利用鑽石中單一氮-空缺中心之混合系統實現量子記憶體

現今已有多種量子系統被提出及實踐作為量子位元並應用於量子資訊領域。人們期望著量子位元能與外加的控制場擁有較強的耦合強度又同時能保有較長的同調(相干)時間：前者可使得量子操作更快速；後者則可使量子位元保有較好的量子態。然而，此兩者通常無法同時兼顧，容易與其他量子系統耦合的系統亦容易受到環境的影響而失去其同調性，反之亦然。利用耦合能力較佳的系統進行量子操作，在其閒置時將量子態移轉到同調時間較長的量子記憶體中儲存以保持其同調性，此種混合量子系統便能解決上述之兩難。在此論文中，我們提出了一種鑽石中單一氮-空缺中心之混合量子系統，用以儲存磁通量超導量子位元之量子態。藉由釔鐵石榴石(YIG)強化兩系統間之耦合強度，我們將量子態轉存到鑽石中單一氮-空缺中心而實現量子記憶體，且在增強耦合強度的同時，釔鐵石榴石不會像使用群體氮-空缺中心一樣引入其他導致同調性流失的效應。我們推導了磁通量超導量子位元與單一氮-空缺中心是如何藉由釔鐵石榴石而達到耦合，接著詳述量子記憶體中量子態轉換和儲存的過程，最後考慮磁通量超導量子位元與單一氮-空缺中心的能量與相位耗散效應，我們以量子主方程式(Lindblad 形式)來模擬整個過程。利用真實的實驗參數估算，我們所提出的量子記憶體在完成量子態移轉、儲存(達 10 毫秒)及回傳等所有步驟後，仍可保持超過 90%的保真度。此混合量子系統不只能作為可靠的量子記憶體，亦可作為往後欲利用以集體激發之磁振子來增強各個不同量子系統間之耦合強度的範例與參考。

關鍵字

量子計算；磁通量超導量子位元；釔鐵石榴石；氮-空缺中心；量子記憶體； Holstein-Primakoff 轉換； Schriffer-Wolff 轉換

並列摘要

There are many kinds of physical quantum systems that have been proposed and realized as qubits to implement quantum computation and information processing. One may wish to have both the strong coupling strength between the qubit and an external control field and long coherence times for qubits: the former leads to fast and easy qubit operations; the latter maintains the coherence of the quantum state of the qubit. However, it is hard to have a qubit with both advantages. The systems, which can couple to other system strongly , are normally also easily influenced by the environment resulting in decoherence, and those with good coherence property due to the isolation from their environment cannot interact with other system well. So the idea of hybrid quantum system taking advantages of their constituents’ strengths has been proposed. Using the qubit with excellent coupling ability in the operating stage, and assisted by a quantum memory, which can transfer the quantum state between the operating qubit and storage qubit, one can avoid the decoherece in the idle time of the whole quantum processes. Here we propose a quantum memory scheme to transfer and store the quantum state of a superconducting flux qubit (FQ), as an operating unit, into the electron spin of a single nitrogen-vacancy (NV) center in diamond, as a storage unit, via a ferromagnet transducer, yttrium iron garnet (YIG). 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 by a master equation in Lindblad form. 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.

並列關鍵字

quantum computation ； superconducting flux qubit (FQ) ； yttrium iron garnet (YIG) ； nitrogen-vacancy (NV) center ； quantum memory ； HolsteinPrimakoff transformation ； Schriffer-Wolff transformation

參考文獻

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利用鑽石中單一氮-空缺中心之混合系統實現量子記憶體

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