
English: 
It is often said that the power of quantum
computers comes from quantum parallelism
which means that instead of processing each input
one by one, quantum mechanics allows us to
do operations on a super position of all the
possible inputs at the same time.
So let me give you an example of a quantum algorithm
that uses quantum parallelism
to arrive at the solution in a much smaller number of steps compared to a classical computer.
It's called the quantum search algorithm and it's used every time I have a large database
and I'm trying to recognise a certain entry within that database.
Imagine I have a telephone number and I want to find out who it belongs to using an old telephone book.
The names are in order, but the numbers are not.
So in the classical world, the only option I have is to start checking all the entries in the book
until I recognise my number and find the name associated with it.

Chinese: 
安德里亚·莫雷洛教授（Andrea Morello）
为您讲述量子计算机的概念
都说量子计算机的能力来源于量子平行性
意味着可以不必依次处理每一条输入
我们可以利用量子力学的态叠加
同时操作所有可能的输入
让我举一个量子算法的例子
来说明与传统计算机相比
如何运用量子平行性
通过较少步骤得到解法
这被称作量子搜索算法
如果我的数据库过于庞大
可以用它从中识别出一个特定条目
假如我得到了一个电话号码
想用一本老式电话簿查询这个号码的主人
里面的人名是按顺序排列的 但号码不是
所以在传统世界里
我只能把电话簿里的所有条目翻个遍

Chinese: 
直到找出我想要的号码和对应的人名
如果这本电话簿里有100万个号码
我需要尝试平均50万次
才能找到想要的号码
而量子计算机能做的远超于此
为了从中找到我想要的人名
它所需的运行步数等于条目总量的平方根
所以只用试1000次而不是50万次
来看看它是怎么运作的
在这个简单的实例中
我会用三个量子比特编码八串数字
每串数字都与电话簿中的一个人名相关
我想找到代码1-0-1对应的那个人名
首先 我为这三个量子比特的值
所有可能的组合创建了一个量子态叠加
在量子力学中 
我们给态叠加的每一部分赋值
它可以显示出态叠加里包含了多少特定代码
每一个代码幅值的平方
就是在我测量过所有比特之后

English: 
If there are one million names in the book, it will take me on average, half a million attempts
before I find the one I'm looking for.
A quantum computer can do much better than this.
It will find the name I'm looking for using a number of steps
that is only the square root
of the number of items in the list.
So in this case, one thousand steps instead of half a million.
Let's see how it works.
I will show a really simple example where I use three quantum bits to encode eight numbers,
each one associated to a name in the telephone book.
I want to find which name goes with the code 1-0-1.
First of all, I create a quantum super position of all the possible combinations of values for the three qubits.
In quantum mechanics, we assign to every part of the super position
an amplitude that tells us how much of that particular code is included in the super position.
The square of of the amplitude for each code is the probability that if I measure the qubits,

English: 
I find them in that particular code.
So in this case, all the amplitudes are 1 over the square root of 8
so that the probability to measure each  one of the eight codes would be one eighth.
Next thing we do is apply a global operation to all the qubits,
such that the amplitude
of the code I'm looking for changes sign.
Whereas all the others are left untouched.
Then I apply another operation that amplifies
the difference between each amplitude and
those of the equal super position states.
Now you see that the amplitude of the 1-0-1
code is greatly amplified
because it's very far from the average, whereas the others are slightly reduced.
If I repeat the sequence  just one more time,
the amplitude is almost completely concentrated on the 1-0-1 code
so that if I measure my register of qubits
after these two steps,
I will find the 1-0-1 code with about 95% probability.

Chinese: 
能够找到那个特定代码的概率
在这个例子中
所有幅值都是1除以8的平方根
所以测量八个代码得到的概率是1/8
接下来
只用对所有的比特进行总体作业
我要找的那个代码幅就会改变正负号
而其余的都不会有变化
然后我进行另一项操作
放大每个幅值和相同态叠加之间的差别
现在代码1-0-1的幅值被明显放大
因为它远高于离平均值
而其他的略有减少
我只用重复一次操作步骤
幅值几乎全部集中在了代码1-0-1上
所以如果在这两个步骤之后
我再测量之前寄存的那些比特
有95%的概率能够找到代码1-0-1
如果按照传统方法 我需要尝试四次

English: 
Classically, I would have needed four attempts to find that code with better than 50% probability.
This simple example should give you a sense of what makes quantum algorithm special.
They operate globally on a super position of quantum codes
and they are designed in such a way
that at every step,
the initial quantum super position converges towards the correct answer
which is no longer a super position
so we can actually measure it.

Chinese: 
有大于50%的概率才能找到那个代码
这个简单的例子应该让你
对量子算法的特别之处有所了解
它们被广泛地应用在量子编码的态叠加之中
被设计成可以通过每一步操作
使最初的量子态叠加趋向于正确答案
最终不再是态叠加
让我们可以进行实际测量
本视频由澳大利亚新南威尔士大学电视台制作
许梦鸽翻译
