Abstract
Among the known particles, the neutron is special because it provides experimental access to all of the four fundamental forces and a wide range of hypothetical interactions. Despite being unstable, free neutrons live long enough to be used as test particles in interferometric, spectroscopic and scattering experiments probing low-energy scales. Recognized already in the 1970s, fundamental concepts of quantum mechanics can be tested in neutron interferometry using silicon perfect single crystals. Besides enabling tests of uncertainty relations or Bell inequalities, neutrons offer the opportunity to observe the effects of gravity and hypothetical dark forces acting on extended matter wavefunctions. Such tests gained importance in the light of recent discoveries of inconsistencies in the understanding of cosmology and the incompatibility between quantum mechanics and general relativity. Experiments with low-energy neutrons are, thus, indispensable tools for probing fundamental physics and represent a complementary approach to particle colliders. In this Review, we discuss the history and experimental methods used at this low-energy frontier of physics and overview the current bounds and limits on quantum mechanical relations and dark energy interactions.
Key points
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The neutron is an excellent probe of various interactions, because it is sensitive to all four fundamental forces and hypothetical beyond-standard-model interactions.
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Matter-wave interferometry with neutrons offers several advantages, such as macroscopic beam separation, individual control of the sub-beams and long interaction and coherence times at room temperature.
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The neutron as a single-particle quantum system is almost ideal to study fundamental concepts of quantum mechanics, such as entanglement, weak values or uncertainty relations, because it can be prepared, manipulated and detected with high efficiency and accuracy.
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Tensions between observation and the standard model of Big Bang cosmology and the ongoing quest to understand the nature of the dark sector are a strong motivation to search for new physics.
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Neutrons, with their vanishing electric charge and large mass, are ideal test particles, allowing for the most sensitive measurements at sub-micrometre distances for several hypothetical dark sector models.
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Acknowledgements
The authors thank their co-workers and collaborators for their long-term efforts and support, and especially wish to thank the Institut Laue-Langevin (ILL), in Grenoble, France, for ongoing support and hospitality at instruments S18 and PF2. This work was supported by the Austrian Science Fund (FWF) project nos. P 30677, P 27666 and P 33279. Y.H. is partly supported by KAKENHI.
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Sponar, S., Sedmik, R.I.P., Pitschmann, M. et al. Tests of fundamental quantum mechanics and dark interactions with low-energy neutrons. Nat Rev Phys 3, 309–327 (2021). https://doi.org/10.1038/s42254-021-00298-2
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DOI: https://doi.org/10.1038/s42254-021-00298-2
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