Vacuum Rabi Oscillations in a Macroscopic Superconducting Qubit LC Oscillator System


Jan Johansson1, Shiro Saito1, Hayato Nakano1, Masahito Ueda2 and Kouichi Semba1
1Physical Science Laboratory, 2Tokyo Institute of Technology/NTT Research Professor

 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 (Fig.1). 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 (photon), in a superconducting flux qubit LC harmonic oscillator coupled system [5]. The observed vacuum Rabi frequency 140 MHz is roughly 3×103 (1×107) times larger than that of Rydberg (ordinary) atom coupled to a single photon in a high-Q cavity [6]. This is a direct evidence that a strong coupling condition can be rather easily established in the case of macroscopic superconducting quantum circuit. It is explained by the reasons that the circuit is huge compared with the atomic scale and also the super-current of ?A order flows in the qubit. We have also obtained evidence of level quantization of the superconducting LC circuit by observing the change in the quantum oscillation frequency when the LC circuit was not initially in the vacuum state (Fig.2). 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 by a set of microwave pulses just like the method used in the quantum optics.

[1] F. Wilhelm and K. Semba, in "Physical Realizations of Quantum Computing: Are the Divincenzo Criteria Fulfilled in 2004?", (World Scientific Publishing Company; April, 2006)
[2] J. E. Mooij et al., Science 285 (1999) 1036.
[3] T. Kutsuzawa et al., Appl. Phys. Lett. 87 (2005) 073501.
[4] S. Saito et al., Phys. Rev. Lett. 96 (2006) 107001.
[5] J. Johansson et al., Phys. Rev. Lett. 93 (2006) 127006.
[6] J. M. Raimond, M. Brune, and S. Haroche, Rev. Mod. Phys. 73(2001) 565.

Fig. 1. (a) Scanning Electron Micrograph of the sample (qubit, SQUID, and an on-chip LC harmonic oscillator). (b) close-up view of a qubit and a SQUID detector (c) Equivalent circuit of the sample.
Fig. 2. Rabi oscillations as a function of the duration of the flux bias shift pulse and the amplitude of an LC-oscillator weak resonant pulse. The lowest two quantized Rabi periods are observed.

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