Page 9 - Science Focus (Issue 017)

never determine all physical properties of a The characteristics of a pair of entangled particles,
particle . Measurement inevitably alters an object. A and B, can be described as a single system, such
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For example, to see an object and know its position, that the state of each member can be measured
we need to shine light onto it. Light is composed of and used to infer that of the other. Moreover,
tiny particles called photons , and while a photon the change induced in A is also mirrored in its
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is too small to influence a macroscopic object, as empathetic twin B even though the two particles
we reduce the object's scale to atomic level, the are separated far apart — that’s why Einstein called
force of a photon becomes significant. Therefore, this “spooky action in a distance.” Imagine a pair of
once we determined the position using the photon, mischievous, telepathic twins who hate giving the
it would have kicked the particle away. Figure 1 same answer to the same question — if one answers
is an analogy in our macroscopic world: if we are yes, the other will answer no — and this is how a
only allowed to find out the position of a football pair of entangled particles behave. Measurements
in a dark room by shooting billiard balls, we can can be made to gather information on the system,
imagine that, even if we knew where the football but they inevitably introduce further changes to the
was from the trajectory of the billiard ball, the system.
football would have already been struck to a new Entanglement is the main principle behind the
position. This, then, seems to rule out any hope of quantum teleportation achieved by a group of six
remotely transmitting a particle’s data.
researchers at IBM in 1993 [2]. Here’s an abridged
Fortunately, we do not need to explicitly know version of their procedure: Suppose, as Figure 2
everything about a particle in order to transmit its shows, Alice and Bob each gets one particle from
state. The key is a property known as entanglement. the entangled pair A-B. Alice, who wants to transmit

Quantum Teleportation, or How to
量子遙傳 ─

Fax a Particle to the Orbit 粒子傳真大法 By Terrence Tai 戴煒庭

到一顆粒子的位置後，這顆粒子已經被光子彈走了。 圖一 IBM 的六個研究者在 1993 年設計的量子遙傳方法
是一個在宏觀世界的比喻：如果我們只能以桌球撞擊足球 [2]，就是運用了量子糾纏作為基本原理。以下是其簡略版
的方式在暗室中找出足球的位置，我們可以想像，就算我 的過程：如圖二所示，一開始 Alice 和 Bob 從一對相互糾
們從桌球的軌跡計算到足球原本的位置，足球已經被撞到 纏的粒子 A-B 中分別獲得了一顆粒子。 Alice 想把另一顆
一個新的位置了。這樣看起來，遙距傳送粒子資訊的方法似

乎變得沒有可能。

幸好的是，我們可以在不需要知道粒子的所有資訊下
傳送它的量子態，其關鍵在於名為量子糾纏（quantum
entanglement）的特性。相互糾纏的粒子 A 和 B 的特性
可以被描述成一整個系統，我們可以只量度其中一顆粒子
的特性，從而推斷出另外一顆的特性。除此之外，更加奇怪
的是粒子 A 和 B 可以相隔萬里，但我們對粒子 A 的施加的
改變仍然會共鳴到粒子 B — 這就是愛因斯坦把其稱為「詭
異的超距作用（spooky action at a distance）」的原因。
想像一對有心靈感應能力、喜歡鬥嘴的雙胞胎，他們從不
會就同一條問題給相同的答案 — 如果一個說是，另一個
就會說不 — 這正是相互糾纏粒子的行為模式。我們可以
對其進行測量，蒐集這個系統的資訊，不過所有測量都無
可避免地會引起更多的改變。

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