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科學家最近發現了將量子位(quantum-bits，qubits)由所糾纏的光子(photons)轉移到固態結晶內存組件的方法，讓寬帶量子網的實現又前進了一小步。通過采用過冷晶體(super-cooled crystal)，科學家證實了量子網波導(quantum network waveguide)的糾纏態量子位，能轉移到固態內存，而且此過程是可逆轉的。 以上是加拿大卡爾加里大學(University of Calgary)以及德國帕德博恩大學(University of Paderborn)的合作研究成果；他們發現了光子-光子糾纏與光子還有固態原子激發(excitation of atoms)之間的可逆性轉移。以稀土元素(thulium)摻雜鈮酸鋰(lithium niobate)制成的波導，則用以做為該種光子回波量子內存的通信協議。 另一個來自瑞士日內瓦大學(University of Geneva)的研究團隊，也通過50米的光纖鏈接完成類似的實驗；證實了量子中繼器(quantum repeaters)可能將量子網的超高安全性通信，擴展到任何距離。 
卡爾加里大學的研究團隊已經證實，他們所采用的鈮酸鋰波導(已經廣泛應用在光纖通信領域)，能處理5MHz~5GHz的信號，內存保留時間為7納秒(nanosecond)；該團隊的寬帶量子內存利用現成的鈮酸鋰晶體，并需要超冷卻至零下攝氏270度。接下來，研究團隊打算制作一個及時讀寫通道，采用遠距傳輸(teleportation)來將量子位移進/出固態內存。 “我們已經證實了光子與晶體的原子之間會產生糾纏；下一步我們將以第三個光子進行交互作用，將其狀態通過糾纏傳輸到固態內存中。”卡爾加里大學量子信息科學研究所(the Institute for Quantum Information Science)教授Wolfgang Tittel表示：“這種傳輸步驟可望實現未來的超高安全性長距離通信量子網。” 除此之外，研究團隊也計劃延長內存保留時間，目標是由7納秒拉長到1秒
——這也是采用該種中繼器來制作更大型的量子網的必要條件。 Solid-state quantum memory unveiled R. Colin Johnson Broadband quantum networks inched closer to reality recently when researchers demonstrated the ability to transfer quantum-bits (qubits) from entangled photons to solid-state crystalline memory devices. Using a super-cooled crystal the researchers were able to demonstrate the reversible transfer of entangled qubits from a quantum network waveguide to the solid-state memory and back again. Researchers at the University of Calgary (Canada) collaborated with the University of Paderborn (Germany) in the reversible transfer of photon-photon entanglement into entanglement between a photon and the solid-state excitation of atoms. The rare-earth (thulium) doped lithium niobate waveguide made use of the photon-echo quantum memory protocol. Separately, another research group at the University of Geneva (Switzerland) demonstrated a similar capability over a 50-meter fiber optical link, paving the way for quantum repeaters that could extend the ultra-secure communications of a quantum network to any distance. The University of Calgary team demonstrated that their lithium niobate waveguides, which are already widely used for fiber optic communications, can handle signals from five megahertz to five gigahertz, with a memory retention time of seven nanoseconds. Its broadband quantum memory used off-the-shelf lithium-niobate crystals which needed to be supercooled to minus 270 degrees Celsius. Next, the group plans to create a real-time read-write channel using teleportation to transfer the qubits into and out-of its solid-state memory. "We have already demonstrated entanglement between a photon and the atoms of the crystal. Our next step will be to use interactions with a third photon to teleport its state into our solid-state memory by virtue of that entanglement," said University of Calgary professor Wolfgang Tittel at the Institute for Quantum Information Science. "This teleportation step will enable future quantum networks that provide ultra-secure long-distance communications." For the future, besides perfecting teleportation as a means of transferring qubits to and from its quantum memories, the researchers are also planning to extend the memory retention time from seven nanoseconds toward a goal of one second—a necessary condition for using repeaters to create large quantum networks. |
