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Distant Nanomechanical Resonators Carry Strong Indirect Coupling

Jan 28, 2018

With the advantages of small size, stability and high quality factors, nanomechanical resonators are considered as the promising candidate to storage, manipulate and transfer information. Both classical and quantum information can be encoded on phonon states of mechanical resonators. Phonon states can also transfer such information.
The most common way to realize tunable phonon interaction at long distance is to use optical cavities or superconducting microwave resonators as mediators. However, the difference between resonant frequencies of mechanical resonators and optical cavities or microwave resonators is too large. The coupling strengths between them are relatively small, and hard to reach strong coupling regime.
The joint group from the University of Science and Technology of China (USTC) of Chinese Academy of Sciences and University of California, Merced realized strong coupling between distant phonon modes by introducing a third resonator as a phonon cavity mode. Varying the resonant frequency of the phonon cavity mode, the coupling strength between distant phonon modes can be continuous tuned. The study was published in Nature Communications.

Previously, GUO Guoping’s group from USTC realized strong coupling between neighboring mechanical resonators and coherent manipulation of phonon modes. Based on this, scientists designed and fabricated a linear chain of three graphene-based nanomechanical resonators, as shown in the figure. In this device, the resonant frequency of each resonator can be tuned in a wide range via local bottom metal gates.

Such tunability provided the possibility to realize and modulate the coupling between resonators in different frequency ranges. The mode splitting of each neighboring resonators was observed, and neighboring resonators were found being strongly coupled, which contribute to the study on the coupling between the first and the third resonator. When the resonant frequency of the center resonator is tuned near to that of the side resonators, large mode splitting can be observed. Also, the splitting can be widely tuned via tuning the resonant frequency of center resonator.

This phenomenon was similar to Raman process in optics. The center resonator can be regarded as a mediating state, and the phonon modes of side resonators can achieve effective coupling via exchanging virtual phonon with the mediating state. Using the theoretical model of optical Raman process, the researchers got the relation between effective coupling strength and detuning. The experiment data agreed well with the theoretical results.

It is the first time to experimentally realize non-neighboring coupling in graphene-based nanomechanical resonators, which sheds light on the studies of nanomechanical resonators. This study provides the foundation for storage and transfer of quantum information via phonon modes.

This work was supported by the Ministry of Science and Technology, National Natural Science Foundation of China, Chinese Academy of Sciences, Ministry of Education, National Natural Science Foundation of United States, University of California and the USTC Center for Micro and Nanoscale Research and Fabrication.

 

Figure: Schematic and scanning electron microscopy image of device architecture with a chain of three graphene-based nanomechanical resonators (Image by LUO gang and ZHANG Zhuozhi)

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