SIGNAL (programming language)

Quantum mechanics

Computational physics

Photon counting

Operating system

Computer science

Classical mechanics

Rotation (mathematics)

Free space

Photon

Physics

Orbital angular momentum of light

Total angular momentum quantum number

Mathematics

Optics

Geography

Angular momentum

Environmental science

Angular velocity

Artificial intelligence

Geometry

Remote sensing

Rotational symmetry

Programming language

Symmetry (geometry)

Mechanics

Space (punctuation)

Rotational speed

Computer vision

Doppler effect

Physical Review Applied

In principle the rotational Doppler effect could be used to detect the turning of a distant object, but the approach has not progressed beyond the laboratory, due to low photon-collection efficiency and orbital angular momentum (OAM) mode crosstalk. The authors address these challenges and successfully observe the effect over a 120-m free-space link between two rooftops. By analyzing the photon counts for the relevant OAM components, they deduce the speed and symmetry of rotating bodies, even at the photon-counting level. This work is a significant step toward remote sensing in a realistic environment based on the rotational Doppler effect of ``twisted light''.

Free-Space Remote Sensing of Rotation at the Photon-Counting Level