Name: 量子計算機是如何工作的？ (How Does a Quantum Computer Work?)
Uploaded: 2021-01-14T10:25:28.000Z
Duration: 6 min 47 s

A classical computer performs operations using classical bits, which can be either zero or

一台傳統的計算機使用傳統的可以是0或者1的比特來做運算。

相比之下，一臺量子計算機用量子裏的比特或者quibits(量子比特)。而它們可以同時是0和1.

Now in contrast, a quantum computer users quantum bits or qubits.

正是這點給與一台量子計算機優越的計算能力。

And they can be both zero and one at the same time.

有一定數量的物體可以用作一個量子比特。一個單獨的光子，原子核或者一個電子。

And it is this that gives a quantum computer its superior computing power.

我遇到過一些研究人員使用燐元素

There are a number of physical objects that can be used as a qubit.

最外層電子當作一個量子比特。那麽這是怎麽工作的呢？哼，所有的電子

A single photon, a nucleus or an electron.

都有磁場，這樣他們基本上就像一根根的很小的磁鐵條。而這個性質稱爲spin（自旋）。

I met up with researchers who were using the outermost electron in phosphorous as a qubit.

如果你把它們放進一個磁場它們將順著磁場排列，就像

一個指南針順著地球的磁場排列。

Well, all electrons have magnetic fields, so they are basically like tiny bar magnets.

這樣這就是最低的能量狀態，因此你可以稱它0狀態或者我們對這個電子稱它

朝下自旋。現在你可以把它放到1的狀態，但那要些能量。

If you place them in a magnetic field they will align with that field, just like a compass

如果你把指南針的玻璃拿掉你可以把指針轉到另一個發現，但是

needle lines up with the magnetic field of the earth.

你得會在指針是加些力。你不得不把它推到另一面。

Now this is the lowest energy state, so you could call it the zero state or we call it

而那是最高的能量狀態。在原理是，如果你是那麽精細

真的把它正好是磁場相反的，它會停在那裏的。

Now you can put it in a one state, or spin up, but that takes some energy.

到現在為止這基本是只不過像是一個傳統的比特。它有兩個狀態，自旋朝上和

>> If you took out the glass from your compass you could turn the needle the other way, but

自旋朝下， 這就像是傳統的1和0。但是對有量子性的

you would have to apply some force to it.

物體是它們可以一下子處於兩種狀態。而你在測量自旋時

You have to push it to flip to the other side.

它或者是朝上或者是朝下。但是在你測量它的之前，這個電子可以存在於

一種量子的重叠，那裏的這些係數指示

In principle, if you were so delicate to really put it exactly against the magnetic field,

找到這電子處於一種或者另一種狀態的相對概率。

現在是很難來想象怎樣用這個來使這個量子計算機不可置信的計算能力

>> Now so far this is basically just like a classical bit.

而沒有對交互作用著的2個量子比特作個考慮。

It has got two states, spin up and spin down, which are like the classical one and zero.

But the funny thing about quantum objects is that thy can be in both states at once.

現在這2個電子有4種可能的狀態。

Now when you measure the spin it will be either up or down.

你也許和想，好吧，那就是像一臺傳統計算機的2個比特，對嗎？

But before you measure it, the electron can exist in what is called a quantum super position,

如果你有2個比特你可以寫0,0; 0,1; 1,0; 1,1。對嗎？

where these coefficients indicate the relative probability of finding the electron in one

但仍然衹有2比特的信息。對嗎？我只需要說的是來決定在你計算機編碼

Now it is hard to imagine how this enables this incredible computing power of quantum

4個數字中的那一個第一個比特（位）的值和第二位的值。

computers without considering two interacting quantum bits.

在這裏不是這樣， 量子力學允許我有

這4種狀態的每一種做重叠的位置。一次我可以寫一個量子力學的狀態，

這完全是合理的，某個係數乘以這個加上某個係數乘以那個加上某個係數

Now there are four possible states of these two electrons.

乘以那個加上某個係數加上某個係數乘以那個。

>> You could think that, well, that is just like two bits of a classical computer, right?

因此決定這個兩自旋系統，我需要給你4個數字，4個係數，

If you have two bits you can write zero, zero; zero, one; one, zero; one, one.

就像在經典的2個比特的例子中，我只需要給你2個比特。因此

這是你怎樣理解為什麽2個量子比特實際上包含著信息的4個比特。

我需要來給你4個數字來告訴你這個

But these are still just two bits of information.

