Multi-Scale Cluster Interference Experiment

The wave-particle duality of matter belongs to the most fundamental features of quantum physics. It illustrates the conceptual contradiction between a continuous, wave-like evolution of probability amplitudes and the particle-like reduction to well-defined positions during a measurement. The quantum wave nature of matter has been visualized in many experiments before for electrons, neutrons, atoms and even molecules. It has, however, remained an open question how non-local quantum superpositions merge into the local phenomena that we are used to see around us, every day. An unbiased way of approaching this question is to systematically increase the mass and size of objects in advanced quantum experiments. The Vienna QNP group holds the current mass world record in matter-wave interferometry and here proposes to increase this by another order of magnitude. While previous high-mass interference experiments were performed with organic materials with limited abundance and thermal stability, we here propose to establish experiments with large metal clusters that may even have a mass-velocity product of 2107 amu m/s. This allows for 60 m/s fast objects with a mass of around 300 kDa. Metal clusters have never been employed in de Broglie studies before and their low work function will enable the use of three continuous photodepletion gratings. A scalable cold cluster source shall fulfill the demanding requirements for high-mass experiments in the 104-106 amu range. An intense beam of cluster anions will be generated by collisional aggregation in a cold noble gas vapor. The ions will be mass selected in a quadrupole mass filter, internally and translationally cooled in a cryogenic ion guide and then neutralized by ultraviolet electron detachment in ultra-high vacuum. The low work function of metals is the key to realizing three continuous photodepletion gratings with a standing wave period of 133 nm and it will facilitate single-photon ionization mass spectrometry detection.First tests can even be run with alkali clusters that are even susceptible to photo-depletion by 532 nm. The MUlti-Scale CLuster Interference Experiment (MUSCLE) will provide charged and neutral clusters with a wide atom number distribution from a few to beyond 10000 atoms per particle. The cryogenic setting will allow to prepare structurally simple cold nanoparticles and effectively suppress thermal quantum decoherence. MUSCLE will push the experimental limits in the search for non- standard quantum phenomena that are all predicted to scale with the square of the particle mass. MUSCLE will also define a new mile stone in matter wave physics because it opens a door to quantum experiments with a new material class, effectively comprising third of the entire periodic table, from alkaline over alkaline earth clusters, rare earth and coinage metals but also decorated and mixed nanoparticles. MUSCLE will render their size and conformation-dependent electronic and magnetic properties accessible to quantum-assisted measurements.

The goal of MUSCLE was to push the frontiers of high-mass interferometry. The novelty of the approach was to address a new material class for quantum superposition experiments: massive metal clusters. This requires pushing the limits of source and detection technologies, the realization of high power deep ultraviolet laser beams and optics in ultra-high vacuum. The interferometer needs vibrational isolation to better than < 10 nm motion and alignment of three standing light waves to gravity at the level of a few microradians. The physics of metal clusters has been known but the boundary conditions for matter-wave interferometry are special: even using advanced near-field matter-wave effects the conditions are exigent, as the minimally accepted de Broglie wavelength is about 30 fm. A high required count rate is key because quantum coherence needs to be prepared on the fly by virtue of Heisenberg's uncertainty during diffraction at a the first photodepletion grating with low transmission. The idea for our high mass cluster interferometers is published in AVS Quantum Science (2022). The setup consists of a magnetron sputtering cluster source with quadrupole mass filter, followed by a flight path for v-selection and cluster analysis in mass spectrometry. A frequency-doubled laser was set up and tested to generate deep UV light at 266 nm to generate the light required for a novel matter-wave beam splitter. We were able to produce, sort and detect large metal clusters with a source brilliance of up to 10^11 srad^-1 s^-1 which is good for interferometer applications (Phys. Rev. A. 2022) and currently optimized for durability. A new detection unit was simulated, built and tested and found to generate high ion counts. We have worked on an aerodynamic focussing stage to maximize the use of the cluster signal, to operate it longer and with larger and slower particles. This is being tested. A similar matter-wave interferometer was used to perform and complete magnetic measurements which led to the interesting new finding that certain molecules (here fullerenes) have a clearly measurable paramagnetism at high temperature, even though they have none in the nucleus nor in the electron shell. This was published and highlighted as Editor's choice in Phys. Rev. Lett. 129, 123001 (2022). With source and detectors available a new interferometer was setup (LUMI 2.0) with 3 photo-depletion gratings. Its alignment is a highly sophisticated procedure which requires extreme precision in many degrees of freedom as described in Proc. of SPIE 12447, 124470K-1 (2023). This alignment process is still under way together with continued work to extend the source longevity, even higher detection efficiency and a much-improved control over the grating alignment.