量子计算机实现的关键：铋元素

英国的伦敦大学学院(University College London，UCL)与位于美国佛罗里达州的国家高磁场实验室(National High Magnetic Field Lab，NHMFL)，合作开发出一种能在硅芯片中更有效率地将量子信息(quantum information)编码的方法。

磷(phosphorus)一直是传统 IC制程中做为掺杂物融入硅材料中的元素，铋(bismuth)虽然也能与硅兼容，但到目前为止却一直被忽视；不过UCL与NHMFL的研究人员发现，铋在对量子态进行编码时的表现比磷还要好。

铋是最重的稳定原子，并有相当大的核子自旋(nuclear spin)──其量子自旋就像是小小的罗盘针，会相应不同程度的倾斜，呈现出十种状态中的一种，而不是像磷核子那样只有两种方向。这种特性让铋核子比磷原子能储存更多的量子信息，因为量子态空间(quantum state space)现在是十度空间(ten dimensional)而非二度空间。

以上的观察引出了一个可在硅中结合铋与磷原子的“梦幻组合”构想──因为它们是不同的元素，所以能单独被运用；在铋储存量子信息的同时，磷则能提供访问控制与信息流。“需克服的实验性障碍，包括在硅芯片中使用铋来做为量子信息的准备、控制与储存。”为上述研究论文主笔、来自UCL伦敦奈米科技中心(LCN)的Gavin Morley解释。

Morley接着指出，在这个案例中的原则是越大越好，因为较大的铋原子核能提供更多空间来储存量子信息。论文的共同作者，同一研究单位的Marshall Stoneham补充：“如果能用以上概念打造出一部量子计算机，就能解决一些长久以来被认为的问题。”

Stoneham表示，使用一种原子在硅芯片中储存量子信息，再用另一种原子来进行控制，就像是在一场单人独白中加入另一个人与之对话，会有趣得多。NHMFL总监Greg Boebinger则表示：“这个结果对于采用硅芯片来进行量子科技研究，是很大的鼓舞。”

点击进入参考原文：Research improves silicon for quantum computing，by Julien Happich, EE Times Europe

《电子工程专辑》网站版权所有，谢绝转载

{pagination}

Research improves silicon for quantum computing

by Julien Happich

EE Times Europe

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.

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. 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.

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 could provide access control and 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. 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. 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."