Single spontaneous photon as a coherent beamsplitter for an atomic matterwave

TL;DR

This paper demonstrates that a single spontaneous photon emission near a mirror can create motional coherence in an atom, acting as a beamsplitter for atomic matterwaves, extending fundamental quantum thought experiments.

Contribution

It shows that spontaneous emission can generate motional coherence in free atoms near surfaces, acting as a coherent beamsplitter for atomic matterwaves, which was not previously demonstrated.

Findings

Motional coherence verified by atom interferometry.

Single spontaneous emission can act as a coherent beamsplitter.

Extension of Einstein's recoiling slit experiment to quantum superpositions.

Abstract

In spontaneous emission an atom in an excited state undergoes a transition to the ground state and emits a single photon. Associated with the emission is a change of the atomic momentum due to photon recoil. Photon emission can be modified close to surfaces and in cavities. For an ion, localized in front of a mirror, coherence of the emitted resonance fluorescence has been reported. In free space experiments demonstrated that spontaneous emission destroys motional coherence. Here we report on motional coherence created by a single spontaneous emission event close to a mirror surface. The coherence in the free atomic motion is verified by atom interferometry. The photon can be regarded as a beamsplitter for an atomic matterwave and consequently our experiment extends the original recoiling slit Gedanken experiment by Einstein to the case where the slit is in a robust coherent superposition of the two recoils associated with the two paths of the quanta.