Stardust

Published online: 7 February 2007; | doi:10.1038/news070205-8
Turning light into matter
How do you make a light wave vanish and then reappear elsewhere?

Philip Ball


It sounds like a conjuring trick. You shine a light into a gas, and the light gets swallowed. Then you pump the gas into another container, say the magic words, and the light comes out again.
听起来就像是魔术，把光照进气体，光被吸收掉，当你把把气体注入其他容器后说出咒语光会再次出现。

But this trick, demonstrated by physicist Lene Vestergaard Hau and her co-workers at Harvard University in Cambridge, Massachusetts, doesn't use magic. Instead, the researchers have harnessed the strangeness of quantum mechanics to conduct their vanishing act1.
但是由物理学家 Lene Vestergaard Hau 与其合作者在哈佛大学进行的这个演示没有用魔法。取而代之的是研究者用奇妙的量子机器制造了消失表演。

First, they slow a light pulse — travelling, naturally, at the speed of light — to a crawl by beaming it into an ultracold cloud of about two million sodium atoms. Then they destroy the light beam entirely, but imprint a memory of it in the sodium.
首先，他们把以正常光速传播的一个光脉冲射入一团约200万钠原子的超低温云团中使其减速。随后将光束完全消灭，在钠云中留下其印记。

They shunt some of these atoms into a second cloud, and tickle them with another laser beam. This triggers their 'memory' of the original pulse, which emerges, much weaker but otherwise unchanged, from the second cloud.
他们将这些原子分流一部分到第二个云团中，并用另一激光束刺激云团，此举触发了钠原子对原先脉冲的“记忆”，从第二云团中显现出非常微弱但是没有改变的光。

Hau says that the 'messenger' atoms that move between clouds are basically a 'matter copy' of the original light pulse: a piece of light cast in atoms, you could say.
Hau 表示在两云团间移动的“信使”原子是原始光脉冲的“物质拷贝”，可以说是投射到原子中的一份光。

This process could be used for manipulating information in quantum computers, which would be potentially much more powerful than conventional devices. It might also prove useful in conventional optical telecommunications, for example for storing information held in light beams.
这一过程可以用来操作量子计算机中的信息，量子计算机的潜在能力比传统设备强大得多。这同样也能在传统光电通信中应用，比如存储光脉冲的信息。

Mexican wave
墨西哥波

The experiment relies on the way that, according to quantum mechanics, atoms may behave as waves as well as particles. This enables atoms to do some counterintuitive things, such as passing through two openings at once.
实验是建立在原子具有波粒两相性的量子力学之上，这种性质使原子能做出一些诸如同时穿过两个通道这样违反常理的事情。

Usually, each atom displays its wavelike behaviour independently of all its neighbours, like a crowd of soccer fans each waving their arms about at random. But if a group of atoms is cooled to very low temperatures — a mere fraction of a degree above absolute zero — then they may all come into step, like the fans conducting a Mexican wave.
通常，每一个原子表现出各自与周围原子独立的波动特性，就像球迷随机的挥动自己的手臂一样。但是如果一群原子被冷却到非常低的温度——仅在绝对零度之上不到一度——那么它们全都进入相同的步调，就像球迷们制造的“墨西哥波”一样。

In this state, called a Bose-Einstein condensate, information encoded in a light pulse can be transferred to the atom waves. Because all the atoms move coherently, the information doesn't dissolve into random noise and get lost.
这种情况称作波色—爱因斯坦凝聚态，光脉冲中编码的信息能转变成原子波。因为所有原子运动的相关性，信息不会在随机噪声中消失也就不会丢失。

Hau and her team have previously shown that a Bose-Einstein condensate — which they make here by cooling sodium atoms to about 600 billionths of a degree (colder than deep space) — can slow down a light beam and even bring it to a standstill.
Hau 和他的团队已经演示过波色-爱因斯坦凝聚态——就是他们在这儿把钠原子冷却到6千亿分之度（比深空还要冷）——能将光束减速甚至能让其停下来。

In this case, the Bose-Einstein condensate slows the light speed to a mere 24 kilometres an hour. This means that a pulse lasting less than a millionth of a second, which normally travels about a kilometre in that time, covers only about 20 micrometres (thousandths of a millimetre) in the sodium gas. So one of these pulses fits comfortably inside the cloud, Hau says.
在这种情况下，波色-爱因斯坦凝聚态物质能将光减速到时速仅24千米。这意味着一个不到百万分之一秒的脉冲，在钠气体中只能前进20微米，而通常情况下可以传播一千米。所以这些脉冲中的一员会很舒适的呆在云团中，Hau 认为。

Spreading the news
新闻扩展

A second 'control' laser then writes the shape of the pulse into the atom waves. When this control beam is turned off and the light pulse disappears, the 'matter copy' remains.
一秒的“控制”激光把脉冲的轮廓写入到原子波中，当控制光束关闭后光脉冲消失，留下了“物质拷贝”。

At the same time, the light's momentum is transferred to the atoms, so they move out of the Bose-Einstein condensate cloud to a second, similar cloud that the researchers hold suspended in a magnetic trapping field less than a millimetre away.
与此同时，光的动量传递给了原子，所以它们离开波色-爱因斯坦凝聚态运进入第二个类似的——研究人员用磁陷阱在不到一毫米远处悬浮住的云团。

By turning the control beam on this second cloud, the added atoms are encouraged to spread their 'message' throughout the whole cloud. All the atoms come into step with each other again, giving this second cloud a memory of the original laser pulse. They then re-radiate this pulse, albeit with only about one-fiftieth as much of the original light energy. The pulse crawls through the second cloud and speeds up once it leaves.
在第二云团上打开控制光，加入的原子被激励在整个云团中传播它们的“消息”。所有的原子再一次步调一致，给与第二云团以原始激光脉冲的记忆。它们随后重新辐射出这一脉冲，尽管只有原始光能量的五十分之一。脉冲缓慢的通过第二云团一旦离开云团便加速前进。

The end result is rather like telling a story to a crowd of people, some of who move off to another crowd and spread the story there.
最终结果非常类似于对一群人讲故事，其中一些人又跑到别的人群中散布故事。

Making 'matter copies' of light could be valuable in optical communications, Hau adds. "Matter is much easier to manipulate than light. For example, we could grab these copies and store them on the shelf." Alternatively, matter copies of light pulses arriving at an overworked hub could be put into a holding pattern, like aircraft at a busy airport.
发展光的“物质拷贝”可能对光通信有重要价值，Hau 补充道。“物质比光更容易控制。举例来说，我们可以夺取一份拷贝并存储在机架上。”作为选择，光脉冲抵达过载的集线器时其物质拷贝可以被放入一个保持样本中，就像在一个繁忙机场中的飞机一样。