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Final Report Summary - CQOM (Cavity Quantum Optomechanics)

See attached Publishable Summary in PDF with project logo and banner illustrating the work of the project.

Cavity optomechanics is a field in physics that lies at the junction of two areas that drove the technological advances of the last decades: quantum mechanics and nano-science. Cavity optomechanics couples nano-devices – tiny mechanical oscillators – to laser light and other electromagnetic fields. Such coupling allows exquisite control of both the mechanics and the light, such as decreasing the amplitude of thermally driven mechanical vibrations, performing extremely sensitive measurements of position, low noise amplification of microwave signals and generating ‘quantum states’ of light or mechanical oscillations.

Ultrasensitive position measurements have been brought to the attention of the public in the recent observation of gravitational waves in the LIGO experiment. While LIGO operates at the macro scale, the ability to precisely measure nanomechanical motion is also of fundamental importance as it allows exploration of the limits imposed by quantum mechanics. The position and momentum of a mechanical oscillator cannot be known with arbitrary precision but are constrained by Heisenberg’s Uncertainty Principle. This fundamental measurement limit of quantum backaction is indeed appreciable in experiments performed in the reported program. Fundamental insights from cavity optomechanics are expected to inform the development of future sensing technologies. Indeed, precise measurement of motion is already used in commercial micro/nano-electromechanical systems (MEMS) sensors to measure acceleration and angular momentum. Cell phones use piezo-mechanical resonators to filter radio frequency signals, and mechanical quartz oscillators are used in timekeeping and navigation. The nano-optomechanical sensors developed through this project offer other potential functions, such as accurately measuring temperature and amplifying weak radio frequency signals. Cavity optomechanics thus provides technology that is fundamentally limited by the laws of quantum mechanics and opens the door for enhanced measurement precision over existing technologies.

The set of skills important for researchers in this field is very diverse, both in theory and experiment, including nanofabrication, cryogenics, quantum optics and structural modeling and simulation of materials. The purpose of this ITN was to provide excellent tools and training for young researchers aspiring to enter the field, as well as to maintain the leading role of Europe in this area. Various workshops were organized by the partners, covering relevant topics of fundamental physics (e.g. “Fundamental Noise Sources” organized by the Leibniz University in Hannover and “Theory of Cavity Optomechanics” by the University of Erlangen), state-of-the-art experimental techniques (“Experimental toolbox for cavity optomechanics” organized by the UPMC in Paris and “Levitation in (quantum) physics: a new tool for addressing foundational questions” by the University of Vienna), on micro- and nanofabrication (“From Photonics Research to the CMOS-fab” jointly organized by the University of Gent and CNRS in Paris, in collaboration with leading partners from industry) as well as on simulation tools (“Finite Element Modeling” organized by EPFL in Switzerland).

Aside from these workshops aimed at preparing the participants for top-quality academic research, the network also provided a platform to interact with high-profile industry partners. On one hand, these partners organized workshops to expose the participants to research in industry and to entrepreneurship (“Taking an idea to a product”, organized by Attocube systems AG in Munich, then by IBM Research in Zurich). These workshops were very popular and insightful among the early stage researchers. Moreover, these industry partners visited all other conferences and workshops where the participants had ample opportunity to exchange ideas and to benefit from the knowledge and experience of these partners.

All of the partners in this ITN were pursuing active research in various aspects of cavity optomechanics - a fact that has considerably leveraged the collaborative effort, resulting in efficient knowledge transfer and exceptional scientific productivity. This is reflected by the prolific scientific output produced by the partners (often in collaboration), summarized in the following main achievements:

1. Demonstration of ground state cooling of a macroscopic object. Partners at UNIVIE have demonstrated cooling of nanomechanical oscillators to below one quantum of energy [1]. Moreover, work at EPFL has demonstrated for the first time the ability to use an external feedback loop to cool a MHz oscillator within a few quanta of the ground state [2]. Partner UMPC developed cryogenic techniques for pre-cooling free-space Fabry-Perot cavities and have also greatly improved the mechanical quality factor and experimental optical setup [3]. This is expected to allow access to the quantum regime of such oscillators.

