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Vacuum Rabi oscillations observed in a flux qubit LC-oscillator system
K. Semba1, 2, * , J. Johansson1,2 , S. Saito1,2, T. Meno1,3 , H. Tanaka1,2, H. Nakano1,2,
M. Ueda1,2,4 , and H. Takayanagi1,2
1
NTT Basic Research Laboratories, NTT Corporation, Atsugi, Kanagawa 243-0198, Japan
2
CREST, Japan Sience and Technology Agency, Kawaguchi, 331-0012, Japan
3
NTT Advanced Technology Corp., NTT Corporation, Atsugi, Kanagawa 243-0198, Japan
4
Department of Physics, Tokyo Institute of Technology, Tokyo 152-8551, Japan
* E-mail:[email protected]
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 coupled
system [5]. The observed vacuum Rabi frequency 140 MHz is roughly 3x103 (1x107) times
larger than that of Rydberg (ordinary) atom coupled to a single photon in a high-Q cavity [6].
This is the direct evidence that strong coupling condition can be rather easily established in
the case of macroscopic superconducting quantum circuit, because of the huge number of
condensed Cooper pairs in a super-current. 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. 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, 1036 (1999).
[3] T. Kutsuzawa et al., Appl. Phys. Lett. 87, 073501 (2005).
[4] S. Saito et al., Phys. Rev. Lett. 96, 107001 (2006).
[5] J. Johansson et al., Phys. Rev. Lett. 93, 127006 (2006).
[6] J. M. Raimond, M. Brune, and S. Haroche, Rev. Mod. Phys. 73, 565 (2001).