An experimental tour-de-force: Entanglement between glass bead and light

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Writing about quantum entanglement is always a challenge. There are so many clichés to avoid: It’s mysterious, ghostly, spooky, and weird. Entanglement is none of those things, and yet it is also all of those things—a superposition of clichés, you might say. So, having gotten all of my clichés out of the way in the second sentence, let’s take a look at how a group of researchers managed to entangle a light beam with a glass bead, which is, frankly, quite an achievement.

撰写关于量子纠缠的文章总是一项挑战。有太多的陈词滥调需要避免：它神秘、诡异、幽灵般且古怪。纠缠并非这些东西，但它同时又是所有这些东西——你可以说它是陈词滥调的叠加态。因此，既然已经在第二句中抛开了所有的陈词滥调，让我们来看看一组研究人员是如何设法将光束与玻璃珠纠缠在一起的，坦率地说，这是一项了不起的成就。

Quantum entanglement is nothing more or less than the idea that if two objects are linked, then their behavior will, in some ways, be correlated. To take a terrible example: My upper and lower arm are very strongly correlated in terms of relative position because they are connected at the elbow. No one is surprised by this because we can see that they are actually a single object called an arm. Two photons can be, in a sense, joined together, meaning that they have correlations, too.

量子纠缠的本质很简单：如果两个物体相互关联，那么它们的行为在某些方面就会产生相关性。举一个糟糕的例子：我的上臂和下臂在相对位置上有着极强的相关性，因为它们在肘部相连。没有人对此感到惊讶，因为我们能看出它们实际上是一个被称为“手臂”的单一物体。从某种意义上说，两个光子也可以结合在一起，这意味着它们也具有相关性。

In this case, we are (naively) surprised for three reasons. First, we think of photons as separate objects that cannot be joined—this is a mistake of understanding. Second, when we connect two photons, we only connect them in limited ways: The two photons may be wholly uncorrelated in terms of polarization, but strongly correlated in terms of energy. Third and critically, the connections that turn two single photons into a single object can only be observed in the results of destructive measurements we make on the photons; we cannot see the connection otherwise.

在这种情况下，我们（天真地）感到惊讶有三个原因。首先，我们认为光子是无法结合的独立物体——这是一种认知错误。其次，当我们连接两个光子时，我们只能以有限的方式连接它们：这两个光子在偏振方面可能完全不相关，但在能量方面却高度相关。第三，也是至关重要的一点，将两个独立光子变成单一物体的连接，只能通过我们对光子进行的破坏性测量结果来观察；否则我们无法看到这种连接。

This last property (and to a lesser extent, the second) is unique to quantum mechanics and completely foreign to our everyday experience. To make it obvious how far outside of our experience that last feature is, there is also no time delay in how this correlation works. Even though two entangled photons may be separated by the diameter of the Universe, a measurement of one photon has an immediate effect on a measurement of the other photon. (No, this cannot be used for instant communication. No, I am not going to explain why—that’s why we have a comments section.)

最后这一特性（以及在较小程度上的第二个特性）是量子力学所独有的，与我们的日常经验完全陌生。为了说明这一特征超出了我们的经验范围有多远，这种相关性的运作方式没有任何时间延迟。即使两个纠缠光子相隔整个宇宙的直径，对其中一个光子的测量也会对另一个光子的测量产生即时影响。（不，这不能用于即时通讯。不，我不会解释原因——这就是为什么我们有评论区。）

Take a single photon that we, through some clever trickery, divide into two. Each new photon carries some of the energy of the original. The specific division of energy is unknown, so each photon is in a superposition of multiple energies—there is a string of probabilities that tells us the chance of finding that the photon has any particular energy. Yet, the sum of the energies of the two photons has to equal that of the original photon. When we measure the energy of one photon, we instantly set the energy of the second photon.

取一个光子，通过一些巧妙的手段将其一分为二。每个新的光子都携带了原始光子的一部分能量。具体的能量分配是未知的，因此每个光子都处于多种能量的叠加态中——有一串概率告诉我们发现光子具有特定能量的可能性。然而，这两个光子的能量之和必须等于原始光子的能量。当我们测量其中一个光子的能量时，我们也就瞬间确定了第二个光子的能量。

Now, you might be thinking this is just a trick: Each photon had the measured energy all along, we just never bothered to check. But if you make this assumption and start making predictions about measurement results, you will get the wrong answers. Conclusion: The two photons have multiple energies before measurement and a single energy after measurement, and measuring one sets the energy of the second.

