Hidden no more 再無‘隱藏’

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世紀(jì)30年代，量子學(xué)中的一個(gè)現(xiàn)象讓愛因斯坦疑惑不解。這個(gè)現(xiàn)象就是量子糾纏現(xiàn)象。不論距離遠(yuǎn)近，它讓兩個(gè)物體如亞原子粒子的命運(yùn)永遠(yuǎn)交纏在一起。如果說，相糾纏的一對(duì)粒子的其中一個(gè)自旋光子發(fā)生改變，根據(jù)該理論，另外一個(gè)自旋光子會(huì)瞬間產(chǎn)生相應(yīng)的反應(yīng)，即使它們?cè)阢y河的兩端。這個(gè)現(xiàn)象對(duì)于愛因斯坦和其他物理學(xué)家來說簡直是噩耗，因?yàn)檫@個(gè)現(xiàn)象表明有些信息傳遞速度超過光速，這完全不符合相對(duì)論。愛因斯坦戲稱其為“幽靈作用”。但是物理學(xué)家們經(jīng)過80多年苦心孤詣，終于在本周報(bào)告了一項(xiàng)精密的實(shí)驗(yàn)，可以證明世界是怎么運(yùn)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.

愛因斯言引用“隱藏變量”，以免物理學(xué)受“幽靈”之言蠱惑（其他物理學(xué)家稱其為“瞎掰的變量”）。稱“隱藏變量”可以不打破宇宙最快的光速，傳遞信息。然而這一模糊不清的理論直到1964年，才變成可以驗(yàn)證的等式。英國物理學(xué)家John Bell，計(jì)算出隱藏變量在實(shí)驗(yàn)中可以達(dá)到的最快速度。因此，任何大于此最快速度的現(xiàn)象，都是因?yàn)橛撵`現(xiàn)象。這就是著名的貝爾不等式，它激發(fā)了科學(xué)家們數(shù)十年契而不舍的實(shí)驗(yàn)。從各處發(fā)射相糾纏光子，也從各種驗(yàn)測(cè)儀上獲得數(shù)據(jù)，只是為了徹底推翻隱藏變量學(xué)說。

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，這些實(shí)驗(yàn)總是存在一些漏洞，雖然科技日益進(jìn)步，這些漏洞依然是隱藏變量影響實(shí)驗(yàn)結(jié)果的途徑。其中一個(gè)便是探測(cè)漏洞。萬無一失探測(cè)每一個(gè)光子，是無稽之談。實(shí)驗(yàn)會(huì)漏掉大部分光子。但是如果無法探測(cè)所有的光子，隱藏變量可能利用漏掉的光子傳遞信息。另一個(gè)就是通信漏洞。若兩個(gè)測(cè)量地太近，那么不可知的隱藏變量信號(hào)可能在兩者中傳遞。（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.

大部分實(shí)驗(yàn)都或多或少避免了這些漏洞，例如使用較光子更易探測(cè)的粒子，或者加長實(shí)驗(yàn)距離因此低于光速的信號(hào)無法及時(shí)產(chǎn)生影響。目前，隱藏變量觀點(diǎn)的錯(cuò)誤已經(jīng)是不爭的事實(shí)。但是，本周之前，還沒有一項(xiàng)實(shí)驗(yàn)可以同時(shí)規(guī)避兩種漏洞。

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大學(xué)的Hanson和他的同事在Nature上介紹了一項(xiàng)實(shí)驗(yàn)：在兩間距離超過1000米的實(shí)驗(yàn)室中，分別使用電子發(fā)射光子，光子沿光纖到達(dá)第三間實(shí)驗(yàn)室，互相糾纏。這樣，原來的電子也產(chǎn)生了同樣的反應(yīng)。這樣就可以得到結(jié)論：相距甚遠(yuǎn)的粒子（電子）絕不會(huì)受到隱藏變量的影響。

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.

實(shí)驗(yàn)小組在18天的實(shí)驗(yàn)中，證實(shí)電子之間關(guān)聯(lián)性之強(qiáng)超出想象，量子力學(xué)也正如愛因斯坦害怕的那樣奇怪。

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博士稱，此實(shí)驗(yàn)不僅僅是隱藏變量的結(jié)束，它也是一個(gè)開端：完全安全，絕不可破解的量子加密技術(shù)。早在1991年，Bell 的實(shí)驗(yàn)就可以檢驗(yàn)量子加密技術(shù)?，F(xiàn)在，無漏洞的Bell實(shí)驗(yàn)，便可以百分百檢測(cè)出任何試圖截取密匙的黑客?！癏anson說，這樣的獨(dú)立量子加密技術(shù)，是黑客的雷池，就算你的安全級(jí)別低到不行的電腦正在接受NSA的數(shù)據(jù)。（NSA國家安全局）

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，還有一點(diǎn)可以解釋這些所有的違反直覺的發(fā)現(xiàn)。也許，從一切的開始，無論是科學(xué)家選擇測(cè)量方法，還是你決定下一篇要讀的文章，都在宇宙誕生之初就注定了，所有這些實(shí)驗(yàn)只不過是演繹所有預(yù)定了的劇本。然而，這一切是玄學(xué)的課題了。