Geometric Phase in Entangled Systems: A Single-Neutron Interferometer Experiment

TL;DR

This experiment demonstrates how geometric phase affects Bell measurements in a neutron interferometer, showing it can be balanced or maximized to observe quantum correlations beyond classical limits.

Contribution

It provides the first experimental observation of geometric phase influence on Bell measurements in a neutron interferometric setup, with novel schemes for controlling Bell correlations.

Findings

Geometric phase causes sinusoidal oscillations in Bell correlation S.

Bell inequality violation can be maximized or balanced by adjusting Bell angles.

The maximal violation S=2√2 is maintained regardless of geometric phase in one scheme.

Abstract

The influence of the geometric phase on a Bell measurement, as proposed by Bertlmann et al. in [Phys. Rev. A 69, 032112 (2004)], and expressed by the Clauser-Horne-Shimony-Holt (CHSH) inequality, has been observed for a spin-path entangled neutron state in an interferometric setup. It is experimentally demonstrated that the effect of geometric phase can be balanced by a change in Bell angles. The geometric phase is acquired during a time dependent interaction with two radio-frequency (rf) fields. Two schemes, polar and azimuthal adjustment of the Bell angles, are realized and analyzed in detail. The former scheme, yields a sinusoidal oscillation of the correlation function S, dependent on the geometric phase, such that it varies in the range between 2 and 2\sqrt{2} and, therefore, always exceeds the boundary value 2 between quantum mechanic and noncontextual theories. The latter scheme results in a constant, maximal violation of the Bell-like-CHSH inequality, where S remains 2\sqrt2 for all settings of the geometric phase.