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Nature Communications

Quantum interference of large organic molecules

Stefan Gerlich, Sandra Eibenberger, Mathias Tomandl, Stefan Nimmrichter, Klaus Hornberger, Paul J. Fagan, Jens Tüxen, Marcel Mayor, Markus Arndt

Published Apr 5, 2011• 389 citations
Publisher

Abstract

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.

DOI
10.1038/ncomms1263

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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.
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.
Matter-wave dualism is a fundamental concept in quantum physics. The observation of wave properties of heavy organic molecules can be used to approach the borderline between classical and quantum physics. Up to which mass and size do particles or molecules still have wave properties? This question is subject to be answered within this thesis and as part of the international nanoquestfit-project where many researchers of different disciplines are involved. \nThis thesis is a summary of the work done within a research collaboration between the group of Prof. Dr. Marcel Mayor (University of Basel) and the group of Prof. Dr. Markus Arndt (University of Vienna). The work is mainly focused on the development of novel quantum interference experiments that imply the development of new synthetic strategies towards tailor-made molecules. Novel interferometers have been developed by the Arndt group and therefore the molecular requirements changed strongly within the past years. The syntheses of these tailor-made molecules were performed by the author in the laboratories of Prof. Mayor in Basel. Quantum interference experiments and the preliminary investigations described within this work, namely the thermal stability test, the desorption studies and the laser-induced acoustic desorption have been performed by the Arndt group in Vienna. \nInvestigations at the interface of quantum and classical physics are a fascinating part of the research. The final goal is to approach the quantum-to-classical transition by the observation of quantum properties of large and complex molecules. In order to study the wave nature of such particles—a purely quantum mechanical phenomenon—synthetic procedures towards tailor-made compounds are developed to improve the observation of this phenomenon through matter-wave experiments. \nAt the starting point of this work, a series of highly fluorinated porphyrin-monomers set a new benchmark in high-mass quantum interference experiments. The highest masses of particles used in quantum interference experiments were around 10’000 g/mol. Thus, our objective was to reach masses up to 25’000 g/mol with the suitable features for the quantum interferometry experiments. \nIn the first chapter of this thesis, the principle of interferometry, the development and evolution of this technology are presented. In the subsequent chapters, the different synthetic approaches of the tailor-made molecules designed to improve the measurements and achieve the goal of this thesis are described. \nThe main part of this thesis is focused on the synthesis of tailor-made molecules for quantum interference experiments. In a first part, the synthesis of a novel, more massive fluorinated phthalocyanine is presented. That phthalocyanine was used in far-field quantum interference experiments. Unfortunately, the molecular requirements could not been tuned to observe wave-particle duality in these experiments. The second part focuses on the synthesis of three different molecular libraries. Molecular libraries are mixtures of molecules having the same core but a to some extent random number of long perfluorinated chains. The individual members within the library were observable in MALDI-TOF mass spectrometry. We attached the chains to three different cores to obtain three different molecular libraries. To produce a first generation of libraries we used diacetylene bond porphyrin-dimer. The rather limited reaction scope of diacetylene bond porphyrins was enhanced by the second generation of tetramers and pentamers. To compare the modularity of the synthesis of oligo-porphyrin systems two additional cores, a porphyrin-dimer and a porphyrin-trimer, have also been synthesized. The possibility to produce a molecular beam with the three molecular-libraries has been investigated by thermal studies, desorption experiments and laser-induced acoustic desorption. \nThe last part of the thesis is focused on the synthesis of photocleavable tags that can be attached to model compounds. These tags are synthesized since novel quantum interference experiments are based on light gratings where a mass difference is supposed to be observed at the detector level upon cleavage. The final idea is, in the near future, to attach these molecules to large particles, such as metal-nanoparticles, clusters or proteins. It is expected to enable the detection of mass differences of the intact clusters and the clusters with the cleaved tags. Three different model compounds have been synthesized. The cleavage of the tags was proven in solution upon irradiation of UV-light in solution, where a constant and reproducible decay was observed. In future quantum interference experiments, the cleavage should be performed in gas-phase as a final prove of concept. The molecules are currently under investigations in the Arndt group.
The quantum superposition principle, a key distinction between quantum physics and classical mechanics, is often perceived as a philosophical challenge to our concepts of reality, locality or space-time since it contrasts with our intuitive expectations with experimental observations on isolated quantum systems. While we are used to associating the notion of localization with massive bodies, quantum physics teaches us that every individual object is associated with a wave function that may eventually delocalize by far more than the body's own extension. Numerous experiments have verified this concept at the microscopic scale but intuition wavers when it comes to delocalization experiments with complex objects. While quantum science is the uncontested ideal of a physical theory, one may ask if the superposition principle can persist on all complexity scales. This motivates matter-wave diffraction and interference studies with large compounds in a three-grating interferometer configuration which also necessitates the preparation of high-mass nanoparticle beams at low velocities. Here we demonstrate how synthetic chemistry allows us to prepare libraries of fluorous porphyrins which can be tailored to exhibit high mass, good thermal stability and relatively low polarizability, which allows us to form slow thermal beams of these high-mass compounds, which can be detected using electron ionization mass spectrometry. We present successful superposition experiments with selected species from these molecular libraries in a quantum interferometer, which utilizes the diffraction of matter-waves at an optical phase grating. We observe high-contrast quantum fringe patterns of molecules exceeding a mass of 10,000 amu and having 810 atoms in a single particle.
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.
Summary form only given, as follows. In a recent experiment we were able to demonstrate the de Broglie wave character of the fullerene molecule C/sub 60/ by diffraction at a free standing material grating. This proves that it is possible to preserve quantum coherence almost completely in the external degrees of freedom of a hot, massive and complex object over a long time and distance. This may seem astonishing in the light of the fact that the fullerenes were produced in a thermal source which was kept at about 900 K. At such a high temperature the de Broglie wavelength is about 400 times smaller than the size of the molecule (/spl lambda//sub dB//spl sim/25 pm), all internal degrees of freedom (174 different vibrational modes, and rotational modes up to J>100) are excited and the molecules may emit a few infrared photons (blackbody radiation and vibrational quanta) during their time of flight through the apparatus. Our published measurements so far have been essentially consistent with the absence of decoherence during the time of flight of the molecules. The interference contrast in all experiments has however so far been limited by the thermal velocity spread and the finite collimation of the fullerene beam.