Decoherence of a Superconducting Flux Qubit


Kosuke Kakuyanagi, Shiro Saito, Hayato Nakano, and Kouichi Semba
Physical Science Laboratory

 In order to carry out quantum operations using qubits, we need to maintain quantum coherence throughout all the quantum gate operations. However, coherence decreases over time because of the interaction between qubits and their environment. This phenomenon is called decoherence, and we must clarify its origins if we are to extend the coherence time.
 The superconducting flux qubit (Fig. 1) is a promising solid-based qubit that offers the advantage of scalability. We attempt to measure the magnetic field dependence of the phase relaxation time (T2) and energy relaxation time (T1) in order to clarify the decoherence of a superconducting flux qubit [1]. Relaxation is generally caused by the energy fluctuation that is generated from the interaction between qubits and their environment. Therefore, we can obtain information about the contribution of magnetic fluctuations to decoherence from field dependence measurements of the relaxations.
 Figure. 2 shows measurement results for the magnetic field dependence of T1 and T2  near the degeneracy point (ΔΦqb=0). The magnetic field is plotted with flux quantum units (Φ0). The T2  values increase as the external magnetic field approaches the degeneracy point. At the degeneracy point, the T2 value reaches 250 ns. In contrast, theT1 values are uniform ( T1〜140 ns).
 In general, T2  includes pure dephasing () and T1 component contributions. The relationship between T1 and T2 is described as . From the observed T1 and T2  values, the coherence time of a superconducting flux qubit at the degeneracy point is mainly suppressed by energy relaxation. Moreover, we can explain the behavior of the magnetic field dependence of pure dephasing in terms of the contribution of  type frequency distributed magnetic fluctuations.
 Energy relaxation is generated from high frequency fluctuations, whose frequency is the same as the qubit energy. These results suggest that the coherence time of a superconducting flux qubit improves when we suppress the high frequency noise. Next, we will attempt to improve the coherence time of the superconducting flux qubit.

[1] K. Kakuyanagi, et al., Phys. Rev. Lett. 98 (2007) 047004.

Fig. 1. Sample image of flux qubit.
Fig. 2. Field dependence of T1 and T2.

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