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Hybridization of superconducting flux qubits and diamond ensembles

William J. Munro, Shiro Saito, Xiaobo Zhu, Yuichiro Matsuzaki, Robert Amsüss, Kosuke Kakuyanagi, Takaaki Shimo-Oka, Norikazu Mizuochi, Kae Nemoto, Kouichi Semba

Published Jan 1, 2013• 0 citations
Publisher

Abstract

The development of moderate scale quantum devices requires stable quantum bits in which we can process, store and transport quantum information. Superconducting flux qubits are well known for their processing ability while electron-spin nitrogen-vacancy centers in diamond are a natural memory candidate. Here we report on strong coupling and coherent exchange of a single quantum of energy between these hybrid systems as well as the realization of a quantum memory operation.

DOI
10.1364/cleo_qels.2013.qm3c.2

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The development of moderate scale quantum devices requires stable quantum bits in which we can process, store and transport quantum information. Superconducting flux qubits are well known for their processing ability while electron-spin nitrogen-vacancy centers in diamond are a natural memory candidate. Here we report on strong coupling and coherent exchange of a single quantum of energy between these hybrid systems as well as the realization of a quantum memory operation.
The development of quantum networks requires stable quantum bits with which we can process, store and transport quantum information. A significant bottleneck in their performance is the ability to perform reliable local gates. It is well known that superconducting flux qubits have excellent processing ability while electron-spin nitrogen-vacancy centers in diamond are a natural memory and optical interface. Hybridization of these two systems thus presents the promise of an effective and efficient way to perform local gates. Here we report on the first step towards this: quantum state transfer between these systems.
We have built a hybrid system composed of a superconducting flux qubit (the processor) and an ensemble of nitrogen-vacancy centers in diamond (the memory) that can be directly coupled to one another, and demonstrated how information can be transferred from the flux qubit to the memory, stored, and subsequently retrieved. We have established the coherence properties of the memory and succeeded in creating an entangled state between the processor and memory, demonstrating how the entangled state's coherence is preserved. Our results are a significant step towards using an electron spin ensemble as a quantum memory for superconducting qubits.
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.
We study a hybrid quantum system consisting of spin ensembles and superconducting flux qubits, where each spin ensemble is realized using the nitrogen-vacancy centers in a diamond crystal and the nearest-neighbor spin ensembles are effectively coupled via a flux qubit. We show that the coupling strengths between flux qubits and spin ensembles can reach the strong and even ultrastrong coupling regimes by either engineering the hybrid structure in advance or tuning the excitation frequencies of spin ensembles via external magnetic fields. When extending the hybrid structure to an array with equal coupling strengths, we find that in the strong-coupling regime, the hybrid array is reduced to a tight-binding model of a one-dimensional bosonic lattice. In the ultrastrong-coupling regime, it exhibits quasiparticle excitations separated from the ground state by an energy gap. Moreover, these quasiparticle excitations and the ground state are stable under a certain condition that is tunable via the external magnetic field. This may provide an experimentally accessible method to probe the instability of the system.
We propose a quantum memory scheme to transfer and store the quantum state of a superconducting flux qubit (FQ) into the electron spin of a single nitrogen-vacancy (NV) center in diamond via yttrium iron garnet (YIG), a ferromagnet. Unlike an ensemble of NV centers, the YIG moderator can enhance the effective FQ-NV-center coupling strength without introducing additional appreciable decoherence. We derive the effective interaction between the FQ and the NV center by tracing out the degrees of freedom of the collective mode of the YIG spins. We demonstrate the transfer, storage, and retrieval procedures, taking into account the effects of spontaneous decay and pure dephasing. Using realistic experimental parameters for the FQ, NV center and YIG, we find that a combined transfer, storage, and retrieval fidelity higher than 0.9, with a long storage time of 10 ms, can be achieved. This hybrid system not only acts as a promising quantum memory, but also provides an example of enhanced coupling between various systems through collective degrees of freedom.
We report the experimental realization of a hybrid quantum circuit combining a superconducting qubit and an ensemble of electronic spins. The qubit, of the transmon type, is coherently coupled to the spin ensemble consisting of nitrogen-vacancy centers in a diamond crystal via a frequency-tunable superconducting resonator acting as a quantum bus. Using this circuit, we prepare a superposition of the qubit states that we store into collective excitations of the spin ensemble and retrieve back into the qubit later on. These results constitute a proof of concept of spin-ensemble based quantum memory for superconducting qubits.