Einstein's brilliant mistake: Entangled states - Chad Orzel

愛因斯坦大智的錯誤：量子糾纏態 - 查德．歐澤

譯者: Yu-Chen Chu 審譯者: 瑞文Eleven 林Lim

Albert Einstein played a key role in launching quantum mechanics

阿爾伯特‧愛因斯坦發表光電效應理論

through his theory of the photoelectric effect

確立他在量子力學中的地位。

but remained deeply bothered by its philosophical implications.

但他始終對其論述有所質疑。

And though most of us still remember him for deriving E=MC^2,

雖然我們大多知道他推導出E=MC^2，

his last great contribution to physics was actually a 1935 paper,

但他最後的偉大貢獻 應是1935年的論文，

coauthored with his young colleagues Boris Podolsky and Nathan Rosen.

與他年輕的同事鮑里斯‧波多爾斯基 和納森‧羅森共同發表(EPR論文)。

Regarded as an odd philosophical footnote well into the 1980s,

到1980年代仍被視為哲學上的悖論，

this EPR paper has recently become central to a new understanding of quantum physics,

但近年來EPR論文的重要性日漸提升， 也為量子力學帶來新視角。

with its description of a strange phenomenon

其中尤為重要的是

now known as entangled states.

對弔詭現象糾纏態的描述。

The paper begins by considering a source that spits out pairs of particles,

論文起頭假設來自同源的成對粒子，

each with two measurable properties.

每個粒子具兩種可測量的屬性，

Each of these measurements has two possible results

兩種屬性測量後各有兩種可能的結果

of equal probability.

且機率一樣。

Let's say zero or one for the first property,

我們假設第一種量測量結果非1即0，

and A or B for the second.

第二種結果可能為A或是B。

Once a measurement is performed,

一經量測，

subsequent measurements of the same property in the same particle

同一粒子同一種量的後續觀測

will yield the same result.

都會得到相同的結果。

The strange implication of this scenario

這情況下衍生出弔詭的論調，

is not only that the state of a single particle

其一為單一粒子的狀態

is indeterminate until it's measured,

在未測量之前都是不確定的，

but that the measurement then determines the state.

其二為測量的行為決定了狀態。

What's more, the measurements affect each other.

甚至發現，測量結果會相互影響。

If you measure a particle as being in state 1,

如果測量一粒子的結果為1，

and follow it up with the second type of measurement,

再進行第二種屬性的測量，

you'll have a 50% chance of getting either A or B,

得到A或B的結果各一半，

but if you then repeat the first measurement,

但如果重複第一種屬性的測量，

you'll have a a 50% chance of getting zero

仍有50%的機會結果會是0，

even though the particle had already been measured at one.

即便先前已經測量過的結果顯示為1。

So switching the property being measured scrambles the original result,

改變測量的屬性，就會重寫原本的測量結果，

allowing for a new, random value.

有機會得到一個不同的隨機測量值。

Things get even stranger when you look at both particles.

更加難懂的是，如果同時考量兩個粒子，

Each of the particles will produce random results,

任一粒子都會有隨機的結果，

but if you compare the two,

但如果比較兩者，

you will find that they are always perfectly correlated.

可以發現兩者相依相存。

For example, if both particles are measured at zero,

例如若兩者量測結果皆為0，

the relationship will always hold.

這個相對關係永遠不變。

The states of the two are entangled.

兩者的狀態糾纏在一起。

Measuring one will tell you the other with absolute certainty.

測量其一，則可確定另一個的結果。

But this entanglement seems to defy Einstein's famous theory of relativity

但這種糾纏現象似乎 牴觸愛因斯坦的相對論

because there is nothing to limit the distance between particles.

因為粒子間的距離沒有上限。

If you measure one in New York at noon,

如果距離拉大，如在紐約正午測量其一，

and the other in San Francisco a nanosecond later,

十億分之一秒後在舊金山測量另一個，

they still give exactly the same result.

