Observation of single-photon azimuthal backflow with weak measurement

TL;DR

This paper reports the first experimental observation of quantum azimuthal backflow in single photons using weak measurement, revealing counterintuitive momentum behavior with high spatial resolution.

Contribution

It introduces a novel weak measurement technique to observe quantum backflow in single photons, surpassing classical optical measurement limitations.

Findings

Single photons with negative orbital angular momentum exhibit positive OAM due to backflow.

Backflow effects depend on mode ratio, propagation distance, and OAM index.

The method enables accurate momentum measurement without slits or lenslet arrays.

Abstract

Quantum backflow, a counterintuitive interference phenomenon where particles with positive momentum can propagate backward, is important in applications involving light-matter interactions. To date, experimental demonstrations of backflow have been restricted to classical optical systems, where momentum is measured using the slit scanning technique or the Shack-Hartmann wavefront sensor technique. However, these techniques have low spatial resolution due to limitations in slit width and Fourier transform lenslet array density. Here, by adopting the technique of weak measurement, we report an observation of azimuthal backflow both theoretically and experimentally. Our results show that a heralded single photon, prepared in specific superposition states with solely negative orbital angular momentum (OAM), exhibits positive OAM. The effects of mode ratio, propagation distance and OAM index on the azimuthal backflow are systematically investigated. Our method avoids using slits and lenslet arrays, allowing for the accurate extraction of photon momentum at each pixel. This work provides new insights and techniques for observing and manipulating backflow in quantum systems.