Coherent Optics for Long-Base-Line Molecule Interferometry

Quantum physics tells us that matter has a much more intriguing character than we usually perceive in our everyday life. While we often think of atoms and molecules as localized building blocks of nature with a well-defined location and momentum, comparable to little Lego blocks, it has been known for 90 years by now that all matter is also associated with a quantum wave function. This wave function carries many characteristics that we know from water waves, acoustics or light: it can spread out, delocalize, diffract and two or more wavelets may also interfere. Quantum physics differs, however, from classical mechanics in that these phenomena even apply to individual atoms or molecules. This surprising fact can be verified by exposing single particles to matter-wave interferometry. Successful early experiments with electrons, neutrons and atoms have stimulated molecule interferometry. Throughout the last years the Quantum Nanophysics Group at the University of Vienna has become home to cluster and molecule diffraction and interference experiments, recently even with molecules composed of more than 800 atoms, weighing as much as 10000 protons, still the current world record. The refined machines needed for tracking the quantum nature of matter, now also turn out to be highly sensitive force sensors, for molecule metrology as well as for novel tests in fundamental physics. Within the project COLMI we propose to build LUMI, the first long-baseline matter-wave interferometer, 10 times longer than all previous molecule experiments, and with a versatile choice of diffraction gratings. The proposed instrument will allow us to study and use the quantum wave nature of atoms over biomolecules to metal clusters, under comparable conditions in the same machine. Its extended length and its small grating periods, close the minimum of what is technologically feasible in continuous wave laser technology, will enable quantum interference with de Broglie wavelengths down to 50 fm i.e. an order of magnitude smaller than in any matter-wave interferometer so far. For this to become reality, we will explore new beam splitting techniques, advanced interferometer concepts, and we will dedicate substantial efforts to fight external perturbations which become systematically more relevant with the expected more than 100-fold increase in force sensitivity. This includes a new interferometer design, vibration isolation, monitoring, compensation and cross- correlation of subtle effects related to the Earths rotation and gravity. The project will contribute substantially to future tests of the quantum superposition principle and it shall become the basis for one of the finest tools for quantum-based molecule metrology.

We have realized LUMI, the world's first and only long-baseline universal matter-wave interferometer. It has a total base line of 2 meters, which makes it 10 times longer than all previous macromolecule experiments. The novel instrument can universally and coherently manipulate complex many-body systems and allows exploring a new mass and complexity regime that had remained inaccessible so far. We were able to demonstrate de Broglie wave interference with a large variety of different atom species, fullerenes, polyaromatic hydrocarbons, biomolecules such as tripeptides, and tailored nanoparticles even including particles beyond 25.000 Da and composed of almost 2000 atoms. The extended length and small grating periods of LUMI allowed us to see quantum interference with de Broglie wavelengths down to _dB 50 fm, i.e. an order of magnitude smaller than in any matter-wave interferometer so far. We have explored new beam splitting techniques, explored UV enhancement cavities, and successfully demonstrated the first Bragg diffraction (at 532 nm) for hot molecules. An optical phase grating was also integrated into the LUMI interferometer. We have advanced our interferometer concepts, designed a new interferometer support for vibration isolation and monitoring and compensated the Coriolis acceleration by gravity. The high thermal and mechanical ruggedness of the novel machine makes it robust for routine operation with fluctuations in fringe visibility of the order of a few percent per day. The interferometer is systematically sensitive to forces well below 10-26 N. The long base line allows operation with long coherent interaction times T which determine the interferometric phase and fringe shift in proportion to T^2 for a three-grating interferometer. In contrast to established atom interferometers our new machine is a genuine multi-species interferometer which accepts vastly more complex particles than any previous interferometer. This has allowed us to calibrate molecular effects to atomic constants. Our project - sets a new world record in mass and complexity in matter-wave interference - sets new bounds on interferometric tests of the continuous spontaneous collapse models - sets the current record in quantum macroscopicity according to the Nimmrichter-Hornberger criterion (which also compares it with SQUIDS, nanocantilevers, etc.) - has become a new standard for matter-wave assisted molecule metrology, with which we have improved polarizability values for fullerenes and were able to measure for the first time the magnetic susceptibility of Ba and Sr atoms in the ground state as well as of hot anthracene and adamantane. The experiments have opened a new window to quantum optics and matter-wave research and shall be scaled to more massive clusters and nanoparticles, operate with complex bio-nanomatter and allow refined molecule metrology in the future.