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Quantum Optomechanics for Fundamental Experiments in Space

Final Report Summary - QOFES (Quantum Optomechanics for Fundamental Experiments in Space)

Quantum optomechanics is a relatively new field that has seen rapid progress over the last years. The central element of optomechanics is the interaction of light with massive mechanical oscillators. Every single photon being absorbed or reflected by a mechanical object exhibits a momentum transfer on that object. This radiation pressure can be increased by a cavity where the mechanical object is a mirror facing a second mirror. In this case, the photons are reflected back and forth many times. If the cavity is good enough, even a single photon can, in sum, exhibit a notable force on the mirrors. Typically, in optomechanics one of the mirrors is placed on a cantilever and the action of the radiation pressure becomes noteable in the center-of-mass (CM) motion of the mirror on the cantilever. The motion of the cantilever is described as an oscillator. Usually, the motion of the oscillator is in thermal equilibrium with its environment but, using radiation pressure, the oscillator's CM motion can be damped, i.e. cooled. If this optomechanical interaction is strong enough, one can cool the motion so far that it becomes necessary to use quantum mechanics to describe the oscillator. Quantum optomechanics deals with optomechanical systems in this regime and, in principle, one can cool the CM motion very close to its absolute minimum, the quantum ground state.

Apart from applications in ultra-high-precision measurements, e.g. in the context of gravitational-wave detection, quantum optomechanics promises the possibility to prepare massive mechanical systems in non-classical quantum states. For example, states similar to Schrödinger's cat where the seemingly the border between the quantum and the classical world becomes blurred. Such states open the path towards novel experiments in the yet unexplored parameter regime between the microscopic and the macroscopic world. Investigating this untested intermediate regime is one of the most exciting tasks in modern physics because it may even allow us to address the interplay between quantum and gravitational physics.

In particular, quantum optomechanics and macroscopic superposition states allows for testing theoretical models that predict a transition from quantum to classical behavior for increasingly massive objects. Such experiments are ideally performed with particles free from any mechanical support that (1) constrains the particle, (2) limits the size of quantum superpositions, and (3) suffers from thermal and vibrational coupling. In QOFES, we therefore did not use optomechanical systems with small cantilevers as the mechanical oscillators but rather, we aimed at using optically trapped nanoparticles. Yet, even optical trapping is a too strong confinement for macroscopic quantum superpositions. Optical trapping is only used in the beginning to allow cooling the motion of the trapped nanosphere close to the quantum ground state. Then the particle is released from the trap, and one can later prepare it in a macroscopic superposition. On Earth, the time one can let a particle fall is limited. For that reason, QOFES investigated the possibility of performing such experiments in a space environment.

In the course of QOFES, we achieved the following:

(1) In a collaboration with Airbus Defense & Space (D&S), Dr. Kaltenbaek and colleagues wrote and submitted a proposal for a medium-sized, fundamental-science mission (MAQRO) for ESA's Cosmic Vision call 2010. In MAQRO, an optically trapped nanosphere is cooled close to the quantum ground state, then it is released for a time t1 (about 1s). After that time, a short and tightly focused laser pulse is used to create a macroscopic superposition of the nanosphere being in clearly distinct positions at the same time. After another time, t2 (about 100s), one measures the position of the particle. If repeated many times, quantum mechanics predicts the formation of an interference pattern. This can be compared with the predictions of alternative theoretical models. Dr. Kaltenbaek and colleagues also investigated the feasibility of using optically trapped particles in space for inertial measurements, e.g. for tests of Newton's inverse-square law, precision measurements of the gravitational constant G and tests of the equivalence principle. The MAQRO proposal was published in Experimental Astronomy in 2012.

(2) In 2011, a collaboration between the Cirac group and the Aspelmeyer group, including Dr. Kaltenbaek, proposed a novel double-slit-type experiment, similar to MAQRO but on the ground, using optically trapped nanospheres.

(3) Together with Airbus D&S, Dr. Kaltenbaek and colleagues performed a detailed theoretical study (MQES, Po P5401000400) funded by ESA to define possible space-based experiments with quantum optomechanics, to define their technical requirements, choose the most promising of the experiments and to suggest a preliminary design for such a space mission. We used this opportunity to investigate the central experiment of the mission proposal MAQRO in detail. Parts of that study were published in several papers in 2013.

(4) the Aspelmeyer group, including Dr. Kaltenbaek, performed first proof-of-principle experiments with optically trapped nanospheres. They trapped nanoparticles within an optical cavity and demonstrated significant cooling of the CM motion of the trapped particle using optomechanical interaction (published in 2013). Since then, the group has been working on improving the experimental setup by implementing an additional feed-back cooling mechanism, by devising and testing novel mechanisms for loading nanoparticles into the optical trap, and by designing more stable optical cavities together with Airbus D&S to increase the technological readiness level (TRL) towards future space experiments. These measures should allow cooling the CM motion of optically trapped particles close to the quantum ground state in the near future.

(5) A central prerequisite for future space experiments is to achieve low environment pressure and temperature. For this purpose, MAQRO uses an optical bench outside the spacecraft with three shields for insulation from the spacecraft. By performing a detailed thermal analysis of the shield design in collaboration with Airbus D&S, the team of Dr. Kaltenbaek could recently show that it is possible to achieve the strict requirements of MAQRO, i.e. low pressures (<=10e-13Pa) and temperatures (ca. 16K). The results were published in early 2014 and were featured in a BBC Future article in September, 2013. Even more detailed analyses are performed at the moment and indicate that the technical requirements of MAQRO can even be surpassed.

(6) In 2012, Dr. Kaltenbaek successfully acquired funding from ESA for the project NanoTrapS (Contract No.: AO/1-6889/11/NL/CBi), which was a collaboration between the Aspelmeyer and the Arndt group at the University of Vienna. The goal of the project was to experimentally investigate novel loading mechanisms for future matter-wave experiments in space. In early April 2014, the project was successfully concluded and showed promising results useful for future matter-wave experiments with massive particles on ground and, eventually, in space.

(7) In 2013, Dr. Kaltenbaek successfully acquired funding from the Austrian Research Promotion Agency, FFG, for the project MAQROsteps (Proj. Nr. 3589434), which he leads as the principle investigator. It is a two-year project to increase the TRL of several technologies central for the proposed MAQRO mission.

(8) In early 2013, Dr. Kaltenbaek initiated the international MAQRO consortium, which so far consists of researchers from more than 30 research groups from 9 countries. A central goal of the consortium will be the preparation of the MAQRO proposal for a Cosmic Vision call expected to be announced in mid-2014.

Overall, the work in QOFES led to 6 publications with respect to the work performed in the course of QOFES, including 1 last-author paper and 1 single-author paper of Dr. Kaltenbaek. Ongoing work should lead to at least 4 more publications.

We are confident that the work done in QOFES helped solidify the leading role of Europe for experiments on the foundations of physics in space. With the initiation of the MAQRO consortium, the work of Dr. Kaltenbaek helped ensuring continued international effort towards consolidating Europe's leading role in this important field also in the future.