科學家發現　鉍元素是實現量子電腦的關鍵

Research improves silicon for quantum computing

Julien Happich, EE Times Europe 8/26/2010 6:47 AM EDT

英國的倫敦大學學院(University College London，UCL)與位於美國佛羅里達州的國家高磁場實驗室(National High Magnetic Field Lab，NHMFL)，合作開發出一種能在矽晶片中更有效率地將量子資訊(quantum information)編碼的方法。

LONDON – A team of scientists from University College London (UCL) and the National High Magnetic Field Lab (NHMFL) in Florida has discovered a more efficient way to encode quantum information in silicon. (The research is described in the journal Nature Materials and in a forthcoming article in Physical Review Letters.)

磷(phosphorus)一直是傳統 IC製程中做為摻雜物融入矽材料中的元素，鉍(bismuth)雖然也能與矽相容，但到目前為止卻一直被忽視；不過UCL與NHMFL的研究人員發現，鉍在對量子態進行編碼時的表現比磷還要好。

Despite being compatible with the silicon, bismuth has been overlooked to date in favor of phosphorus. This is probably because phosphorous is familiar as a dopant and conventional ICs exploit phosphorous dissolved in silicon. However, the researchers in London and Florida have found that bismuth outperforms phosphorus at encoding quantum states. 

鉍是最重的穩定原子，並有相當大的核子自旋(nuclear spin)──其量子自旋就像是小小的羅盤針，會相應不同程度的傾斜，呈現出十種狀態中的一種，而不是像磷核子那樣只有兩種方向。這種特性讓鉍核子比磷原子能儲存更多的量子資訊，因為量子態空間(quantum state space)現在是十度空間(ten dimensional)而非二度空間。

Bismuth is the heaviest stable atom and has a correspondingly large nuclear spin: its quantum spin is like a tiny compass needle that can exist in one of ten states corresponding to different tilts (see illustration) instead of the two directions available to a phosphorus nucleus. This allows bismuth nuclei to store much more quantum information than phosphorous nuclei, because the quantum state space is now ten- rather than two-dimensional.

以上的觀察引出了一個可在矽中結合鉍與磷原子的「夢幻組合」構想──因為它們是不同的元素，所以能單獨被運用；在鉍儲存量子資訊的同時，磷則能提供存取控制與資訊流。「需克服的實驗性障礙，包括在矽晶片中使用鉍來做為量子資訊的準備、控制與儲存。」為上述研究論文主筆、來自UCL倫敦奈米科技中心(LCN)的Gavin Morley解釋。

The observations lead to the suggestion of a 'dream team' using both bismuth and phosphorus atoms in silicon: as they are different, they can be manipulated independently. The bismuth would store quantum information while the phosphorus controls the information flow. "The experimental hurdles overcome include the use of bismuth in silicon for the preparation, control and storage of quantum information" explained lead author Gavin Morley of the London Centre for Nanotechnology (LCN) at UCL. 

Morley接著指出，在這個案例中的原則是越大越好，因為較大的鉍原子核能提供更多空間來儲存量子資訊。論文的共同作者，同一研究單位的Marshall Stoneham補充：「如果能用以上概念打造出一部量子電腦，就能解決一些長久以來被認為的問題。」

He continued: "Bigger is better in this case because the larger nucleus of bismuth provides more room for storing quantum information." Co-author Marshall Stoneham, also of the LCN, added “If a quantum computer could be built, it could solve some problems long regarded as impossible. 

Stoneham表示，使用一種原子在矽晶片中儲存量子資訊，再用另一種原子來進行控制，就像是在一場單人獨白中加入另一個人與之對話，會有趣得多。NHMFL總監Greg Boebinger則表示：「這個結果對於採用矽晶片來進行量子科技研究，是很大的鼓舞。」

Having one type of atom for storing quantum information in silicon, and another type for controlling it is like bringing a second person into a one-man conversation: much more interesting!”Greg Boebinger, the Director of the NHMFL which hosted some of the experiments reported, commented “This result is a big incentive to use silicon for research into quantum technologies”.