现在，你可能会认为这只是个把戏：每个光子一直都具有测量出的能量，只是我们从未去检查过。但如果你做出这种假设并开始对测量结果进行预测，你将得到错误的答案。结论是：两个光子在测量前具有多种能量，在测量后具有单一能量，测量其中一个会设定第二个的能量。

This result upset a lot of people and generated all the loathsome clichés in the first sentence of the article. If quantum mechanics is really like this, why don’t we see it everywhere? Good question. When the cage match between competing theories is over, I will be happy to tell you about the answer. In the meantime, the common point between competing theories is that the bigger something is, the more it interacts (you can read ‘interact’ as a form of measurement) with the rest of the world, which destroys these correlations as quickly as they are initiated.

这一结果让许多人感到不安，并产生了文章第一句中所有令人厌恶的陈词滥调。如果量子力学真的是这样，为什么我们没有随处可见？好问题。当相互竞争的理论之间的“笼斗”结束时，我很乐意告诉你答案。与此同时，竞争理论之间的共同点是：物体越大，它与外界的相互作用（你可以将“相互作用”理解为一种测量）就越多，这会使这些相关性在产生的同时迅速被破坏。

That is why quantum entanglement between a light field and a large bead of glass is so unusual. Under normal circumstances, the bead’s size would wash out the entanglement before it could be measured. The researchers used laser light to suspend and cool a glass bead using optical tweezers. The laser light is confined between a pair of mirrors, called an optical cavity, with the bead located at the center of the cavity.

这就是为什么光场与大玻璃珠之间的量子纠缠如此不同寻常。在正常情况下，玻璃珠的尺寸会在纠缠被测量之前就将其抹去。研究人员使用激光通过光镊悬浮并冷却玻璃珠。激光被限制在一对被称为光学腔的镜子之间，玻璃珠位于腔体的中心。

The optical cavity acts to strongly define the light fields. Essentially, the distance between the two mirrors sets the phase and frequency of the light fields and creates a series of fixed locations between the mirrors where there are high and low light intensities—this is called a standing wave pattern, which you can also see on a guitar string. The standing wave pattern means that the interaction between the bead and the light field is also very well-defined as long as the bead doesn’t move very much, which is another reason to cool the bead.

光学腔的作用是强烈地限定光场。本质上，两面镜子之间的距离设定了光场的相位和频率，并在镜子之间产生了一系列固定的高光强和低光强位置——这被称为驻波模式，你在吉他弦上也能看到这种现象。驻波模式意味着只要玻璃珠移动幅度不大，它与光场之间的相互作用就是非常明确的，这也是冷却玻璃珠的另一个原因。

Cooling in this instance has a very special meaning. The bead is trapped within the laser beam, but the trap is more akin to holding the bead with elastic bands than in metal tweezers. The bead will vibrate back and forth within the laser beam. In that sense, the bead is hot. To cool the bead, the vibration has to be slowed until it is as still as possible (it can never be completely still). This is achieved by slightly reducing the laser frequency to a redder color, such that the bead is always giving up energy to the light field via the Doppler effect.

在这种情况下，冷却有着非常特殊的含义。玻璃珠被困在激光束中，但这种束缚更像是用橡皮筋固定住玻璃珠，而不是用金属镊子。玻璃珠会在激光束内来回振动。从这个意义上说，玻璃珠是“热”的。为了冷却玻璃珠，必须减慢振动，直到它尽可能静止（它永远不可能完全静止）。这是通过将激光频率稍微降低到更偏红的颜色来实现的，这样玻璃珠总是通过多普勒效应向光场释放能量。

The light that scatters from the bead always gains a little energy from the bead in the process. Once cooled, the laser light has to give the bead a bit of a kick in just the right way to entangle the two. This is done by slightly increasing the frequency of the laser (meaning a bluer color). Now, the same Doppler shift that was removing energy is adding it. In summary, we have two laser beams, one cooling and one heating. But the cooling beam can only cool if the bead is moving more than its absolute minimum, which the heating beam ensures happens. That means that the two light fields are correlated.

在此过程中，从玻璃珠散射出的光总是会从玻璃珠那里获得一点能量。一旦冷却，激光必须以恰当的方式给玻璃珠一点“踢力”，从而使两者纠缠在一起。这是通过稍微增加激光频率（意味着更偏蓝的颜色）来实现的。现在，之前移除能量的多普勒频移反而是在增加能量。总之，我们有两束激光，一束用于冷却，一束用于加热。但冷却光束只有在玻璃珠的运动超过其绝对最小值时才能发挥冷却作用，而加热光束则确保了这一点。这意味着这两个光场是相关的。