Coherent phonon dynamics in spatially separated graphene mechanical resonators

  • [2020-03-13]

    The research team led by Prof. GUO Guangcan from University of Science and Technology (USTC) of Chinese Academy of Sciences (CAS), collaborating with Prof. TIAN Lin from University of California, Merced and Origin Quantum Computing Company Limited, realized electrically tunable coherent phonon dynamics between spatially separated graphene mechanical resonators. The study was published in Proceedings of the National Academy of Sciences of the United States of America (PNAS) on March 2nd.

    The rapid development of nanotechnologies enables the possibility to use long-lifetime vibrational phonon modes for classical and quantum information processing. Despite continuous efforts in the past decade proposed that couplings between two spatially separated mechanical oscillators via the exchange of virtual photon pairs can be achieved. However, observation of tunable coherent dynamics of indirectly coupled phonon modes remains a challenge, due to the requirement on the combination of strong coupling, long coherence time, and tunability of phonon modes.

    Taking advantage of the excellent electronic and mechanical properties of graphene, the team reported the observation of coherent Rabi and Ramsey oscillations between spatially separated mechanical resonators in the classical regime via Raman-like indirect coupling. Their results indicated that phonon coherence can be maintained and controlled over distant resonators, which will shed light on scalable phonon-based information processing.

    In the work, the graphene ribbon with a width of 2.2 μm and 7 layers is suspended over three trenches (2 μm in width, 200 nm in depth) between four contact electrodes. This structure defines three electromechanical resonators. They characterized each resonator separately using the one-source frequency-modulation technique and found that the relaxation rate of the resonators shows strong dependency on the driving amplitude.

    Next, they studied the precession dynamics between resonators in a Ramsey interference experiment. The highly tunable coupling strength between resonators offers promising prospect for phonon-state manipulation. They also investigated the coherent dynamics under different coupling strength.

    The results showed that at low driving power with a quality factor up to  105, the zero-temperature cooperativity between the resonators can reach 107. The measured damping times could be improved by optimizing the measurement setup to lower driving power to decrease the influence of nonlinear damping effects in the future.

    The work provides a promising platform for coherent computation using controllable vibrational phonons. Meanwhile, taking advantages of mechanical resonators being an outstanding interface between different physical systems, such as quantum dots engineered in graphene flakes, their architecture can be applied to transferring and storage of information between various systems.

    (a)Scanning electron microscope image of a typical sample with a tilted view angle. (b)The Rabi oscillation versus the microwave amplitude. (c)Average mixing current with coupling strength.

    Their demonstration of coherent dynamics between mechanical modes with indirect interaction and high tunability is a key step toward on-demand state transfer and manipulation in an all-phonon platform. Reviewers gave a highly evaluation: These results clearly go beyond what has been achieved thus far on the coherent manipulation of resonators in the classical regime.

    With the capabilities of system integrations, architecture can be extended to large scale for phonon-based computing and long- distance information exchange in mechanical systems. The work provides the building blocks toward scalable phonon-based information processing.


    Paper link:

    https://www.pnas.org/content/early/2020/02/28/1916978117


    (Written by LI Xiaoxi, edited by LU Hongyu, USTC News Center)


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