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Quantum walkers under a microscope
Generations of physics students have been taught to think of one-dimensional random walks in terms of a drunken sailor taking random steps to the right or to the left. But that doesn't compare with the complexity of a quantum walker, who can propagate down multiple paths at the same time. Preiss et al. detected particles in single sites of an optical lattice to study the dynamics of two interacting atoms of 87Rb performing a quantum walk (see the Perspective by Widera). Depending on the initial conditions and the interaction strength between the atoms, the atoms either ignored each other, stuck to each other, or tried to get as far away from each other as possible.
Full control over the dynamics of interacting, indistinguishable quantum particles is an important prerequisite for the experimental study of strongly correlated quantum matter and the implementation of high-fidelity quantum information processing. We demonstrate such control over the quantum walk—the quantum mechanical analog of the classical random walk—in the regime where dynamics are dominated by interparticle interactions. Using interacting bosonic atoms in an optical lattice, we directly observed fundamental effects such as the emergence of correlations in two-particle quantum walks, as well as strongly correlated Bloch oscillations in tilted optical lattices. Our approach can be scaled to larger systems, greatly extending the class of problems accessible via quantum walks.