Ideal Negative Measurements in Quantum Walks Disprove Theories Based on Classical Trajectories

• Main Authors: Carsten Robens, Wolfgang Alt, Dieter Meschede, Clive Emary, Andrea Alberti
• Format: Article
• Language: English
• Published: American Physical Society 2015-01-01
• Series: Physical Review X
• Online Access: http://doi.org/10.1103/PhysRevX.5.011003

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 interaction-free 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.