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Quantum walks with neutral atoms: from quantum transport phenomena to the falsification of classical trajectory theories

Andrea Alberti, Institut für Angewandte Physik

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Quantum walks with neutral atoms: from quantum transport phenomena to the falsification of classical trajectory theories Andrea Alberti and Dieter Meschede Institut für Angewandte Physik der Universität Bonn, Wegelerstraße 8, 53115 Bonn, Germany The quantum walk is a prime example of quantum transport: a particle undergoing discrete shifts in space and time is delocalized over a very large Hilbert space. In our system we implement this type of transport using cold atoms moving in a deep optical lattice. By creating artificial electric fields, we observe textbook transport phenomena like spin orbit coupling, Bloch oscillations, or Landau-Zener tunneling in a single experiment. We unravel the unique character of electric quantum walks by studying very different transport regimes, which depend on the commensurability of the electric field [1]. We experimentally observe ballistic delocalization for rational fields and dynamical localization for irrational ones [2]. Physical insight into the "quantumness" of the walk is obtained by an analysis of decoherence phenomena [3] and application of ideal negative measurements, i.e. interaction-free measurements. Ideal negative measurements are the essential requisite to obtain a 6-sigma experimental violation of the Leggett-Garg inequality, which falsifies theories of motion based on classical trajectories [4]. The controlled interaction of exactly two quantum walkers remains a daunting but highly attractive experimental challenge. References: [1]: C. Cedzich, T. Rybá r, A. H. Werner, A. Alberti, M. Genske and R. F. Werner, Propagation of quantum walks in electric fields, Phys. Rev. Lett. 111, 160601 (2013) [2]: M. Genske, W. Alt, A. Steffen, A. H. Werner, R. F. Werner, D. Meschede, A. Alberti, Electric quantum walks with individual atoms, Phys. Rev. Lett. 110, 190601 (2013) [3]: A. Alberti, W. Alt, R. Werner and D. Meschede, Decoherence Models for Discrete-Time Quantum Walks and their Application to Neutral Atom Experiments, arXiv:1409.6145 [quant-ph] (2014), accepted in New J. Phys. [4]: C. Robens, W. Alt, D. Meschede, C. Emary, and A. Alberti, Quantum diffusion falsifies the concept of classical trajectories by violation of Leggett-Garg inequality, arXiv 1404:3912 (2014).