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Experimental Evidence for Quantum Phase Dynamics: Non-Exponential Decoherence in Neutron Interferometry

Ayoub Taoussi

Published Jan 10, 2026• 0 citations
Publisher

Abstract

This study presents a reanalysis of high-precision neutron interferometry data from the Institut Laue-Langevin (ILL), revealing a significant deviation from the traditionally expected exponential decoherence. The interference visibility exhibits a stretched-exponential decay, characterized by $V(t) = V_0 \exp[-(t/\tau_{\mathrm{coh}})^{1+\varepsilon}]$ with a measured $\varepsilon = 0.18 \pm 0.08$, in agreement with the theoretical prediction of $\varepsilon = 0.23 \pm 0.02$. These results provide the first experimental support for the Quantum Phase Interpretation (QPI), in which quantum systems evolve through dynamical phases rather than static states. The phase-based model achieves a substantially better fit than conventional collapse models, with a 3.9-fold improvement in $\chi^2$, and aligns closely with the theoretical framework of nonlinear phase diffusion. The study has significant implications for the foundations of quantum mechanics, offering new insights into decoherence mechanisms, the measurement problem, and the emergence of classicality from quantum phase dynamics. Proposed future experiments aim to test phase evolution across different quantum systems, including photons, neutrons, and quantum dots. Keywords: Quantum Phase, Neutron Interferometry, Decoherence, Non-Exponential Decay, Quantum Foundations, Phase Dynamics, Measurement Problem

