AQP Seminars: Enhancement of the optomechanical coupling and Kerr nonlinearity using the Josephson Capacitance of Cooper-pair box
Optomechanics is the study of the interaction between the optical field and mechanical motion mediated by radiation pressure force. Among various optomechanical systems, cavity optomechanics with superconducting microwave circuits [1,2,3] is a very promising platform for studying quantum phenomena. In recent times, this field is garnering a lot of attention because of its compatibility with superconducting qubits which are proving to be a leading platform for quantum computing. Specially, there is a growing research interest for strong coupling regime where the strength of the coupling is comparable to the frequency of the mechanical resonator or the linewidth of the cavity. This single photon strong coupling regime, if reached, will allow rich quantum physics experiments, for example, preparing non-classical states of light and mechanical resonator for potential quantum information processing applications. We investigated a scheme to enhance the optomechanical coupling by embedding a Josephson junction Cooper Pair Box (CPB) into the cavity . The CPB is coupled to the cavity via a movable capacitance. The optomechanical interaction is mediated by this CPB and can be enhanced by tuning its Josephson capacitance. By tuning the charge and flux bias of the CPB, the optomechanical coupling can be enhanced six-seven orders of magnitude, facilitating access to ultrastrong coupling regime with properly chosen parameters. Additionally, with suitable choice of tuning parameters, the optomechanical coupling vanishes and the system exhibits purely a cross-Kerr type of nonlinearity between the cavity and the mechanical resonator. This allows the system to be used for phonon counting.
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 Haque, M., Manninen, J., Vitali, D. & Hakonen, P. Enhancement of the optomechanical coupling and Kerr nonlinearity with the Josephson Capacitance of Cooper Pair Box. arXiv:2010.13493.