系統的狀態，而在這裏我只需要2個。

現在如果我們做3個自旋，我們就會有8個不同的狀態不去這可以給你

All I need to say to determine which one of the four numbers you have in your computer

8個不同的數字來定義那些3個自旋，而在傳統上這做不過是3個比特。

code is the value of the first bit and the value of the second bit.

如果你繼續這樣下去，你所發現的是

Here, instead, quantum mechanics allows me to make super position of each one of these

So I can write a quantum mechanical state, which is perfectly legitimate, that is some

而，當然，指數冪的力量告訴你一旦你有了，比方說，300個那樣的量子比特

coefficient times this plus some coefficient times that plus some coefficient times that

在我們稱為(folient angle)[單葉角度]狀態，因此你一定能夠

來創造這些真有點發瘋的狀態即所有三種角度有一個重叠的位置同時是一個和

So determine the state of this two spin system, I need to give you four numbers, four coefficients,

另一個和另一個一直下去的話，然後就有像2的300次方的傳統比特，

whereas in the classical example of the two bits, I only need to give you two bits.

這就和宇宙中有的粒子一樣多了。

So this is how you understand why two qubits actually contain four bits of information.

但是有一個臆想不到的東西，雖然量子比特可以存在於任何狀態的組合，在

I need to give you four numbers to tell you the state of this system, whereas here I only

對它們作測量的時候他們一定落入這些基本狀態中的一種。而所以有關這狀態

的測量之前其他的信息就失去了。

Now if we make three spins, we would have eight different states and it could give you

所以通常你是不想要你想量子計算的最終結果的，那是一個很復雜的重叠的

eight different numbers to define the state of those three spins, whereas classical it

一個重叠的狀態。你只能測量這些基本

If you keep going, what you find is that the amount of equivalent classical information

狀態中的一個，像下，下，上，上。

contained by N qubits is two to the power N classical bits.

這樣你要的是這樣的一種方法來設計你要到最終計算

And, of course, the power of exponentials tells you that once you have, let’s say,

結果的邏輯運算，其最終的結果你可以

300 of those qubits in what we call the folient angle state, so you must be able to create

來測量，衹是一種獨特的狀態。

these really crazy states where there is a super position of all three angles being one

way and another way and another way and so on, then you have like two to the 300 classical

那不是微不足道的。而這基本上是...我有點在誇大，但我猜想這就是

bits, which is as many particles as there are in the universe.

在某種程度上的為什麽量子計算機不是

>> But there is a catch, although the qubits can exist in any combination of states, when

they are measured they must fall into one of the basis states.

And all the other information about the state before the measurement is lost.

不，它們不是的。它們並不是永遠更快的。  它們只對特殊類型的運算那裏

>> So you don’t want generally to have as the final result of your quantum computation

你同時有著所有這些量子上重叠，

something that is a very complicated super positional state, because our cannot measure

來做某些平行進行的運算。如果你用一種傳統的算法來得到結果，只不過是

想在高清晰度下看一個視頻或者在網上看看或者寫些東西，

You can only measure one of these basis states.

它們將。不會給你特別的改進的。

所以你不應該把量子計算機想成是每種

運算都是更快。事實上，每種運算可能

So what you want is to design the logical operations that you need to get to the final

比你在桌子上的計算機更慢些。而這是一種計算機對得到結果所需的運算次數

computational result in such a way that the final result is something you are able to

是冪級數上很小。所以改進並不是在

單獨的一次運算。這是你需要得到結果縂的運算次數。

而那只是在特定類型的運算，特定的算法下的情況。

這不是全部適用的，這就是為什麽它

And it is essentially ... I am kind of stretching things, but I guess it is to some degree the

不是一臺傳統計算機的取代品。

reason why quantum computers are not a replacement of classical computers.

They are only faster for special types of calculations where you can use the fact that

you have all these quantum super positions available to you at the same time, to do some

If you just want to watch a video in high definition or browse the internet or write

some documenting work, they are not going to give you any particular improvement if

you need to use a classical algorithm to get the result.

So you should not think of a quantum computer as something where every operation is faster.

In fact, every operation is probably going to be slower than in the computer you have

But it is a computer where the number of operations required to arrive at the result is exponentially

So the improvement is not in the speed of the individual operation.

It is in the total number of operations you need to arrive at the result.

But that is only the case in particular types of calculations, particular algorithms.

It is not universally, which is why it is not a replacement of a classical computer.