Ideal Negative Measurements in Quantum Walks Disprove Theories Based on Classical Trajectories
Carsten Robens, Wolfgang Alt, Dieter Meschede, Clive Emary, Andrea Alberti · Physical Review X · 2015
We report on a stringent test of the nonclassicality of the motion of a massive quantum particle, which propagates on a discrete lattice.Measuring temporal correlations of the position of single atoms performing a quantum walk, we observe a 6σ violation of the Leggett-Garg inequality.Our results rigorously excludes (i.e., falsifies) any explanation of quantum transport based on classical, well-defined trajectories.We use so-called ideal negative measurements-an essential requisite for any genuine Leggett-Garg test-to acquire information about the atom's position, yet avoiding any direct interaction with it.The interactionfree measurement is based on a novel atom transport system, which allows us to directly probe the absence rather than the presence of atoms at a chosen lattice site.Beyond the fundamental aspect of this test, we demonstrate the application of the Leggett-Garg correlation function as a witness of quantum superposition.Here, we employ the witness to discriminate different types of walks spanning from merely classical to wholly quantum dynamics.