Quantifying the presence of a neutron in the paths of an interferometer

TL;DR

This study experimentally investigates whether individual neutrons in a two-path interferometer can be physically distributed across both paths, challenging the assumption that path information cannot be obtained during quantum interference.

Contribution

It demonstrates that individual neutrons can exhibit a fractional presence in a path, measured via weak values and feedback compensation, providing new insights into quantum particle distribution.

Findings

Neutrons show a fractional presence in one path before interference is registered.

The path presence corresponds to the weak value of the path projector.

The method applies to individual neutrons, not just statistical averages.

Abstract

It is commonly assumed that no accurate experimental information can be obtained on the path taken by a particle when quantum interference between the paths is observed. However, recent progress in the measurement and control of quantum systems may provide the missing information by circumventing the conventional uncertainty limits. Here, we experimentally investigate the possibility that an individual neutron moving through a two-path interferometer may actually be physically distributed between the two paths. For this purpose, it is important to distinguish between the probability of finding the complete particle in one of the paths and the distribution of an individual particle over both paths. We accomplish this distinction by applying a magnetic field in only one of the paths and observing the exact value of its effect on the neutron spin in the two output ports of the interferometer. The results show that individual particles experience a specific fraction of the magnetic field applied in one of the paths, indicating that a fraction or even a multiple of the particle was present in the path before the interference of the two paths was registered. The obtained path presence equals the weak value of the path projector and is not a statistical average but applies to every individual neutron, verified by the recently introduced method of feedback compensation.