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AIP conference proceedings

Coherence and Decoherence in Neutron Optics

Matthias Baron, Helmut Rauch

Published Jan 1, 2011• 3 citations
Publisher

Abstract

Recent neutron interferometer experiments focused on coherence measurements and have shown that coherence features are determined by the beam preparation only. The wave function contains information about the particle and about the beam preparation simultaneously which signalizes the connection between quantum system and apparatus. Further experiments investigated the question of quantum decoherence and how a quantum system can evolve into a classical system. The experiments have been performed with a perfect crystal neutron interferometer at the high flux reactor of the Institut Laue‐Langevin in Grenoble. The distinction between dephasing and decoherence will be discussed.

DOI
10.1063/1.3567432

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Recent neutron interferometer experiments focused on coherence measurements and have shown that coherence features are determined by the beam preparation only. The wave function contains information about the particle and about the beam preparation simultaneously which signalizes the connection between quantum system and apparatus. Further experiments investigated the question of quantum decoherence and how a quantum system can evolve into a classical system. The experiments have been performed with a perfect crystal neutron interferometer at the high flux reactor of the Institut Laue‐Langevin in Grenoble. The distinction between dephasing and decoherence will be discussed.
Recent neutron interferometer experiments focused on coherence measurements and have shown that coherence features are determined by the beam preparation only. The wave function contains information about the particle and about the beam preparation simultaneously which signalizes the connection between quantum system and apparatus. Further experiments investigated the question of quantum decoherence and how a quantum system can evolve into a classical system. The experiments have been performed with a perfect crystal neutron interferometer at the high flux reactor of the Institut Laue‐Langevin in Grenoble. The distinction between dephasing and decoherence will be discussed.
Neutrons are proper tools for testing basic laws of quantum physics since they are massive and can be handled and measured with high efficiency. Suitable postselection experiments demonstrate coherence features of sub-ensembles even when the whole ensemble seems to have lost its coherence. All experiments have the capacity to explain more details by additional pre- and post-selection methods. It will be shown that specific losses are unavoidable in any interaction. Coherence and decoherence are intrinsic quantum effects and can shed light on the measurement problem. Quantum contextuality is a consequence of the entanglement of different degrees of freedom. This makes quantum phenomena more strongly correlated than classical ones. Most experiments have been performed with perfect neutron interferometers and some others by using ultracold neutrons and spin-echo systems. An event by event based interpretation can also be brought into agreement with the experimental results. In many cases parasitic beams carry the same information as the main beam and this relates such measurements to "weak" measurements. The coupling and entanglement of various parameter spaces guide us to a more elaborate discussion of quantum effects.
Neutron matter-wave optics provides the basis for new quantum experiments and a step towards applications of quantum phenomena. Most experiments have been performed with a perfect crystal neutron interferometer where widely separated coherent beams can be manipulated individually. Various geometric phases have been measured and their robustness against fluctuation effects has been proven, which may become a useful property for advanced quantum communication. Quantum contextuality for single particle systems shows that quantum correlations are to some extent more demanding than classical ones. In this case entanglement between external and internal degrees of freedom offers new insights into basic laws of quantum physics. Non-contextuality hidden variable theories can be rejected by arguments based on the Kochen-Specker theorem.
Quantum theory provides us with the best account of microscopic components of matter as well as of radiation. It was introduced in the twentieth century and has experienced a wide range of success. Although the theory’s probabilistic predictions of final experimental outcomes is found to be correct with high precision, there is no general consensus regarding what is actually going on with a quantum system “en route”, or rather the perceivable intermediate behavior of a quantum system, e.g., the particle’s behavior in the double-slit experiment. Neutron interferometry using single silicon perfect crystals is established as a versatile tool to test fundamental phenomena in quantum mechanics, where an incident neutron beam is coherently split in two or three beam paths with macroscopic separation of several centimeters. Here, we present quantum optical experiments with these matter-wave interferometers, studying the effect of the quantum Cheshire Cat in some variants, the neutron’s presence in the paths of the interferometer as well as the direct test of a commutation relation. To reduce disturbances induced by the measurement, the interaction strength is lessened and so-called weak interactions are exploited by employing pre- and post-selection procedures. All results of the experiments confirm the predictions of quantum theory; the observed behaviors of the neutron between the pre- and post-selection in space and time emphasize striking and counter-intuitive aspects of quantum theory.
The authors discuss the coherence properties of neutron scattering from perfect crystals and apply them to the splitting and coherent recombination of a neutron beam in a Laue-type interferometer. They also point out the difference between neutron and x-ray scattering in such devices and discuss the effect of a relative displacement of the various interferometer slabs on the fringe pattern. All this can be done without using the detailed dynamical theory of x-ray scattering. Various experiments are discussed, especially the neutron-gravity experiment of Colella, Overhauser, and Werner, which is analyzed both in the laboratory frame and in a frame freely falling with the neutron beam. The older electron diffraction experiment of Marton et al., and its relevance to the Aharonov---Bohm effect, is also discussed and reinterpreted, because the standard interpretation is not consistent with the results of the neutron experiment. An Appendix presents a general discussion of the transformation to a uniformly accelerated reference frame, as a guide to help analyze future such experiments.
The neutron interferometer set-up S18 at the ILL reactor has been used for the first neutron quantum state reconstruction experiment. By a simultaneous measurement of the coherence function and of the modulated momentum distribution behind the interferometer, the Wigner function of various quantum states can be reconstructed. This is equivalent to knowledge of the particle wavefunction. Its spatial dimension follows from the measurement of the coherence function, which is the autocorrelation function of the wavefunction and the shape in momentum space follows from momentum post-selection experiments. Non-orthogonal traces through the ( x , k )-phase space have to be measured for an unambiguous reconstruction. Nearly classical (coherent) and non-classical states in the form of Schrödinger cat-like states have been identified. Comparisons of calculated and measured Wigner functions will be shown.