Vacuum Rabi oscillations observed in a flux qubit LC-oscillator system*

Superconducting circuit containing Josephson junctions is one of the promising candidates as a quantum bit (qubit) which is an essential ingredient for quantum computation [1]. A three-junction flux qubit [2] is one of such candidates. On the basis of fundamental qubit operations [3,4], the cavity QED like experiments are possible on a superconductor chip by replacing an atom with a flux qubit, and a high-Q cavity with a superconducting LC-circuit. By measuring qubit state just after the resonant interaction with the LC harmonic oscillator, we have succeeded in time domain experiment of vacuum Rabi oscillations, exchange of a single energy quantum, in a superconducting flux qubit LC harmonic oscillator system [5]. The observed vacuum Rabi frequency 140 MHz is roughly 2800 times larger than that of Rydberg atom coupled to a single photon in a high-Q cavity [6]. This is a direct evidence that strong coupling condition can be rather easily established in the case of macroscopic superconducting quantum circuit. We are also considering this quantum LC oscillator as a quantum information bus by sharing it with many flux qubits, then spatially separated qubits can be controlled coherently by a set of microwave pulses. [1] F. Wilhelm and K. Semba, in Physical Realizations of Quantum Computing: Are the DiVincenzo Criteria Fulfilled in 2004?, (World Scientific; April, 2006) [2] J. E. Mooij \textit{et al.}, Science \textbf{285}, 1036 (1999). [3] T. Kutsuzawa\textit{ et al.}, Appl. Phys. Lett. \textbf{87}, 073501 (2005). [4] S. Saito\textit{ et al.}, Phys. Rev. Lett. \textbf{96}, 107001 (2006). [5] J. Johansson\textit{ et al.}, Phys. Rev. Lett. \textbf{93}, 127006 (2006). [6] J. M. Raimond, M. Brune, and S. Haroche, Rev. Mod. Phys.\textbf{ 73}, 565 (2001).