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科學(xué)家最近發(fā)現(xiàn)了將量子位(quantum-bits，qubits)由所糾纏的光子(photons)轉(zhuǎn)移到固態(tài)結(jié)晶內(nèi)存組件的方法，讓寬帶量子網(wǎng)的實(shí)現(xiàn)又前進(jìn)了一小步。通過采用過冷晶體(super-cooled crystal)，科學(xué)家證實(shí)了量子網(wǎng)波導(dǎo)(quantum network waveguide)的糾纏態(tài)量子位，能轉(zhuǎn)移到固態(tài)內(nèi)存，而且此過程是可逆轉(zhuǎn)的。 以上是加拿大卡爾加里大學(xué)(University of Calgary)以及德國帕德博恩大學(xué)(University of Paderborn)的合作研究成果；他們發(fā)現(xiàn)了光子-光子糾纏與光子還有固態(tài)原子激發(fā)(excitation of atoms)之間的可逆性轉(zhuǎn)移。以稀土元素(thulium)摻雜鈮酸鋰(lithium niobate)制成的波導(dǎo)，則用以做為該種光子回波量子內(nèi)存的通信協(xié)議。 另一個(gè)來自瑞士日內(nèi)瓦大學(xué)(University of Geneva)的研究團(tuán)隊(duì)，也通過50米的光纖鏈接完成類似的實(shí)驗(yàn)；證實(shí)了量子中繼器(quantum repeaters)可能將量子網(wǎng)的超高安全性通信，擴(kuò)展到任何距離。 
卡爾加里大學(xué)的研究團(tuán)隊(duì)已經(jīng)證實(shí)，他們所采用的鈮酸鋰波導(dǎo)(已經(jīng)廣泛應(yīng)用在光纖通信領(lǐng)域)，能處理5MHz~5GHz的信號(hào)，內(nèi)存保留時(shí)間為7納秒(nanosecond)；該團(tuán)隊(duì)的寬帶量子內(nèi)存利用現(xiàn)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(tuán)隊(duì)打算制作一個(gè)及時(shí)讀寫通道，采用遠(yuǎn)距傳輸(teleportation)來將量子位移進(jìn)/出固態(tài)內(nèi)存。 “我們已經(jīng)證實(shí)了光子與晶體的原子之間會(huì)產(chǎn)生糾纏；下一步我們將以第三個(gè)光子進(jìn)行交互作用，將其狀態(tài)通過糾纏傳輸?shù)焦虘B(tài)內(nèi)存中。”卡爾加里大學(xué)量子信息科學(xué)研究所(the Institute for Quantum Information Science)教授Wolfgang Tittel表示：“這種傳輸步驟可望實(shí)現(xiàn)未來的超高安全性長距離通信量子網(wǎng)。” 除此之外，研究團(tuán)隊(duì)也計(jì)劃延長內(nèi)存保留時(shí)間，目標(biāo)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(wǎng)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. |
