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Physical Review A

Neutron-interferometer absorption experiments in the quantum limit

H. Rauch, Johann Summhammer

Published Dec 1, 1992• 28 citations
Publisher

Abstract

Absorbers and other beam attenuators inside a neutron interferometer have different influences on the interference pattern, ranging from a stochastic to a deterministic situation. We show that in the quantum limit a smooth transition between both cases is possible. The quantum limit can be approached when using thin absorption gratings, very fast choppers, or rapidly changing fields. The nonremoval probability of neutrons from the original phase space is identified as a crucial quantity describing these phenomena. Experimental results obtained with thin absorption gratings are in agreement with these formulations.

DOI
10.1103/physreva.46.7284

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AIP conference proceedings•Jan 1, 2005•1 citation
Absorbers and other beam attenuators inside a neutron interferometer have different influences on the interference pattern, ranging from a stochastic to a deterministic situation. We show that in the quantum limit a smooth transition between both cases is possible. The quantum limit can be approached when using thin absorption gratings, very fast choppers, or rapidly changing fields. The nonremoval probability of neutrons from the original phase space is identified as a crucial quantity describing these phenomena. Experimental results obtained with thin absorption gratings are in agreement with these formulations.
Experiments with absorbing foils, beam choppers, or an absorbing lattice in one path of a neutron interferometer to expose the difference between stochastic and deterministic absorption in quantum mechanics are reported. The different amplitudes of the interference patterns in stochastic and deterministic absorption when the absorption probability is the same were observed in agreement with prediction. Also the possibility of a gradual transition from deterministic to stochastic absorption was experimentally investigated.
Here we discuss the limits of the recognizability of neutron interference patterns either observed in low-contrast measurements or when collecting very few neutrons only. Low contrast can be caused by a strong beam attenuation or by a large phase shift applied to one beam path. Stochastic and deterministic cases have different influences on the interference pattern, which can be interpreted as a different wavelike or particlelike behavior of the system. Measurements of interference patterns with very few neutrons only are related to the phase--particle-number uncertainty relation, which is discussed on the basis of stochastic and quantum theory arguments. Analogies between coherent-state behavior known in atomic physics and the behavior of neutrons in an interferometer are discussed also.
Abstract This chapter describes a rather broad selection of neutron interferometry experiments whose theoretical description requires an understanding of the mutual coherence function, and the sensitive experimental techniques utilized. Within the interferometer the coherent beams can be influenced by interactions with fields and material media in space and time causing phase shifts and wave packet delays that substantially affect the outgoing beams. Post-selection experiments involving spectral filtering and time-of-flight analysis are described. The phase-echo effect is described; it is a generalization of the spin-echo effect. Placing statistical or deterministic absorbers within the interferometer affects the outgoing coherence properties of beams quite differently. Low counting rate interference experiments show time pair-correlation effects characteristic of Poissonian statistical beams. Decoherence effects are investigated to study the transition from a quantum to a classical regime.
Neutrons are proper tools for testing quantum mechanics because they are massive, they couple to electromagnetic fields due to their magnetic moment and they are subject to all basic interactions and they are sensitive to topological effects, as well. Related experiments will be discussed. Deterministic and stochastic partial absorption experiments can be described by Bell-type inequalities. Recent neutron interferometry experiments based on postselection methods renewed the discussion about quantum nonlocality and the quantum measuring process. It has been shown that interference phenomena can be revived even when the overall interference pattem has lost its contrast. This indicates a persisting coupling in phase space even in cases of spatially separated Schrödingercat-like situations. These states are extremely fragile and sensitive against any kind of fluctuations and other decoherence processes. More complete quantum experiments also show that a complete retrieval of quantum states behind an interaction volume becomes impossible in principle. First results of neutron quantum state reconstruction experiments will be presented.
Entanglement of two photons, or atoms is a complementary situation to a double slit situation of a single photon, neutron or atom. With neutrons single particle interference phenomena can be observed and the “entanglement of degrees of freedom”, i.e. contextuality can be verified. In this respect, neutrons are proper tools for testing quantum mechanics because they are massive, they couple to electromagnetic fields due to their magnetic moment and they are subject to all basic interactions, and they are sensitive to topological effects, as well. Related experiments will be discussed. Deterministic and stochastic partial absorption experiments can be described by Bell‐type inequalities. Recent neutron interferometry experiments based on postselection methods renewed the discussion about quantum nonlocality and the quantum measuring process. It has been shown that interference phenomena can be revived even when the overall interference pattern has lost its contrast. This indicates a persisting coupling in phase space even in cases of spatially separated Schrödinger cat‐like situations. These states are extremely fragile and sensitive against any kind of fluctuations and other decoherence processes. More complete quantum experiments also show that a complete retrieval of quantum states behind an interaction volume becomes impossible in principle.