Neutron interferometer

A neutron interferometer coherently splits and recombines a neutron matter wave, converting a difference of quantum phase between paths into an output intensity change. A sample in one path can therefore be studied through its phase shift, even when its effect on transmitted intensity is small.

A perfect-crystal implementation uses several diffracting silicon blades machined from one crystal with a common base. Bragg diffraction splits the incident wave into paths, redirects them and recombines them in a geometry analogous to a Mach-Zehnder interferometer. The common base helps preserve relative alignment; thermal and vibration isolation protect phase stability. The institutional apparatus description includes the blade geometry and explains the importance of environmental isolation.^[ref]

Recombination measures a relative phase rather than an individual particle trajectory. Electric, magnetic, gravitational or material interactions can all contribute, so a quantitative result requires controls separating the desired phase from drifts and other path differences. The Colella–Overhauser–Werner experiment is a gravitational example.

The Colella–Overhauser–Werner experiment observed a gravitationally induced relative quantum phase in a neutron interferometer. The 1975 primary report identifies the measured signal as a phase shift caused by the interaction of neutron matter waves with Earth's gravitational field.^[ref]

An idealized two-path model explains the sensitivity. If equal-speed horizontal segments of length $L$ are separated vertically by $h$, their gravitational potential energy difference is $mgh$ and their common traversal time is approximately $L/v$. The corresponding phase-difference magnitude isThe sign depends on path orientation and phase convention. Recombination converts this difference into matter-wave interference, making a tiny potential-energy difference measurable through phase accumulation. The rectangular equal-speed expression is a leading explanatory model, not a complete precision correction for real crystal diffraction, bending and rotation effects.