Storage and Retrieval of Quantum States in a Hybrid Quantum System

Shiro Saito1, Xiaobo Zhu1, Robert Amsüss1,3, Yuichiro Matsuzaki1,
Kosuke Kakuyanagi1, Takaaki Shimo-Oka4, Norikazu Mizuochi4, Kae Nemoto5,
William J. Munro2, and Kouichi Semba1,5
1Physical Science Laboratory, 2Optical Science Laboratory,
3TU Wien, 4Osaka University, 5NII

 Superconducting quantum bits (qubits) are one of the most promising candidates for a future large-scale quantum processor because of their controllability and scalability. However the currently reported coherence times of these has not yet reached the levels associated with those of microscopic systems such as electron spins, nuclear spins etc. On the other hand, it is hard to control and scale up such systems as they are well isolated from their environment. In this context, a superconductor-spin ensemble hybrid system has attracted considerable attention. We have utilized a nitrogen vacancy (NV) spin ensemble in diamond to act as a quantum memory for a superconducting flux qubit. By bonding the diamond crystal on top of a gap tunable flux qubit [1], we had realized strong coupling between the qubit & spin ensemble and further observed vacuum Rabi oscillations between them [2]. However we couldn’t store quantum information in the spin ensemble because its coherence time was too short. To overcome this we applied an in-plane magnetic field of 2.6 mT to the diamond crystal to reduce strain effects on decoherence and thus succeeded in performing quantum memory operations [3].
 The inset of Fig. 1(a) shows a pulse sequence for storage of a single excitation present in the flus qubit. First we detuned the qubit from the spin ensemble and applied a microwave π pulse to prepare the qubit in the excited state |1>qb|0>ens. Next to transfer the excitation to the spin ensemble |0>qb|1>ens, we applied an iSWAP pulse making the qubit on resonance with the spin ensemble for 30 ns. Then we keep the excitation in the spin ensemble for a storage time T and retrieved it by applying the iSWAP pulse again. Finally we measured the state of the qubit. From this experiment, a decay time of the excitation was determined as 20.8 ns (Fig. 1(a)). In a similar manner, we can store a superposition state in the spin ensemble by using two microwave π/2 pulses instead of the π pulse (Inset of Fig. 1(b)). From this Ramsey interference experiment on the spin ensemble, we could evaluate the decay time of the superposition state at 33.6 ns (Fig. 1(b)). These results show that we can store information of a population and a phase, namely an arbitrary quantum state in the spin ensemble. Our results are a significant first step to realize the long lived quantum memory and we will improve the diamond property and a coupling scheme to realize it.
 This work was supported by FIRST and NICT.

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S. Saito, et al., Phys. Rev. Lett. 111 (2013) 107008.
Fig. 1.
Quantum memory operations. (a) storage of a single excitation. (b) storage of a superposition state.