The field of optomechanics was founded by the pioneers of LIGO, who were exploring the limit of interferometric displacement measurement [1]. According to Heisenberg uncertainty principle, measurement inevitably perturbs the state of the test mass, which is called backaction. Backaction will be evident in the measurement record, when the interaction between meter and test mass is strong enough. After half-century of effort, some optomechanical systems have finally reached this interesting regime. This opens the gate to a rich quantum world on meso/macroscopic objects, such as observation of quantum fluctuation, backaction evasion measurement, preparation of non-classical states of mechanics and optics, realizing quantum transducer, test of fundamentals of quantum mechanics, and even observation of low energy signatures of quantum gravity [2] [3].

One of the biggest challenges in quantum experiments in meso/macroscopic scale, compared to atomic scale counter parts, is the fast decoherence of the quantum state. We address this challenge with a technique called soft-clamping [4]. Putting a soft-clamped mechanical resonator in a Fabry–Pérot cavity, we have a membrane-in-the-middle system. With the system, we have achieved the first feedback cooling of a mechanical mode to its quantum ground state [5], observed the first quantum trajectory of mechanical motion [6], conducted the first displacement and force measurement below the standard quantum limit [7], and entangled two optical fields through optomechanical interaction. In this talk, I will introduce the basics of optomechanics, and talk about the results mentioned above.

[1] V. B. Braginsky, Classical and quantum restrictions on the detection of weak disturbances of a macroscopic oscillator. J. Exp. Theor. Phys 26, 831–834 (1968)

[2] M. Aspelmeyer, Cavity Optomechanics. Rev. Mod. Phys. 86, 1391 (2014)

[3] R. Penrose, On gravity's role in quantum state reduction. Gen Relat Gravit 28, 581 (1996)

[4] Y. Tsaturyan, et. al. Ultracoherent nanomechanical resonators via soft clamping and dissipation dilution. Nat. Nanotech. 12, 776–783 (2017)

[5] M. Rossi, et. al. Measurement-based quantum control of mechanical motion. Nature 563, 53–58 (2018)

[6] M. Rossi, et. al. Observing and Verifying the Quantum Trajectory of a Mechanical Resonator. arXiv:1812.00928

[7] D. Mason, et. al. Continuous Force and Displacement Measurement Below the Standard Quantum Limit. Nat. Phys. DOI: 10.1038/s41567-019-0533-5

Bio:  Junxin Chen is currently a final year PhD student at Niels Bohr Institute of University of Copenhagen, under the supervision of Prof. Dr. Albert Schliesser. During his PhD study, Junxin is mainly working on membrane-in-the-middle system with a soft-clamped SiN membrane. Before he moved to Denmark, Junxin got his bachelor’s degree from Hong Kong University of Science and Technology, and master’s degree from ETH Zürich.