兩者該得到相同的結果。

But if the measurement does determine the value,

但如果量測的行為決定狀態，這表示

then this would require one particle sending some sort of signal to the other

一方得傳送某種訊息給另一方 來確定彼此狀態，

at 13,000,000 times the speed of light,

傳遞速度還得是 光速的一千三百萬倍，

which according to relativity, is impossible.

以相對論來說這是不可能的。

For this reason, Einstein dismissed entanglement as "spuckafte ferwirklung,"

因此，愛因斯坦稱為 「spukhafte fernwirkung」，

or spooky action at a distance.

也就是，鬼魅似的遠距作用。

He decided that quantum mechanics must be incomplete,

他堅信量子力學有不完備處，

a mere approximation of a deeper reality in which both particles

所以不足以揭露並解釋

have predetermined states that are hidden from us.

兩個粒子具有的深層先决狀態。

Supporters of orthodox quantum theory lead by Niels Bohr

支持正統量子理論的尼爾斯‧波耳一派

maintained that quantum states really are fundamentally indeterminate,

認為量子狀態確實是不確定的，

and entanglement allows the state of one particle

而糾纏現象讓某一粒子的狀態，

to depend on that of its distant partner.

與距離甚遠的另一粒子緊密相關。

For 30 years, physics remained at an impasse,

三十年來，物理陷入僵局，

until John Bell figured out that the key to testing the EPR argument

直到約翰‧貝爾想出試驗EPR論點的方法，

was to look at cases involving different measurements on the two particles.

關鍵在於討論涉及兩個粒子 不同測量的狀況。

The local hidden variable theories favored by Einstein, Podolsky and Rosen,

愛因斯坦、波多爾斯基和羅森 支持的局域隱變數理論

strictly limited how often you could get results like 1A or B0

嚴格限制得到的固定結果， 如1A或是B0的次數，

because the outcomes would have to be defined in advanced.

因為這些結果事先已經固定。

Bell showed that the purely quantum approach,

貝爾從純量子的方法著手，

where the state is truly indeterminate until measured,

也就是說測量前屬性確實無法確定，

has different limits and predicts mixed measurement results

這樣的狀況下有不同的限制 且預示混雜的測量結果，

that are impossible in the predetermined scenario.

其結果不可能在事先 安排好的狀況下達成。

Once Bell had worked out how to test the EPR argument,

貝爾一提出驗證EPR論點的方法，

physicists went out and did it.

物理學家紛紛出馬試驗。

Beginning with John Clauster in the 70s and Alain Aspect in the early 80s,

起於70 年代的約翰·克勞澤 及80年代早期的阿蘭‧阿斯佩，

dozens of experiments have tested the EPR prediction,

許許多多的實驗 反覆驗證EPR預測，

and all have found the same thing:

全數得到相同的結論：

quantum mechanics is correct.

證明量子力學是正確的。

The correlations between the indeterminate states of entangled particles are real

具不確定性的兩糾纏粒子間 確實存在相關性。

and cannot be explained by any deeper variable.

沒有更進一步的變數可以解釋此現象。

The EPR paper turned out to be wrong but brilliantly so.

EPR論文雖然不成立，但是個精彩的錯誤。

By leading physicists to think deeply about the foundations of quantum physics,

它引領物理學家深究量子力學的根基，

it led to further elaboration of the theory

使理論得以更為完備，

and helped launch research into subjects like quantum information,

也有更多研究投入各個面向， 好比量子資訊，

now a thriving field with the potential to develop computers of unparalleled power.

現在是個熱門的領域， 有潛力發展出效能無敵的電腦。

Unfortunately, the randomness of the measured results

但礙於測量結果的隨機性，

prevents science fiction scenarios,

科幻場景還是不可能成真，

like using entangled particles to send messages faster than light.

好比利用粒子糾纏現象以超光速傳遞訊息。

So relativity is safe, for now.

所以相對論暫時還站得住腳。

But the quantum universe is far stranger than Einstein wanted to believe.

但量子的世界，確實難搞， 不像愛因斯坦堅信的客觀實在。