Atomic "bomb testing": the Elitzur-Vaidman experiment violates the Leggett-Garg inequality

TL;DR

This paper demonstrates that the Elitzur-Vaidman bomb testing scheme, implemented with a single atom, violates the Leggett-Garg inequality, providing experimental evidence against macro-realistic theories using a novel spin-position entanglement measurement.

Contribution

The authors experimentally realize a quantum bomb test with a single atom, introducing a new measurement method that entangles spin and position to test macro-realism.

Findings

Violation of Leggett-Garg inequality observed

Novel spin-position entanglement measurement developed

Decoherence induces a quantum-to-classical transition

Abstract

Elitzur and Vaidman have proposed a measurement scheme that, based on the quantum superposition principle, allows one to detect the presence of an object --- in a dramatic scenario, a bomb --- without interacting with it. It was pointed out by Ghirardi that this interaction-free measurement scheme can be put in direct relation with falsification tests of the macro-realistic worldview. Here we have implemented the "bomb test" with a single atom trapped in a spin-dependent optical lattice to show explicitly a violation of the Leggett-Garg inequality --- a quantitative criterion fulfilled by macro-realistic physical theories. To perform interaction-free measurements, we have implemented a novel measurement method that correlates spin and position of the atom. This method, which quantum mechanically entangles spin and position, finds general application for spin measurements, thereby avoiding the shortcomings inherent in the widely used push-out technique. Allowing decoherence to dominate the evolution of our system causes a transition from quantum to classical behavior in fulfillment of the Leggett-Garg inequality.