Parametric Control of a Superconducting Flux Qubit


Shiro Saito1, Masahito Ueda2, Hirotaka Tanaka1 and Kouichi Semba1
1Physical Science Laboratory, 2Tokyo Institute of Technology/NTT Research Professor

 Quantum state engineering has become one of the most important areas in quantum physics. In particular, the coherent control of quantum two-state systems, which are applicable to quantum bits (qubits), has attracted increasing interest in the context of quantum computing and quantum information processing. Of the recently realized solid-state qubits, the superconducting flux qubit has the advantages of scalability and a long coherence time. Multiphoton transitions have been demonstrated in the flux qubit [1]. In this study, we achieved the parametric control of a flux qubit by using two-frequency microwave pulses [2]. We observed Rabi oscillations stemming from parametric transitions between the qubit states when the sum of the two microwave frequencies or the difference between them matched the qubit Larmor frequency fqb.
 Our device is fabricated by lithographic techniques that define the structure of an inner aluminum loop forming a qubit and an outer enclosing dc-SQUID loop for the readout (see Fig. 1). Two currents circulationg in opposite directions in the qubit loop correspond to the ground state |g> and an excited state |e> of the qubit. First a qubit operation is performed by applying a two-frequency (fMW1 = 11.1 GHz, fMW2 = 18.5 GHz) microwave pulse of length tp to the qubit (fqb = 7.4 GHz). After the operation, we immediately apply a dc readout pulse to the dc-SQUID and measure whether or not the SQUID switches to a voltage state. By repeating the measurement 8000 times, we obtain the SQUID switching probability Psw, which is directly related to the probability with which we find the qubit in |e>. Figure 2(a) shows the microwave amplitude of the MW1 VMW1 dependence of Rabi oscillations when the microwave amplitude of the MW2 VMW2 is fixed at 50.1 mV. Figure 2(b) shows Rabi frequencies WRabi/2p at various VMW1 and VMW2 values, which are well reproduced by Rabi frequencies derived by using a semiclassical model.
 By utilizing the parametric processes, we can control a qubit with much higher frequencies than the qubit Larmor frequency. This implies that we can filter out the noise around and lower than the qubit frequency from the microwave line by using a high-pass filter. This will greatly improve qubit coherence.

[1] S. Saito, et al., Phys. Rev. Lett. 93 (2004) 037001.
[2] S. Saito, et al., Phys. Rev. Lett. 96 (2006) 107001.

Fig. 1. Electron microscope image of superconducting flux qubit.
Fig. 2. (a) Rabi oscillations observed in the flux qubit. (b) Microwave amplitude dependence of Rabi frequency. Inset is schematic of parametric control.

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