Neutron spin echo is a "quantum tale of two paths''

TL;DR

This paper provides experimental and theoretical evidence supporting a quantum interferometric model for neutron spin echo, challenging the traditional semi-classical approach by showing that neutron spin states propagate along separate paths coherently.

Contribution

The study demonstrates that a quantum two-path interferometric model accurately describes neutron spin echo signals, contradicting the semi-classical single-path Larmor precession model.

Findings

No damping observed in SESANS signal with a periodic phase grating.

Quantum model predicts signal independence from wave packet width.

Experimental results support the two-path quantum interpretation.

Abstract

We describe an experiment that strongly supports a two-path interferometric model in which the spin-up and spin-down components of each neutron propagate coherently along spatially separated parallel paths in a typical neutron spin echo small angle scattering (SESANS) experiment. Specifically, we show that the usual semi-classical, single-path treatment of Larmor precession of a polarized neutron in an external magnetic field predicts a damping as a function of the spin echo length of the SESANS signal obtained with a periodic phase grating when the transverse width of the neutron wave packet is finite. However, no such damping is observed experimentally, implying either that the Larmor model is incorrect or that the transverse extent of the wave packet is very large. In contrast, we demonstrate theoretically that a quantum-mechanical interferometric model in which the two mode-entangled (i.e. intraparticle entangled) spin states of a single neutron are separated in space when they interact with the grating accurately predicts the measured SESANS signal, which is independent of the wave packet width.