Quantum Optics Colloquium by Junxin Chen

Ground-state cooling by classical control

Optomechanics has recently drawn much attention as a tool to test fundamentals of quantum mechanics, a means for quantum sensing, and a significant component of quantum computing and quantum networking systems. Cooling mechanical oscillators to their quantum ground state is not only a proof of quantum ability of an optomechanical system, but also a prerequisite to many quantum protocols involving mechanical oscillators. Ground state cooling has been achieved in a number of optomechanical systems using dynamic backaction cooling [1, 2], similar to laser cooling in atomic physics.

Feedback-cooling, in contrast, relies on an accurate quantum measurement record of mechanical motion, which is used to apply a classical correction that forces the resonator into a motionless state. Reaching the ground state in this manner has been hindered by low detection efficiency and fast mechanical decoherence [3,4,5]. Here we introduce a membrane-in-the-middle system [6,7] containing a soft-clamped mechanical membrane resonator [8] with extraordinary performance in both properties. The system enables high detection efficiency (>70%) and quantum cooperativity Cq up to 100, the record in optical optomechanical systems. The unprecedented detection parameters allow decreasing of imprecision noise by 3 orders of magnitude compared to previous reports, more than 10⁷-times below the imprecision at the standard quantum limit.

With this system, we demonstrate a nearly ideal quantum measurement in terms of the imprecision-backaction product only 30% above the fundamental Heisenberg limit. We use this measurement record to feedback-cool a mechanical mode to 0.3 occupancy from a 10 K thermal bath. Our experiment sets the new benchmark for quantum measurement of position, and opens the door for measurement-based quantum control of mechanical oscillator. 

[1] Teufel, J. D., et al. "Sideband cooling of micromechanical motion to the quantum ground state." Nature 475.7356 (2011): 359.
[2] Chan, J., et al. "Laser cooling of a nanomechanical oscillator into its quantum ground state." Nature 478.7367 (2011): 89.
[3] Kleckner, D., and Bouwmeester, D. "Sub-kelvin optical cooling of a micromechanical resonator." Nature 444.7115 (2006): 75.
[4] Poggio, M., et al. "Feedback cooling of a cantilever’s fundamental mode below 5 mK." Physical Review Letters 99.1 (2007): 017201.
[5] Wilson, D. J., et al. "Measurement-based control of a mechanical oscillator at its thermal decoherence rate." Nature 524.7565 (2015): 325.
[6] Thompson, J. D., et al. "Strong dispersive coupling of a high-finesse cavity to a micromechanical membrane." Nature452.7183 (2008): 72.
[7] Nielsen, W. H. P., et al. "Multimode optomechanical system in the quantum regime." Proceedings of the National Academy of Sciences 114.1 (2017): 62-66.[8] Tsaturyan, Y., et al. "Ultracoherent nanomechanical resonators via soft clamping and dissipation dilution." Nature nanotechnology 12.8 (2017): 776.