DOI
10.5281/zenodo.18207716

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This study presents a reanalysis of high-precision neutron interferometry data from the Institut Laue-Langevin (ILL), revealing a significant deviation from the traditionally expected exponential decoherence. The interference visibility exhibits a stretched-exponential decay, characterized by V(t)=V0exp⁡[−(t/τcoh)1+ε]V(t) = V_0 \exp[-(t/\tau_{\mathrm{coh}})^{1+\varepsilon}]V(t)=V0​exp[−(t/τcoh​)1+ε] with a measured ε=0.18±0.08\varepsilon = 0.18 \pm 0.08ε=0.18±0.08, in agreement with the theoretical prediction of ε=0.23±0.02\varepsilon = 0.23 \pm 0.02ε=0.23±0.02. These results provide the first experimental support for the Quantum Phase Interpretation (QPI), in which quantum systems evolve through dynamical phases rather than static states. The phase-based model achieves a substantially better fit than conventional collapse models, with a 3.9-fold improvement in χ2\chi^2χ2, and aligns closely with the theoretical framework of nonlinear phase diffusion. The study has significant implications for the foundations of quantum mechanics, offering new insights into decoherence mechanisms, the measurement problem, and the emergence of classicality from quantum phase dynamics. Proposed future experiments aim to test phase evolution across different quantum systems, including photons, neutrons, and quantum dots. Keywords: Quantum Phase, Neutron Interferometry, Decoherence, Non-Exponential Decay, Quantum Foundations, Phase Dynamics, Measurement Problem
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Quantum mechanics exhibits a broad collection of theoretical results in complete agreement with experimental evidence. Besides showing unquestionable success in the description of isolated systems, it can be also successfully used to characterize non-isolated quantum system. In such a case, phenomena like dissipation, diffusion or decoherence. The theory of open quantum systems provides the framework where such features can be conveniently explained. \nThis thesis is about decoherence and collapse models. Albeit conceptually they are very far from each other, they both belong the framework of open quantum systems. Indeed, they consider systems interacting with an external entity: an environment for decoherence models, a noise for collapse models. Although the external influence has different origin, they can be described by similar dynamical equations and, to confirm or falsify them, the same experimental tests can be performed. \nThe quantum Brownian motion model can be considered the paradigm of decoherence models. We provide an exact and analytic equation for the time evolution of the operators and we show that the corresponding equation for the states is equivalent to well-known results in the literature. Our derivation allows to compute the time evolution of physically relevant quantities in a much easier way than previous formulations. Moreover, we are not bound to compute the time evolution of the state of the system, which in general is a complicated task. The explicit dependence on the initial state appears only in the initial expectation values and not in the dynamics. This makes possible the derivation of expectation values also for nontrivial states. \nAnother decoherence model we considered is a recently proposed model, based on the mass-energy equivalence. The model describes a decoherence source acting universally on every system whose superposition is extended on positions experiencing different gravitational potentials. We studied the conditions under which this mechanism becomes the dominant decoherence effect in typical interferometric experiments. We show that current experiments are off by several orders of magnitude. New ideas are needed to achieve the necessary requirements. \nThe second part of this thesis concerns collapse models, in particular the Continuous Spontaneous Localization (CSL) model. We focus on experimental tests that can probe it. In this respect, experiments can be grouped in two classes: interferometric tests and non-interferometric ones. The first class includes those experiments, which directly create and detect quantum superpositions of the center of mass of massive systems. The strongest bounds on the CSL parameters come from the second class of non-interferometric experiments, which are sensitive to small position displacements and detect CSL-induced diffusion in position. \nWe investigate how we can benefit from the non-interferometric perspective given by optomechanical setups, which have reached high sensitivities as force and position sensors. Three examples are considered.\nFirst, we compute the upper bounds on the CSL parameters, which can be inferred by the gravitational wave detectors LIGO, LISA Pathfinder and AURIGA.\nSecond, we report new results from an experiment based on a high-quality cantilever cooled to millikelvin temperature. High accuracy measurements of the cantilever thermal fluctuations reveal a nonthermal force noise of unknown origin. This excess noise is compatible with the CSL heating predicted by Adler. Several physical mechanisms able to explain the observed noise have been ruled out.\nThird, we propose an unattempted non-interferometric test aimed to investigate the still unexplored region of the CSL parameter space. Our proposal relies on torsional degrees of freedom rather than the usual vibrational ones. We believe that the test that has been put forward here, will eventually probe the unexplored CSL parameter space.
The quantum interference of de Broglie matter waves is probably one of the most startling and fundamental aspects of quantum mechanics. It continues to tax our imaginations and leads us to new experimental windows on nature. Quantum interference phenomena are vividly displayed in the wide assembly of neutron interferometry experiments, which have been carried out since the first demonstration of a perfect silicon crystal interferometer in 1974. Since the neutron experiences all four fundamental forces of nature (strong, weak, electromagnetic, and gravitational), interferometry with neutrons provides a fertile testing ground for theory and precision measurements. Many Gedanken experiments of quantum mechanics have become real due to neutron interferometry. Quantum mechanics is a part of physics where experiment and theory are inseparably intertwined. This general theme permeates the second edition of this book. It discusses more than 40 neutron interferometry experiments along with their theoretical motivations and explanations. The basic ideas and results of interference experiments related to coherence and decoherence of matter waves and certain post-selection variations, gravitationally induced quantum phase shifts, Berry's geometrical phases, spinor symmetry and spin superposition, and Bell's inequalities are all discussed and explained in this book. Both the scalar and vector Aharonov-Bohm topological interference effects and the neutron version of the Sagnac effect are presented in a self-contained and pedagogical way. Interferometry with perfect crystals, artificial lattices, and spin-echo systems are also topics of this book. It includes the theoretical underpinning as well as connections to other areas of experimental physics, such as quantum optics, nuclear physics, gravitation, and atom interferometry. The observed phase shifts due to the Earth's gravity and rotation indicate a close connection to relativity theory. Neutron interferometry can be considered as a central technique of quantum optics with massive particles. It has stimulated the development of interferometry with atoms, molecules and clusters. The book is written in a style that will be suitable at the senior undergraduate and beginning of graduate level. It will interest and excite many students and researchers in neutron, nuclear, quantum, gravitational, optical, and atomic physics. Lecturers teaching courses in modern physics and quantum mechanics will find a number of interesting and historic experiments they may want to include in their lectures.
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.