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Research in Optical Sciences

Coherent coupling of a superconducting flux qubit to an electron spin ensemble in diamond

Xiaobo Zhu, Shiro Saito, Alexander Kemp, Kosuke Kakuyanagi, Shin–ichi Karimoto, H. Nakano, William J. Munro, 康弘 都倉, Mark S. Everitt, Kae Nemoto, Makoto Kasu, Norikazu Mizuochi, Kouichi Semba

Published Jan 1, 2012• 3 citations
Publisher

Abstract

We report evidence of coherent strong coupling, observation of vacuum Rabi oscillations, between a superconducting artificial atom (a flux qubit) and a macroscopic number of electron spins in the form of nitrogen–vacancy color centres in diamond.
DOI
10.1364/qim.2012.qt1a.4

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Bulletin of the American Physical Society•Mar 3, 2014•0 citations
We propose a method to achieve coherent coupling between nitrogen-vacancy (NV) centers in diamond and superconducting (SC) flux qubits. The resulting coupling can be used to create a coherent interaction between the spin states of distant NV centers mediated by the flux qubit. Furthermore, the magnetic coupling can be used to achieve a coherent transfer of quantum information between the flux qubit and an ensemble of NV centers. This enables a long-term memory for a SC quantum processor and possibly an interface between SC qubits and light.
The hybridization of distinct quantum systems has opened new avenues to exploit the best properties of these individual systems. Superconducting circuits and electron spin ensembles are one such example. Strong coupling and the coherent transfer and storage of quantum information has been achieved with nitrogen vacancy centres in diamond. Recently, we have observed a remarkably sharp resonance (~1 MHz) at 2.878 GHz in the spectrum of flux qubit negatively charged nitrogen vacancy diamond hybrid quantum system under zero external magnetic field. This width is much narrower than that of both the flux qubit and spin ensemble. Here we show that this resonance is evidence of a collective dark state in the ensemble, which is coherently driven by the superposition of clockwise and counter-clockwise macroscopic persistent supercurrents flowing in the flux qubit. The collective dark state is a unique physical system and could provide a long-lived quantum memory. Recently, a sharp resonance was observed in the spectrum of a flux-qubit nitrogen-vacancy-centre hybrid quantum system that is much narrower than that of either the flux qubit or the spin ensemble. Zhu et al.investigate this resonance and find evidence of a coherently driven collective dark state.
Entanglement between macroscopic systems is a basic resource for many quantum information processing, and so it is crucially important to investigate the possibility of achieving entanglement between macroscopic systems. In this work, we propose a novel mechanism for achieving entanglement between two spin ensembles in diamond nanostructures. The interaction between two spin ensembles, associated with the nitrogen-vacancy centers in diamond, are mediated by their coupling with a flux qubit. Using numerical methods, we show that two spin ensembles are entangled with the time evolution under realistic experimental conditions. This macroscopic entanglement between two spin ensembles has important implications for ongoing efforts to realize quantum computation and communication.
Abstract The hybridization of distinct quantum systems has opened new avenues to exploit the best properties of these individual systems. Superconducting circuits and electron spin ensembles are one such example. Strong coupling and the coherent transfer and storage of quantum information has been achieved with nitrogen vacancy centres in diamond. Recently, we have observed a remarkably sharp resonance (~1 MHz) at 2.878 GHz in the spectrum of flux qubit negatively charged nitrogen vacancy diamond hybrid quantum system under zero external magnetic field. This width is much narrower than that of both the flux qubit and spin ensemble. Here we show that this resonance is evidence of a collective dark state in the ensemble, which is coherently driven by the superposition of clockwise and counter-clockwise macroscopic persistent supercurrents flowing in the flux qubit. The collective dark state is a unique physical system and could provide a long-lived quantum memory.