pumping particles and for absorption refrigerators these bounds become independent even of the number of terminals. On a technical level, these results follow from an original algebraic analysis of the asymmetry index of doubly substochastic matrices and their Schur complements.