Influence of conformational molecular dynamics on matter wave interferometry
Michael Gring, Stefan Gerlich, Sandra Eibenberger, Stefan Nimmrichter, Tarik Berrada, Markus Arndt, Hendrik Ulbricht, Klaus Hornberger, Marcel Müri, Marcel Mayor, Marcus Böckmann, Nikos L. Doltsinis
We investigate the influence of thermally activated internal molecular dynamics on the phase shifts of matter waves inside a molecule interferometer. While de Broglie physics generally describes only the center-of-mass motion of a quantum object, our experiment demonstrates that the translational quantum phase is sensitive to dynamic conformational state changes inside the diffracted molecules. The structural flexibility of tailor-made organic particles is sufficient to admit a mixture of strongly fluctuating dipole moments. These modify the electric susceptibility and through this the quantum interference pattern in the presence of an external electric field. Detailed molecular dynamics simulations combined with density-functional theory allow us to quantify the time-dependent structural reconfigurations and to predict the ensemble-averaged square of the dipole moment which is found to be in good agreement with the interferometric result. The experiment thus opens a different perspective on matter wave interferometry, as we demonstrate here that it is possible to collect structural information about molecules even if they are delocalized over more than 100 times their own diameter.
We investigate the influence of thermally activated internal molecular dynamics on the phase shifts of matter waves inside a molecule interferometer. While de Broglie physics generally describes only the center-of-mass motion of a quantum object, our experiment demonstrates that the translational quantum phase is sensitive to dynamic conformational state changes inside the diffracted molecules. The structural flexibility of tailor-made organic particles is sufficient to admit a mixture of strongly fluctuating dipole moments. These modify the electric susceptibility and through this the quantum interference pattern in the presence of an external electric field. Detailed molecular dynamics simulations combined with density-functional theory allow us to quantify the time-dependent structural reconfigurations and to predict the ensemble-averaged square of the dipole moment which is found to be in good agreement with the interferometric result. The experiment thus opens a different perspective on matter wave interferometry, as we demonstrate here that it is possible to collect structural information about molecules even if they are delocalized over more than 100 times their own diameter.
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.
We investigate the influence of different electric moments on the shift and dephasing of molecules in a matter wave interferometer. Firstly, we provide a quantitative comparison of two molecules that are non-polar yet polarizable in their thermal ground state and that differ in their stiffness and response to thermal excitations . While C 25 H 20 is rather rigid, its larger derivative C 49 H 16 F 52 is additionally equipped with floppy side chains and vibrationally activated dipole moment variations. Secondly, we elucidate the role of a permanent electric dipole moment by contrasting the quantum interference pattern of a (nearly) non-polar and a polar porphyrin derivative. We find that a high molecular polarizability and even sizeable dipole moment fluctuations are still well compatible with high-contrast quantum interference fringes. The presence of permanent electric dipole moments, however, can lead to a dephasing and rapid degradation of the quantum fringe pattern already at moderate electric fields. This finding is of high relevance for coherence experiments with large organic molecules, which are generally equipped with strong electric moments.
The chapter discusses advances in matter-wave optics with complex molecules, generalizing Young’s double slit to high masses. The quantum wave-particle duality is visualized by monitoring the arrival patterns of molecules diffracted at nanomechanical masks. Each molecule displays particle behavior when it is localized on the detector; however, the overall interference pattern requires their delocalization in free flight. Internal particle properties influence the de Broglie waves in the presence of surfaces or fields—even in interaction with atomically thin gratings. To probe the quantum nature of high-mass molecules, universal beam splitters are combined in a multi-grating interferometer to observe high-contrast matter-wave fringes even for 500 K hot molecules, containing 810 atoms with a mass of 10 000 amu. The high sensitivity of the nanoscale interference fringes to deflection in external fields enables non-invasive measurements of molecular properties. The chapter concludes by discussing research on beam techniques that extend molecular quantum optics to large biomolecules.
