描述量子纠缠向来是一项颇具挑战性的任务。有许多陈词滥调需要避免：比如神秘、诡异、不可思议之类的说法。实际上，量子纠缠既不属于这些范畴，又似乎全都符合——可以说它是各类陈词滥调的集合体。既然我在第二句话里把那些陈词滥调都列举完了，接下来就让我们看看一群研究人员是如何成功让一束光与一个玻璃珠产生量子纠缠的，这确实是一项了不起的成就。
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.
所谓量子纠缠，简单来说就是：如果两个物体存在关联，那么它们的行为在某些方面就会呈现出相关性。举个不太恰当的例子：我的上臂和下臂在相对位置上高度相关，因为它们通过肘关节相连。没人会对此感到惊讶，因为我们能清楚地看到它们其实是一个整体，也就是所谓的“手臂”。
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.
从某种意义上来说，两个光子也可以相互关联，也就是说它们之间也存在相关性。不过在这种情况下，我们往往会感到意外，原因主要有三点。第一，我们通常认为光子是彼此独立的个体，无法相互关联——这种认知其实是错误的。第二，当我们让两个光子产生关联时，这种关联也仅存在于特定维度：比如在偏振特性上两个光子可能毫无相关性，但在能量方面却高度相关。
Two photons can be, in a sense, joined together, meaning that they have correlations, too. 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.