Controllable Coupling between Flux Qubit and Nanomechanical Resonator by Magnetic Field


Y.D Wang, Hajime Okamoto, Hiroshi Yamaguchi, and Kouichi Semba
Physical Science Laboratory

 Flux qubit, also known as persistent current qubit, is micro-meter sized superconducting loop interrupted by several (usually three) Josephson junctions. Flux qubit is one of the most promising candidates for physical realizations of quantum information processing. On the other hand, nanoelectromechanical systems (NEMS), whose scale is smaller size than micro electromechanical system (MEMS), are promising to improve abilities of measuring small displacements and forces at a molecular scale. Meanwhile, nanomechanical resonator (NAMR) with sufficient high oscillation frequency at low temperature is supposed to behave as quantized harmonic oscillator. Due to their comparative sizes and energy scales as well as low dissipation, the coupling of NAMR and superconducting qubits has attracted a lot of interest to realize a novel solid-state cavity QED architecture. With this architecture, NAMR can serve as a data bus for multi-qubits operations of flux qubits. Coupling mechanism between flux qubit and NAMR also enables production and detection of quantum states of NAMR. However, existing theoretical investigations only concentrated on the coupling of NAMR with Josephson charge qubit while experimentally the coherence of flux qubit is undergoing rapid development.
 We thus proposed an active mechanism to couple the mechanical motion mode of a NAMR to the persistent current in the loop of superconducting Josephson junction (or phase slip) flux qubit [1]. As shown in Fig.1, the coupling can be controlled in situ by an external classical magnetic field. According to our numerical estimation, the whole system forms a new solid-state cavity QED architecture in "strong coupling limit". This architecture can be used to demonstrate quantum optics phenomena as well as coherently manipulate the qubit for quantum information processing. The coupling mechanism is applicable for more generalized situations where the superconducting Josephson junction system is a multi-level system.

[1] F. Xue, Y. D. Wang, C. P. Sun, H. Okamoto, H. Yamaguchi, and K. Semba, New J. Phys. 9 (2007) 35.


Fig. 1

A mechanical beam (the gray bar) is incorporated in a superconducting loop of a 3-Josephson-junction (each Josephson junction is indicated by a cross) flux qubit. Under a magnetic field B0 along y direction, the persistent super-current in the qubit loop produces Lorentz force F0 on the beam (NAMR) along z direction. Therefore, the z-direction motion of the beam is coupled with the persistent current of flux qubit.

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