IGM Colloquium: The Physics and Applications of high Q optical microcavities: Cavity Quantum Optomechanics

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The mutual coupling of optical and mechanical degrees of freedom via radiation pressure has been a subject of interest in the context of quantum limited displacements measurements for Gravity Wave detection for many decades(1, 2). The pioneering work of Braginsky predicted that radiation pressure can give rise to dynamical backaction, which allows cooling and amplification of the internal mechanical modes of a mirror coupled to an optical cavity and moreover establishes a fundamental measurement limit via radiation pressure quantum fluctuations. Experimentally these phenomena remained however inaccessible many decades due to the faint nature of the radiation pressure force. A decade ago, it was discovered that optical microresonators with ultra high Q, not only possess ultra high Q optical modes, but moreover mechanical modes that are mutually coupled via radiation pressure(3). The high Q of the microresonators, not only enhances nonlinear phenomena – which enables for instance optical frequency comb generation(4, 5) as well as temporal soliton formation(6, 7)– but also enhances the radiation pressure interaction. This has allowed the observation of radiation pressure phenomena in an experimental setting and is an underlying principle of the research field of cavity quantum optomechanics(8, 9).

In this talk, I will describe a range of optomechanical phenomena that we observed using high Q optical microresonators. Radiation pressure back-action of photons is shown to lead to effective cooling(1, 2, 10, 11) of the mechanical oscillator mode using dynamical backaction. Sideband resolved cooling, combined with cryogenic precooling enables cooling the oscillators such that it resides in the quantum ground state more than 1/3 of its time(12). Increasing the mutual coupling further, it is possible to observe quantum coherent coupling(12) in which the mechanical and optical mode hybridize and the coupling rate exceeds the mechanical and optical decoherence rate (7). This regime enables a range of quantum optical experiments, including state transfer from light to mechanics using the phenomenon of optomechanically induced transparency(13). Moreover, the optomechanical coupling can be exploited for measuring the position of a nanomechanical oscillator in the timescale of its thermal decoherence(14), a basic requirement for preparing its ground-state using feedback as well as (Markovian) quantum feedback. This regime moreover enables to explore quantum effects due to the radiation pressure interaction, notably quantum correlations in the light field that give rise to optical squeezing or sideband asymmetry(15).

The optomechanical toolbox developed in the past decades enables to extend quantum control, first developed for atoms, and recently for superconducting quantum circuits, to be extended to solid state mechanical oscillators. New frontiers that are now possible include for example the generation of non-classical states of motion via post-selection(16), mechanical quantum squeezing, or interfaces from radio-frequency to the optical domain(17). Time, permitting, recent experiments that probe cavity optomechanics reserved dissipation regime in a microwave opto-mechanical system will be discussed, which provide a means to realize a cold dissipative reservoir for microwave light(18) a building block for non-reciprocal devices(19).

1. V. B. Braginsky, S. P. Vyatchanin, Low quantum noise tranquilizer for Fabry-Perot interferometer. Physics Letters A 293, 228 (Feb 4, 2002).
2. V. B. Braginsky, Measurement of Weak Forces in Physics Experiments. (University of Chicago Press, Chicago, 1977).
3. T. J. Kippenberg, H. Rokhsari, T. Carmon, A. Scherer, K. J. Vahala, Analysis of Radiation-Pressure Induced Mechanical Oscillation of an Optical Microcavity. Physical Review Letters 95, 033901 (2005).
4. T. J. Kippenberg, R. Holzwarth, S. A. Diddams, Microresonator-based optical frequency combs. Science 332, 555 (Apr 29, 2011).
5. P. Del'Haye et al., Optical frequency comb generation from a monolithic microresonator. Nature 450, 1214 (Dec 20, 2007).
6. T. Herr et al., Temporal solitons in optical microresonators. Nature Photonics 8, 145 (2013).
7. V. Brasch et al., Photonic chip–based optical frequency comb using soliton Cherenkov radiation. Science 351, 357 (2016).
8. M. Aspelmeyer, T. J. Kippenberg, F. Marquardt, Cavity optomechanics. Reviews of Modern Physics 86, 1391 (2014).
9. T. J. Kippenberg, K. J. Vahala, Cavity optomechanics: back-action at the mesoscale. Science 321, 1172 (Aug 29, 2008).
10. A. Schliesser, P. Del'Haye, N. Nooshi, K. J. Vahala, T. J. Kippenberg, Radiation pressure cooling of a micromechanical oscillator using dynamical backaction. Physical Review Letters 97, 243905 (Dec 15, 2006).
11. A. Schliesser, R. Rivière, G. Anetsberger, O. Arcizet, T. J. Kippenberg, Resolved-sideband cooling of a micromechanical oscillator. Nature Physics 4, 415 (2008).

Tobias J. Kippenberg is Full Professor in the Institute of Physics and Electrical Engineering at EPFL in Switzerland since 2013 and joined EPFL in 2008 as Tenure Track Assistant Professor. Prior to EPFL, he was Independent Max Planck Junior Research group leader at the Max Planck Institute of Quantum Optics in Garching, Germany. While at the MPQ he demonstrated radiation pressure cooling of optical micro-resonators, and developed techniques with which mechanical oscillators can be cooled, measured and manipulated in the quantum regime that are now part of the research field of Cavity Quantum Optomechanics. Moreover, his group discovered the generation of optical frequency combs using high Q micro-resonators, a principle known now as micro-combs or Kerr combs.
For his early contributions in these two research fields, he has been recipient of the EFTF Award for Young Scientists (2011), The Helmholtz Prize in Metrology (2009), the EPS Fresnel Prize (2009), ICO Award (2014), Swiss Latsis Prize (2015), as well as the Wilhelmy Klung Research Prize in Physics (2015) and the 2018 ZEISS Research Award. Moreover, he is 1st prize recipient of the "8th European Union Contest for Young Scientists" in 1996 and is listed in the Highly Cited Researchers List of 1% most cited Physicists in 2014-2019. He is founder of the startup LIGENTEC SA, an integrated photonics foundry.

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IGM Colloquium: The Physics and Applications of high Q optical microcavities: Cavity Quantum Optomechanics