Hannover 2003 – wissenschaftliches Programm

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Q: Quantenoptik

Q 7: Quanteninformation 2

Q 7.5: Vortrag

Montag, 24. März 2003, 15:00–15:15, F102

Evolution of a qubit under the influence of a succession of weak measurements with unitary feedback — •Thomas Konrad¹, Jürgen Audretsch¹, and Lajos Diósi² — ¹Fachbereich Physik Uni Konstanz — ²Research Institute for Particle and Nuclear Physics, Budapest, Hungary

We investigate the evolution of a single qubit subject to a continuous unitary No-dqfreeNo-dq dynamics and an additional interrupting influence which occurs periodically.One may imagine a dynamically evolving closed quantum system which becomes open at certain times. The interrupting influence is represented by an operation, which is assumed to equivalently describe a non-selective weak measurement. It may be decomposed into the action of a positive operator, which in case of a measurement represents the pure measurement part, followed by a unitary back-action depending on the result of the measurement (feedback). Equations of motion for the state evolution are derived in the form of difference equations. Apart from a modification of the free Hamiltonian and decoherence due to the positive part of the back-action, we find decoherence induced by the stochastic feedbacks and a kind of generalized friction. In order to bridge the gap between sequential and continuous measurements the continuum limit to a master equation is performed. The talk is based on Phys. Rev. A 66 0223010(1-11), e-print: quant-ph/0201078.