Quantum correlations in a noisy neutron interferometer

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 0, however the quantum discord remains nonzero for all noise values. Hence even in the presence of strong phase noise nonclassical correlations persist between the spin and the 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 postselected 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 behavior.

Spin-orbit states of neutron wave packets

“…Neutron interferometers have been used to demonstrate single-particle entanglement between different degrees of freedom, such as spin + path and spin + energy, and have supported extensive studies of quantum contextuality [23][24][25][26][27]. A useful measure of entanglement for a bipartite quantum system is the concurrence [28][29][30], which is equal to 1 when the entanglement is maximum and 0 when the state is separable.…”

A new polarized neutron interferometry facility at the NCNR

“…9) surround the neutron guides for radiation safety and to minimize backgrounds. The interferometer is supported by a rotation 8 , translation 9 and height Systematic uncertainties of calibrated fission chamber owing to the deposit particulars are estimated to be ≈ 10 %. Fluence rates for both NIOFa and NIOF included neutrons with wavelengths λ/n where n = 2, 3, .…”

“…neutron phase effects (see Table 1). fisherThese include demonstration of the 4π periodicity of neutron wave function [3], precision coherent and incoherent scattering length measurements [4][5][6][7], measuring vertical coherence [8], demonstrating contrast in the presence of zero entanglement but non-zero Quantum Discord [9], and more applied experiments including phase contrast imaging [10,11] and measuring the density of thin films [12]. More recently, proposed experiments have needed both high neutron polarization and precise spin manipulation.…”

Neutron Interferometry at the National Institute of Standards and Technology

“…To investigate this effect we considered a configuration where we entangled the spin and path degrees of freedom of the neutron by placing a spin-rotator in one path of the interferometer and then observed the output path contrast and spin contrast with and without postselected spin measurements [24]. In the absence of spin measurements, our theoretical calculations found that strong phase noise caused the entanglement to reduce to zero; however the quantum discord remained nonzero.…”