Randomized Dynamical Decoupling StrategiesandImproved One-Way Key Rates for QuantumCryptographyVom Fachbereich Physikder Technischen Universitat Darmstadt¨zur Erlangung des Gradeseines Doktors der Naturwissenschaften(Dr. rer. nat.)genehmigte Dissertation vonDipl.-Phys. Oliver Kernaus MainzDarmstadt 2009D17Referent: Prof. Dr. G. AlberKorreferent: Prof. Dr. J. BergesTag der Einreichung: 29.01.09Tag der Prufung: 25.05.09¨Randomized Dynamical Decoupling Strategies andImproved One-Way Key Rates for Quantum CryptographyAbstractThe present thesis deals with various methods of quantum error correction. It is di-vided into two parts. In the first part, dynamical decoupling methods are consideredwhich have the task of suppressing the influence of residual imperfections in a quantummemory. Such imperfections might be given by couplings between the finite dimensionalquantum systems (qudits) constituting the quantum memory, for instance. The sup-pression is achieved by altering the dynamics of an imperfect quantum memory withthe help of a sequence of local unitary operations applied to the qudits. Whereas up tonow the operations of such decoupling sequences have been constructed in a determin-istic fashion, strategies are developed in this thesis which construct the operations byrandom selection from a suitable set. Formulas are derived which estimate the averageperformance of such strategies.