Cooling of macroscopic and mesoscopic objects to the quantum ground states are of great interests not only for fundamental study of quantum theory but also for the broad applications in quantum information processing and high-precision metrology. However, the cooling limit is subjected to the quantum backaction, and ground state cooling is possible only in the resolved sideband limit, which requires the resonance frequency of the mechanical motion to be larger than the cavity decay rate.

This sets a major obstacle for the ground state preparation and quantum manipulation of macroscopic and mesoscopic mechanical resonators, since more massive resonators typically have lower mechanical resonance frequencies. Therefore, it is essential to overcome this limitation, so that ground state cooling can be achieved for massive objects.

Very recently, Professor Yun-Feng Xiao and Ph.D student Yong-Chun Liu at Peking University, collaborated with Columbia University, have proposed an unresolved sideband ground-state cooling scheme in a generic optomechanical system, by taking advantage of the destructive quantum interference in a cavity optomechanical system with two mechanical modes coupled to the same optical cavity mode (Figure 1), where optomechanically-induced transparency phenomenon occurs.

They find that using the multiple inputs, the cascaded cooling effect further suppresses the quantum backaction heating. They show that ground state cooling of the mechanical mode beyond the resolved sideband limit by nearly three orders of magnitude can be achieved.

"This cooling approach adds little complexity to the existing optomechanical system, which is crucial in the experimental point of view." said Yong-Chun Liu, the first author of the paper. Compared with the conventional backaction cooling approach, the additional requirement here is a control mechanical mode and one (or more) input laser.

It is experimentally feasible for various optomechanical systems within current technical conditions. On one hand, many optomechanical systems possess abundant mechanical modes with different resonance frequencies, since the oscillation have different types and orders.

This situation can be found in optomechanical systems using whispering-gallery microcavities, photonic crystal cavities, membranes, nanostrings and nanorods amongst others. On the other hand, composite optomechanical systems, containing two independent mechanical resonators, are also conceivable.

For example, in Fabry-Perot cavities, the motion of one mirror acts as a control mechanical mode while the other mirror is to be cooled (Figure 1).

"This study paves the way for the manipulation of macroscopic mechanical resonators in the quantum regime." said Yun-Feng Xiao.

Yong-Chun Liu, Yun-Feng Xiao, Xingsheng Luan, and Chee Wei Wong, Optomechanically-induced-transparency cooling of massive mechanical resonators to the quantum ground state. Sci China-Phys Mech Astron, 2015, 58: 050305, doi: 10.1007/s11433-014-5635-6