Superconducting Quantum Physics Research Group  [NTT Basic Research Laboratory]
Success in Observation of Vacuum Rabi Oscillations in a
Superconducting Artificial Atom Microwave-Resonator 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 (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 µA 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.
Success in controlling multi-photon transitions of
superconducting flux qubit
- One step closer to realizing a quantum computer -
NTT Basic Research Laboratories in collaboration with Japan Science and Technology Agency (JST-CREST) have succeeded in flipping supercurrent flow (Fig. 1) (quantum state transition) in each case of one-, two-, and three-photon absorption process by irradiating resonant microwave photons (Fig.2) to the superconducting flux qubit. From Ramsey interference experiment, we have also observed characteristic oscillating pattern (Fig. 3) therefore, we have succeeded in coherent control of a qubit which is essential if quantum computation is to work. We have confirmed that quantum mechanics, which is usually applied to microscopic objects such as elementary particles and atoms, can also be applied to the macroscopic state(consisting of millions of Cooper pairs) in a superconducting flux qubit of six micrometers in size. From these experiments, this device has proved to be a promising candidate as a quantum computer building block.

Fig.1 Fig.2
Fig. 1 A superconducting fulx qubit (inner loop) and a quantum detector SQUID (outer loop). Both are made of aluminum. Arrows represent two states of the qubit, supercurrents in the opposite directions.
Fig. 2 SQUID response (a) without (qubit in the ground state) and (b) with resonant microwave. The ordinate is the change in the maximum supercurrent that we can transmit through the SQUID (1 nA scale). Resonant peaks and dips of up to three photon transitions are clearly observed.
Fig. 3 Coherent oscillation of the qubit observed in a Ramsey interference experiment. Within the coherence time, using a pair of π/2 resonant pulses, we can coherently return the qubit state to the ground state or the excited state. Then we obtain a decayed sinusoidal oscillation.

[Reference: S. Saito et al., Phys. Rev. Lett. 93, 037001 (2004). ]
NTT Basic Research Lab