科學家發現MEMS元件有助於實現量子電腦

MEMS shown to enable quantum computing

R. Colin Johnson - 12/10/2010 6:03 PM EST

美國某大學的研究人員發現，微機電系統(MEMS)越來越有助於量子運算的實現；該研究團隊證實，微反射鏡(micro-mirror)能讀取並寫入編碼在懸浮於透明媒介中的超冷原子雲(clouds of ultra-cold atoms)上的量子位元(qubits)。

PORTLAND, Ore.—Micro-electro-mechanical systems (MEMS) moved closer to enabling quantum computing with university researchers demonstrating that micro-mirrors can read and write qubits encoded on clouds of ultra-cold atoms suspended in a transparent media. 

今日的半導體記憶元件，需要位元線(bit-lines)在讀取或是寫入之前進行定址(address)，但根據美國杜克大學(Duke University)與威斯康辛大學麥迪遜分校(University of Wisconsin-Madison)的研究團隊發現，量子位元也能藉由 MEMS 微反射鏡，以兩道聚焦於其上的雷射進行類似的定址。

Semiconductor memories today need bit-lines to address them before reading or writing, but according to Duke University and the University of Wisconsin-Madison, qubits can be likewise addressed with two lasers focused on them by MEMS micro-mirrors.

上述研究團隊證實，由5個銣87 (rubidium-87)原子組成、間距8.7微米的原子雲，能以兩道分別對準其位置的雷射進行定址，因此也可望以雷射來進行量子位元的讀取與寫入。雖然目前的實驗只是概念驗證，未來的量子電腦可使用透明媒介來儲存量子位元，使它們儘可能靠近彼此，而它們之間的互動將可執行今日看來很棘手的超複雜運算，如破解長密碼。

In the demonstration setup, clouds of five rubidium-87 atoms were spaced at 8.7 micron intervals and addressed by two lasers which independently targeted their location, potentially allowing qubits to be written and read out by lasers. The current experimental setup merely proved the concept, but future quantum computers could use a transparent media to store qubits close enough to each other that their interactions could perform ultra-complex calculations that are intractable today—such as cracking long encryption codes. 

美國威斯康辛大學麥迪遜分校Mark Saffman研究團隊實驗室，以超冷原子雲來儲存量子位元

Ultracold cloud of atoms used to store quantum qubits in the laboratory of Mark Saffman's group at University of Wisconsin-Madison. 

根據研究人員表示，在量子位元之間進行切換的接取時間約5微秒(microseconds)，比目前在光學開關元件中所使用的微反射鏡速度快1,000倍；該研究團隊接下來計劃打造他們相信將成為未來量子電腦基本功能區塊的，也就是侷限在平面2D陣列中的雙量子位元閘極(two-qubit gate)。

An access time of about five microseconds, to switch between qubits, was reported by the researchers to be about 1,000-times faster than today's micro-mirrors used in optical switches. Next the group plans to construct what they believe will become the basic building block for future quantum computers--two-qubit gates confined in planar two-dimensional arrays. 

Nature 422 (6934), 24 Apr 2003

研究證明：鉍元素是不穩定的

1949年發表在《自然》雜誌上的一篇論文，引發了關於鉍元素天然放射性的一場長期爭論。唯一天然出現的鉍同位素209Bi通常被認為是最穩定的同位素。然而，理論上，209Bi是亞穩態的，應通過阿爾法粒子（由兩個質子和兩個中子組成的粒子）的輻射衰變。但是，所預測出的衰變概率極低，1949年以來所做的實驗未能檢測到鉍的阿爾法輻射。最新的閃爍輻射熱測量技術已經克服了這些困難：現在209Bi被正式宣佈是不穩定的，半衰期為2×1019年。第二個最重的穩定同位素208Pb有一個非常穩定的核結構，所以也許從來不會發生衰變。但在55年後結論是否還是這樣就很難說了。

Letters p.876