Hidden no more 再无‘隐藏’
2015-10-22 文今
IN THE 1930s Albert Einstein was greatly troubled by a phenomenon that emerged from quantum theory. Entanglement, as it is called, forever intertwines the fates of objects such as subatomic particles, regardless of their separation. If you measure, say, “up” for the spin of one photon from an entangled pair, the theory suggests that the spin of the other, measured an instant later, will surely be “down”—even if the two are on opposite sides of the galaxy. This was anathema to Einstein and others: it looked as if information was travelling faster than light, a no-no in the special theory of relativity. Einstein was quotably derisive, calling the idea “spooky action at a distance”. But after 80 years of physicists’ fretting, a cunning experiment reported this week proves that such action is in fact how the world works.
20世纪30年代，量子学中的一个现象让爱因斯坦疑惑不解。这个现象就是量子纠缠现象。不论距离远近，它让两个物体如亚原子粒子的命运永远交缠在一起。如果说，相纠缠的一对粒子的其中一个自旋光子发生改变，根据该理论，另外一个自旋光子会瞬间产生相应的反应，即使它们在银河的两端。这个现象对于爱因斯坦和其他物理学家来说简直是噩耗，因为这个现象表明有些信息传递速度超过光速，这完全不符合相对论。爱因斯坦戏称其为“幽灵作用”。但是物理学家们经过80多年苦心孤诣，终于在本周报告了一项精密的实验，可以证明世界是怎么运作的。
To save physics from the spooky, Einstein invoked what he called hidden variables (though others might describe them as fiddle factors) that would convey information without breaking the universal speed limit. It took until 1964, though, to tame this woolly idea into testable equations. John Bell, a British physicist, worked out the maximum effect hidden variables could have on a given test. Any influence beyond that, his equations suggested, must be down to spooky action. The Bell inequality, as it became known, sparked decades of clever experiments—sending entangled photons or atoms hither and thither with detectors triggered by this or that—each designed to catch nature out, to banish hidden variables once and for all.
爱因斯言引用“隐藏变量”，以免物理学受“幽灵”之言蛊惑（其他物理学家称其为“瞎掰的变量”）。称“隐藏变量”可以不打破宇宙最快的光速，传递信息。然而这一模糊不清的理论直到1964年，才变成可以验证的等式。英国物理学家John Bell，计算出隐藏变量在实验中可以达到的最快速度。因此，任何大于此最快速度的现象，都是因为幽灵现象。这就是著名的贝尔不等式，它激发了科学家们数十年契而不舍的实验。从各处发射相纠缠光子，也从各种验测仪上获得数据，只是为了彻底推翻隐藏变量学说。
Yet a number of loopholes remained—ways that hidden variables might exert some influence, though the purported mechanisms became increasingly contrived as years and experimental finesse advanced. One was the detection loophole. Reliably catching a single photon, for example, is tricky; lots of them go amiss in a given experiment. But if an experiment does not capture all of its participants, the loophole idea goes, perhaps hidden variables convey information through the missing ones. Another was the communication loophole. If the two measurements happen near enough to one another, some invisible hidden-variable signal might be passing between them (as long as that signal does not go faster than light).
然而，这些实验总是存在一些漏洞，虽然科技日益进步，这些漏洞依然是隐藏变量影响实验结果的途径。其中一个便是探测漏洞。万无一失探测每一个光子，是无稽之谈。实验会漏掉大部分光子。但是如果无法探测所有的光子，隐藏变量可能利用漏掉的光子传递信息。另一个就是通信漏洞。若两个测量地太近，那么不可知的隐藏变量信号可能在两者中传递。（只要速度不高于光速）
Plenty of experiments have closed one or the other of these loopholes, for example by detecting particles that are more reliably caught than photons, or by sending photons so far apart that no slower-than-light signal could flit between them in time to have an effect. By now, most physicists reckon the hidden-variable idea is flawed. But no test had closed both loopholes simultaneously—until this week, that is.
大部分实验都或多或少避免了这些漏洞，例如使用较光子更易探测的粒子，或者加长实验距离因此低于光速的信号无法及时产生影响。目前，隐藏变量观点的错误已经是不争的事实。但是，本周之前，还没有一项实验可以同时规避两种漏洞。
Ronald Hanson of the University of Delft and his colleagues, writing in Nature, describe an experiment that starts with two electrons in laboratories separated by more than a kilometre. Each emits a photon that travels down a fibre to a third lab, where the two photons are entangled. That, in turn, entangles the electrons that generated the photons. The consequence is easily measured particles (the electrons) separated by a distance that precludes any shifty hidden-variable signalling.
Delft大学的Hanson和他的同事在Nature上介绍了一项实验：在两间距离超过1000米的实验室中，分别使用电子发射光子，光子沿光纤到达第三间实验室，互相纠缠。这样，原来的电子也产生了同样的反应。这样就可以得到结论：相距甚远的粒子（电子）绝不会受到隐藏变量的影响。
Over 18 days, the team measured how correlated the electron measurements were. Perhaps expectedly, yet also oddly, they were far more so than chance would allow—proving quantum mechanics is as weird as Einstein had feared.
实验小组在18天的实验中，证实电子之间关联性之强超出想象，量子力学也正如爱因斯坦害怕的那样奇怪。
Though this experiment marks an end to hidden variables, Dr Hanson says it is also a beginning: that of unassailably secure, quantum-enabled cryptography. It was shown in 1991 that the very Bell tests used to probe hidden variables could also serve as a check on quantum cryptography. A loophole-free Bell test, then, could unfailingly reveal if a hacker had interfered with the fundamentally random, quantum business of generating a cryptographic key. So-called device-independent quantum ciphers would, Dr Hanson says, be secure from hackers “even if you don’t trust your own equipment—even if it’s been given to you by the NSA”.
Hanson博士称，此实验不仅仅是隐藏变量的结束，它也是一个开端：完全安全，绝不可破解的量子加密技术。早在1991年，Bell 的实验就可以检验量子加密技术。现在，无漏洞的Bell实验，便可以百分百检测出任何试图截取密匙的黑客。“Hanson说，这样的独立量子加密技术，是黑客的雷池，就算你的安全级别低到不行的电脑正在接受NSA的数据。（NSA国家安全局）
There remains, alas, one hitch that could explain all these counterintuitive findings. Just maybe, every single event that will ever be, from experimenters’ choices of the means of measurement to the choice of article you will read next, were all predetermined at the universe’s birth, and all these experiments are playing out just as predetermined. That, however, is one for the metaphysicists.
然而，还有一点可以解释这些所有的违反直觉的发现。也许，从一切的开始，无论是科学家选择测量方法，还是你决定下一篇要读的文章，都在宇宙诞生之初就注定了，所有这些实验只不过是演绎所有预定了的剧本。然而，这一切是玄学的课题了。