​The Sagnac effect is seen in an interferometer, when two beams of light are sent around a closed path in opposite directions. If the path is rotated, the beams become out of phase when they reunite back at the start. If the interferometer is large enough, the rotation of the Earth can be enough to shift the phases of the two beams.

The effect has also been seen for particles, including electrons and neutrons. The phase shift for neutrons is proportional to the frequency of the rotation and the orbital angular momentum (OAM) of the neutron.

Detection of neutron OAM remains difficult and is the subject of debate and discussion in the scientific literature.

In this paper, published in Physical Review Research, the researchers used this phase shift to work backwards and measure the OAM of the neutron using the Sagnac effect, and the rotation of the Earth. On the Larmor instrument at ISIS, they used a spin echo interferometer to produce entangled neutrons, with any path dependent phase shift detected in changes to the neutron spins.

They were able to measure the OAM to within 5% of its theoretical value. This demonstrates the feasibility of using the Sagnac effect to definitively measure neutron OAM and paves the way towards a future observation of quantum OAM states.

The full paper can be found at DOI: 10.1103/PhysRevResearch.7.013046