Energy Relaxation Process of a Quantum Memory
Coupled with a Superconducting Qubit


Yuichiro Matsuzaki and Hayato Nakano
Physical Science Laboratory

   For quantum information processing, each physical system has a different advantage as regards implementation and so hybrid systems that benefit from the advantage of several systems would provide a promising approach. One common hybrid approach involves combining a superconducting qubit as a controllable qubit and another quantum system with a long coherence time as a memory qubit [1]. The use of a superconducting qubit gives us an excellent controllability of the quantum states and the memory qubit is capable of storing information for a long time. It has been believed that selective coupling can be realized between a superconducting qubit and a memory qubit by tuning the energy splitting between them.
   However, we have shown that this detuning approach has a fundamental drawback as regards energy leakage from the memory qubit [2]. Even if the superconducting qubit is effectively separated by reasonable detuning, energy relaxation time in the memory qubit decreases via residual weak coupling when the superconducting qubit is affected by severe dephasing (Fig. 1). This energy transport from the memory qubit to the control qubit can be interpreted as the appearance of the anti Zeno effect induced by the fluctuation in the superconducting qubit. We have suggested possible ways to avoid this energy relaxation process by using decoherence free subspaces, which is feasible with existing technology [2].
   This work was supported by KAKENHI.

[1] X. Zhu et al., Nature 478 (2011) 221.
[2] Y. Matsuzaki et al., Phys. Rev. B 86 (2012) 184501.

Fig. 1. Relationship between relaxation time of the memory qubit and the dephasing time of the superconducting qubit. We set the coupling strength as 25 MHz and set the detuning energy as 1 GHz.