2. Measurements beyond the standard quantum limit. Partner EPFL, using active feedback cooling, has successfully demonstrated measurement of a mechanical oscillator at a rate approaching its thermal decoherence rate, with sensitivity of 44 dB below the standard quantum limit [1]. The LUH/UHAM partner has developed ultra-low noise cryogenic system for kilogram-scale oscillators (such as suspended mirrors). Both LUH/UHAM and UNIVIE developed experimental platforms based on membrane resonators for ultra-sensitive displacement sensing. UCAM has demonstrated frequency-noise cancellation intended for displacement measurement using ponderomotive squeezing of light [4, 5].

3. Observation of Quantum Measurement Backaction. Partner EPFL has observed quantum measurement backaction at cryogenic temperatures and has initiated the effort for room-temperature experiments [6].

4. Development of Nano-Optomechanical systems. All partners performed research on a diverse set of optomechanical oscillators, including theoretical work, simulations and nanofabrication. In particular, UGENT has studied and developed optomechanical systems based on Brillouin scattering in the context of silicon photonics [7]. CNRS has developed photonic crystal membranes integrated with an electrostatic actuator [8]. IBM has developed an optomechanical crystal cavity with ultra-small mode volume [9].

5. Development of optomechanical transducers and milli-Kelvin temperatures. Partners UPMC, EPFL, UCAM and UNIVIE have further developed techniques for optical readout and quantum measurement of optomechanical systems in cryogenic setups [10].
These achievements span a wide array of systems and methods, with the focal point being the observation of quantum behavior of macroscopic objects. Utilizing the quantum regime in this new context is expected to greatly impact future technologies, such as novel sensors and quantum information technology. A full list of publications and supplementary information is available on the cQOM ITN website:

Sharing the knowledge and enthusiasm for the ITN fellows, especially the early stage researchers, was a specific goal for the network. The fellows as ambassadors of cQOM have been involved in numerous outreach activities to raise public awareness of their research work in cavity optomechanics, including attendance at conferences, demonstration of their experiments to younger students at local community events and television broadcasts. Noteworthy events included the development of the Wikipedia article, a collaborative effort carried out among the ESRs which has a significant impact on disseminating the central topic of the cQOM ITN, with over 30 unique visitors a day on average. The article is also available from the cQOM ITN website ( Nuits de la Science, an annual event organized across Europe to promote science to young students, witnessed cQOM ESRs participate within their local communities and contributed booths showcasing custom made photonics-related experiments. The booths were visited by hundreds of people from the general public and the ESRs were engaged in discussions with attendees. Fellows were also active in their local Scientific Open Days, annual events where students and the general public visit the research labs to get first-hand experience on the scientific activities. ESRs presented posters at their local universities featuring their scientific work, and assisted with lab tours for high school students and university freshmen.

The ITN cQOM program has spurred collaboration between academic institutions within Europe and has resulted in publications in prestigious journals such as Nature and PRL, which greatly benefit the development of the field. Collaborations with industrial partners, such as that between EPFL and IBM, have also been nurtured by the program and will continue to benefit the partners going forward. Finally, ITN trainees have made a concerted effort to reach out to the general public and share excitement about the burgeoning field of cavity optomechanics. The combination of these efforts has resulted in a broadly successful program that benefits the students with a wide range of skills that goes far beyond a typical PhD program.

[1] R. Riedinger et al., Nature 530, 313-316 (2016)
[2] D. J. Wilson et al., Nature 524, 325 (2015)
[3] Kuhn et al., Appl. Phys. Lett. 104, 044102 (2014)
[4] A. Pontin et al., Phys. Rev. A 89, 033810 (2014)
[5] S. Barzanjeh et al., Phys. Rev. Lett. 114, 080503 (2015)
[6] V. Sudhir et al., arXiv, 1608.00699 (2016)
[7] R. Van Laer et al., New J. Phys. 17, 115005 (2015)
[8] A. Chowdhury et al., Phys. Lett. 108, 163102 (2016)
[9] K. Schneider et al., Optics Express 24(13), 13850-13865 (2016)
[10] R. Schilling et al., Phys. Rev. Applied 5, 054019 (2016)

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