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Coherence and Decoherence in High-mass Matter-wave Interferometry

Kai Walter

Published Dec 9, 2016• 2 citations

Abstract

The present thesis deals with two main topics. First, I study in detail the interaction of a delocalized molecular matter-wave with a standing-wave laser grating. Due to absorption of photons in an optical grating, molecules can get excited and numerous internal photophysical processes are induced. For the theoretical description of the molecule-light interaction I take three relevant internal molecular processes into account: intersystem crossing, fluorescence and internal conversion. Furthermore, the fact that molecules can change their optical properties because of the photoexcitation is shown to have a measurable effect on matter-wave interferometry. The second topic of this thesis is about collisions between a polar molecule and non-polar environmental gas atoms, and their effects on the coherence of the center-of-mass state. Because the scattering potential depends on the molecular orientation the molecular rotational state must also be considered. Assuming a slow rotation of the molecule compared to the scattering time and that the molecular mass fairly exceeds the mass of a gas atom, I present a master equation of collisional decoherence, which depends on the molecular orientation. Its predictions for matter-wave interferometry are discussed both in the near and far field.

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The present thesis deals with two main topics. First, I study in detail the interaction of a delocalized molecular matter-wave with a standing-wave laser grating. Due to absorption of photons in an optical grating, molecules can get excited and numerous internal photophysical processes are induced. For the theoretical description of the molecule-light interaction I take three relevant internal molecular processes into account: intersystem crossing, fluorescence and internal conversion. Furthermore, the fact that molecules can change their optical properties because of the photoexcitation is shown to have a measurable effect on matter-wave interferometry. The second topic of this thesis is about collisions between a polar molecule and non-polar environmental gas atoms, and their effects on the coherence of the center-of-mass state. Because the scattering potential depends on the molecular orientation the molecular rotational state must also be considered. Assuming a slow rotation of the molecule compared to the scattering time and that the molecular mass fairly exceeds the mass of a gas atom, I present a master equation of collisional decoherence, which depends on the molecular orientation. Its predictions for matter-wave interferometry are discussed both in the near and far field.

We show that matter-wave diffraction off a single standing laser wave can be used as an accurate measurement scheme for photophysical molecular parameters. These include state-dependent optical polarizabilities and photon-absorption cross sections, the relaxation rates for fluorescence, internal conversion, and intersystem crossing, as well as ionization or cleavage probabilities. We discuss how the different photophysical processes manifest as features of the interference pattern, and we determine the accuracy of molecular parameters estimated from a realistic measurement with finite particle numbers. The analysis is based on an analytic calculation in Wigner representation, which accounts for the laser-induced coherent and incoherent dynamics, for the finite longitudinal and transverse coherence in the matter-wave beam, the gravitational and Coriolis acceleration, and an imperfect standing laser wave. Published by the American Physical Society 2024

Matter-wave interferometry can be used to probe the foundations of physics and to enable precise measurements of particle properties and fundamental constants. It relies on beam splitters that coherently divide the wave function. In atom interferometers, such elements are often realised using lasers by exploiting the dipole interaction or through photon absorption. It is intriguing to extend these ideas to complex molecules where the energy of an absorbed photon can rapidly be redistributed across many internal degrees of freedom. Here, we provide evidence that center-of-mass coherence can be maintained even when the internal energy and entropy of the interfering particle are substantially increased by absorption of photons from a standing light wave. Each photon correlates the molecular center-of-mass wave function with its internal temperature and splits it into a superposition with opposite momenta in addition to the beam-splitting action of the optical dipole potential.

We extend the theory of matter-wave interferometry of pointlike particles to nonspherical objects by taking the orientational degrees of freedom into account. In particular, we derive the grating transformation operator, which maps the impinging state onto the outgoing state, for a general, orientation-dependent interaction potential between the grating and the molecule. The grating operator is then worked out for the diffraction of symmetric top molecules from standing light waves, and the resulting interference pattern is calculated in the near field. This allows us to identify a signature of the orientational degrees of freedom in near-field matter-wave experiments.

We discuss new de Broglie coherence experiments in a near-field molecule interferometer with a variety of large organic molecules. While de Broglie coherence is to first order only associated with the center-of-mass motion of the diffracted particle, we here investigate the question as to which extent the internal molecular property is relevant for, compatible with or detrimental for maintaining coherence in the translational degree of freedom. External perturbations, such as inhomogeneous external electric fields, may influence the delocalized molecules inside the matter wave interferometer and they allow us to access the molecular polarizabilities, permanent or even fluctuating electric dipole moments. We extract information about the molecular states from the interference phase shifts as well as from the observed dephasing in a Kapitza-Dirac Talbot-Lau-Stark-deflectometer. We discuss the relevance of our experiments with regard to both molecule metrology and the foundations of quantum mechanics, i.e. the preservation of coherence for highly complex particles in external fields.

Recent progress and future prospects of matter-wave interferometry with complex organic molecules and inorganic clusters are reviewed. Three variants of a near-field interference effect, based on diffraction by material nanostructures, at optical phase gratings, and at ionizing laser fields are considered. The theoretical concepts underlying these experiments and the experimental challenges are discussed. This includes optimizing interferometer designs as well as understanding the role of decoherence. The high sensitivity of matter-wave interference experiments to external perturbations is demonstrated to be useful for accurately measuring internal properties of delocalized nanoparticles. The prospects for probing the quantum superposition principle are investigated in the limit of high particle mass and complexity.