Quantum interference with clusters and complex molecules

Quantum theory is the central pillar of modern physics. No other theory in the natural sciences has been confirmed so precisely, and hardly any other models of nature have yielded so many practical consequences. The atomic clock, nuclear magnetic resonance instruments, neutron-based materials research, modern quantum information processing, ultracold quantum matter, superconductors and the quantum physics of condensed matter are all based on the same model, which can largely be traced back to the ideas of Austrian physicist Erwin Schrödinger around 1926 and which has been confirmed again and again in the experimental setting. The 2008 Wittgenstein Award, which went to physicist Markus Arndt, will serve to catalyse a research programme devoted to conducting quantum research at a new level of complexity. One focus area of these efforts is the matter-wave physics of nanomaterials, which is sometimes described in colloquial (albeit slightly misleading) terms as the ability of a large particle to be "in different places at the same time." The world`s first successful quantum interference experiments with large molecules and clusters were carried out in Vienna, and these experiments are now set to make another major advance: Based on the quantum interference of supermassive cold metal clusters, Arndt plans to address the question of whether there are any indications that quantum physics needs to be revised for large particle masses. One of the key goals in the coming years is to test the mass limits of quantum interference experiments. The link between quantum physics and gravitation is a crucial question in physics in general, and the research being conducted in Vienna is designed to make a major contribution to our understanding of this connection. At the same time, quantum interferometry with organic molecules is expected to establish a new interdisciplinary field of research at the intersection between physics, chemistry and biology: Experiments in the Vienna laboratories have revealed that it is possible to use quantum methods in order to measure the characteristics of isolated organic molecules in a new way and with sound levels of accuracy. The Wittgenstein Award will help the researchers to control the states of these complex systems more precisely and to develop the new field of "quantum-assisted molecule metrology." For many of these questions, it will be necessary to establish new technologies in Vienna, especially new processes for cooling and for the optical manipulation of large molecules and clusters. At the same time, the Wittgenstein endowment will provide the "seed money" necessary to pursue unconventional ideas (which require both courage and temporary independence from external reviewers) at a stage where it is not yet possible to say whether the research will ever deliver meaningful results. This also refers to the question of whether it is possible to measure quantum entanglement in large molecules, which itself includes the question of new ways of nanostructuring molecular surfaces using quantum interference. In addition, Arndt also sees a need to examine quantum phenomena in organic or even living systems, again primarily at the molecular level. The Wittgenstein Award will thus create many years of fascinating work for an entire group of young researchers.

Within the Wittgenstein Project Z149 we have studied new ways of manipulating massive quantum systems in a quantum coherent way. Far-field diffraction at nanomechanical gratings was pushed to the extreme limits: We managed to visualize the wave-particle duality down to the level of single molecules in wide-ield fluorescence imaging. We have studied diffraction at ultrathin nanoengineered masks and the coherent evolution of organic molecules through a nanoporous alga. Kapitza-Dirac-Talbot-Lau interferometry (KDTLI) was turned into a tool for quantum assisted metrology that is capable of measuring forces down to the yocto-Newton level. It can read internal properties of molecules that are delocalized by hundred times their own size. Research on quantum-assisted metrology will be intensified in the future, in particular on molecules of biological interest. KDTL interferometry sets the current world mass record in matter-wave experiments, with particles as massive as m=10123 amu. Quantum interferometry with pulsed ionization gratings (OTIMA) was realized for the first time, here not only with single molecules but even with clusters of molecules. The new concept reduces dispersive phase shifts and is independent of the detailed electronic structure of the diffracted nanoparticles. New molecular beam methods have been explored: On the source side progress was made with laser vaporization, desorption and laser-induced acoustic desorption. Aiming at sources for future nanoparticle experiments in the ultra-high mass range we have been able to demonstrate the first cavity cooling of silicon nanoparticles (1010amu) in high vacuum. On the detector side we explored various photo-ionization schemes. Superconducting detectors were studied for the first time on neutral nanoparticle beams. Scanning probe microscopy in Talbot Lau interferometry was used to explore molecule lithography. Our experimental research was performed in close collaboration with international partners and at a highly interdisciplinary interface between experimental and theoretical quantum optics, chemistry, nanotechnology and molecular biology. All interference experiments and many beam methods are still world-wide unique. They have created a long-term perspective for quantum optics with complex molecules that is now increasingly extended towards biomolecules and ultra-high mass particles, also in new European projects.