Quantum correlations in a noisy neutron interferometer

TL;DR

This study explores how quantum correlations like discord persist in a noisy neutron interferometer, revealing that non-classical correlations remain even when entanglement is lost due to phase noise, enabling quantum eraser and which-way measurements.

Contribution

It demonstrates that quantum discord persists under noise in neutron interferometry, and shows how spin measurements can implement quantum eraser and which-way experiments despite phase noise.

Findings

Quantum discord remains non-zero despite entanglement loss.

Spin measurements can induce quantum eraser effects.

Robustness of spin-filtered which-way measurement to phase noise.

Abstract

We investigate quantum coherences in the presence of noise by entangling the spin and path degrees of freedom of the output neutron beam from a noisy three-blade perfect crystal neutron interferometer. We find that in the presence of dephasing noise on the path degree of freedom the entanglement of the output state reduces to zero, however the quantum discord remains non-zero for all noise values. Hence even in the presence of strong phase noise non-classical correlations persist between the spin and path of the neutron beam. This indicates that measurements performed on the spin of the neutron beam will induce a disturbance on the path state. We calculate the effect of the spin measurement by observing the changes in the observed contrast of the interferometer for an output beam post-selected on a given spin state. In doing so we demonstrate that these measurements allow us to implement a quantum eraser, and a which-way measurement of the path taken by the neutron through the interferometer. While strong phase noise removes the quantum eraser, the spin-filtered which-way measurement is robust to phase noise. We experimentally demonstrate this disturbance by comparing the contrasts of the output beam with and without spin measurements of three neutron interferometers with varying noise strengths. This demonstrates that even in the presence of noise that suppresses path coherence and spin-path entanglement, a neutron interferometer still exhibits uniquely quantum behaviour.