The wave nature of matter is a key ingredient of quantum physics and yet it defies our classical intuition. First proposed by Louis de Broglie a century ago, it has since been confirmed with a variety of particles from electrons up to molecules. Here we demonstrate new high-contrast quantum experiments with large and massive tailor-made organic molecules in a near-field interferometer. Our experiments prove the quantum wave nature and delocalization of compounds composed of up to 430 atoms, with a maximal size of up to 60 Å, masses up to m=6,910 AMU and de Broglie wavelengths down to λdB=h/mv≃1 pm. We show that even complex systems, with more than 1,000 internal degrees of freedom, can be prepared in quantum states that are sufficiently well isolated from their environment to avoid decoherence and to show almost perfect coherence. Observing superposition states of mesoscopic quantum systems is an ongoing challenge. Gerlichet al. report quantum interference of large tailor-made organic compounds, demonstrating delocalization and the quantum wave nature of entire molecules composed of up to 430 atoms.
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.
In order to explore the quantum-to-classical transition, the ambition of this thesis was the design and synthesis of tailor-made molecules for matter-wave interference experiments. Firstly, the focus was set on a deeper understanding of the influence of a molecule’s internal structure, especially its polarizability, on quantum interference. Furthermore, the limits of quantum mechanics were addressed by increasing the size of the interfering objects. The metrological aspect of molecule interferometry - to measure influences of molecule properties on the wave nature - is described in the first part of this thesis. Two fourfold perfluorohexyl functionalized constitutional isomers were synthesized in order to demonstrate that different molecular conformations can be distinguished in quantum interference experiments. A tetrahedral isomer and a rod-like oligo(phenylene ethynylene) derivative were applied in a near-field Kapitza-Dirac-Talbot-Lau interferometer. Although de Broglie quantum interference describes the center of mass motion of a massive body, it was shown to be sensitive to the internal molecular structure of the constitutional isomers. The different total susceptibilities of both isomers lead to different de Broglie interference shifts in the presence of external electric fields. The isomers thus become distinguishable in spite of their identical mass and chemical sum formula. Besides metrological applications, a main goal of this thesis was to synthesize model compounds for molecule interferometry to increase the preexisting mass record for quantum interferometry, which was held by the fluorofullerene C60F48 with a molecular weight of 1 633 g/mol. Near-field interferometry is a promising concept to set new records and thus to approach the quantum-to-classical transition. To meet the requirements of quantum studies with complex organic molecules in a Kapitza-Dirac-Talbot-Lau near-field interferometer – high volatility, high stability, and high molecular mass – we chose the functionalization of tetraarylporphyrins with perfluoroalkyl moieties. A modular synthesis of seven perfluoroalkyl-substituted porphyrin derivatives with up to 16 fluorous alkyl chains was presented. With this series in hand, it was possible to observe interference patterns for three of the seven derivatives. The heaviest molecule of this series for which the wave nature has been observed has a 3.3 fold higher molecular weight than the preexisting mass record for quantum interference. These experiments open a new window for quantum experiments with nanoobjects in a complexity class comparable to that of small proteins. The experimental setup of the near-field Kapitza-Dirac-Talbot-Lau interferometer is not restricted to monodisperse and pure starting compounds. It also allows for experiments with compound mixtures because individual components of compound libraries can be detected in the mass spectrometric detection unit. The preparation of porphyrin libraries gave access to compounds in a new mass region (> 10 000 g/mol). Preliminary investigations in near-field interferometers with these structures revealed suitable molecule beam properties. With these porphyrin libraries in hand, it is expected to exceed for the first time the ceiling of 10 000 g/mol for an interfering object in the future. Large, stable, highly volatile and fluorescent molecules were the goal of synthetic work that was performed for novel far-field interference experiments in a setup based on fluorescence detection. For this purpose three fluorescent fluorous phthalocyanine derivatives were prepared. With the help of a new laser-controlled micro-evaporation source it was possible to produce a beam of molecules with the required intensity and coherence. The observation of the wave nature of a free base phthalocyanine having a molecular weight of 1 299 g/mol set a new mass record for far-field molecule interferometry. The preexisting record for far-field interferometry was held by the fullerene C70 with a mass of 841 g/mol. To conclude, major contributions to the exploration of the frontiers of quantum mechanics were achieved by matter-wave interferometry using the perfluoroalkyl-functionalized molecules described in this thesis. \n