经济学人下载:密码术 量子之慰

Science and technology
科学技术

Cryptography
密码术

The solace of quantum
量子之慰

Eavesdropping on secret communications is about to get harder
现在想窃听秘密联系会越来越难

CRYPTOGRAPHY is an arms race between Alice and Bob, and Eve.
艾莉斯，鲍伯和伊芙之间正在进行一场密码术的军备竞赛。

These are the names cryptographers give to two people who are trying to communicate privily, and to a third who is trying to intercept and decrypt their conversation.
这里所说的艾莉斯，鲍伯和伊芙是译解密码者的代号，前两位在努力进行秘密联系，而第三位伊芙则试图阻止他们的联系，并且在尽力破解他们的联系密码，希望能够知道联系的具体内容。

Currently, Alice and Bob are ahead—just.
目前，艾莉斯和鲍伯只是暂时领先，

But Eve is catching up.
但是伊芙正在迎头赶上。

Alice and Bob are therefore looking for a whole new way of keeping things secret.
因此，艾莉斯和鲍伯正在寻找另一种全新的联系方式，以保证他们联系的秘密性。

And they may soon have one, courtesy of quantum mechanics.
借助于量子力学，他们可能很快就会找到。

At the moment cryptography concentrates on making the decrypting part as hard as possible.
眼下，密码系统正集中力量让解密部分尽可能地难以实施。

The industry standard, known as RSA , relies on two keys, one public and one private.
被称为RSA的行业标准，取决于两种密钥，一种是公开密钥，另一种是秘密密钥。

These keys are very big numbers, each of which is derived from the product of the same two prime numbers.
这此密钥都是非常庞大的数字，每一个都来源于两个相同素数的乘积。

Anyone can encrypt a message using the public key, but only someone with the private key can decrypt it.
任何人都可以使用公开密钥加密消息，但只有拥有秘密密钥的人才能解密。

To find the private key, you have to work out what the primes are from the public key.
为了找到这个秘密密钥，人们不得不进行大量的运算，从公开密钥中找到这两个素数。

Make the primes big enough—and hunting big primes is something of a sport among mathematicians—and the task of factorising the public key to reveal the primes, though possible in theory, would take too long in practice.
虽然，从理论上讲，1、要保证素数足够大—发现大的素数是数学家们热衷的一种游戏，2、对公开密钥进行因式分解希望找到这些素数，这两项任务是可能的，但是在实际中将会花费很长时间。

Since the 1970s, though, the computers that do the factorisation have got bigger and faster.
但是，从二十世纪七十年代以来，能够进行因式分解的电脑越来越大，速度越来越快。

Some cryptographers therefore fear for the future of RSA.
一些密码破译者因此开始担心RSA的未来。

Hence the interest in quantum cryptography.
因此，他们把眼光放到了量子密码术上。

Alice, Bob and Werner, too?
艾莉斯，鲍伯，沃莫也这样吗？

The most developed form of quantum cryptography, known as quantum key distribution, relies on stopping interception, rather than preventing decryption.
量子密钥分配是量子密码学最流行的方式，它主要是依靠阻止拦截，而不是防止解密上。

Once again, the key is a huge number—one with hundreds of digits, if expressed in the decimal system.
这个系统的密钥还是一个庞大的数字—如果以十进制表示的话，一个就有几百个数字。

Alice sends this to Bob as a series of photons before she sends the encrypted message.
艾莉斯在发送加密信息之前，会先发送一系列的光量子给鲍伯。

For Eve to read this transmission, and thus obtain the key, she must destroy some photons.
因为伊芙要想读到这个信息，从而获得密钥，她必须要破坏一些光量子。

Since Bob will certainly notice the missing photons, Eve will need to create and send identical ones to Bob to avoid detection.
因为鲍伯肯定会注意到这些消失的光子，伊芙就必须创造并向鲍伯发送相同的光子，才能不会被发觉。

But Alice and Bob can stop that by using two different quantum properties, such as the polarities of the photons, to encode the ones and zeros of which the key is composed.
但是艾莉斯和鲍伯通过将两种不同属性—比如不同极性的光子--的量子转译成1和0，以阻止伊芙的拦截。

According to Werner Heisenberg's Uncertainty Principle, only one of these two properties can be measured, so Eve cannot reconstruct each photon without making errors.
根据海森堡的不确定性原理，伊芙只能捕捉到这两种不同属性光子中的一种，所以她无法完全再造出一模一样的光子。

If Bob detects such errors he can tell Alice not to send the actual message until the line has been secured.
如果鲍伯发现这种错误的话，他就会通知艾莉斯，在通信渠道安全之前，不要发送真正的信息。

One exponent of this approach is ID Quantique, a Swiss firm.
运用这种编码方式的典型代表是一家叫ID Quantique的瑞士公司。

In collaboration with Battelle, an American one, it is building a 700km fibre-optic QKD link between Battelle's headquarters in Columbus, Ohio, and the firm's facilities in and around Washington, DC.
在与一家名称巴特尔的美国公司合作过程中，该公司在巴特尔位于俄亥俄州哥伦布市的总部，和该公司在华盛顿特区附近及市区的工厂之间，建立了一条700公里的光纤量子密码的链接。

Battelle will use this to protect its own information and the link will also be hired to other firms that want to move sensitive data around.
巴特尔将使用这个光子链接来保护它的自身信息，同时这个链接也会在其它打算频繁交换信息的公司中使用。

QuintessenceLabs, an Australian firm, has a different approach to encoding.
一家名叫QuintessenceLabs的澳大利亚公司的编码方式却不一样。

Instead of tinkering with photons' polarities, it changes their phases and amplitudes.
这家公司并没有在光子的极性上做文章，而是改变了它们的相位和振幅。

The effect is the same, though: Eve will necessarily give herself away if she eavesdrops. Using this technology, QuintessenceLabs is building a 560km QKD link between the Jet Propulsion Laboratory in Pasadena, California, which organises many of NASA's unmanned scientific missions, and the Ames Research Centre in Silicon Valley, where a lot of the agency's scientific investigations are carried out.
但是结果却是一样：如果伊芙窃听的话，她肯定会露出马脚的。该公司正使用这个技术，在加利福尼亚州帕萨迪纳市的喷气推进实验室—该实验室组织了多次美国航空航天局无人驾驶的科研任务，和进行大量机构科学调查的硅谷艾姆斯研究中心之间建立一条560公里的QKD链接。

A third project, organised by Jane Nordholt of Los Alamos National Laboratory, has just demonstrated how a pocket-sized QKD transmitter called the QKarD can secure signals sent over public data networks to control smart electricity grids.
量子运用于密码术上的第三个项目，则是由洛斯阿拉莫斯国家实验室的简-纳德特组织的。这个项目演示了，一个口袋大小的称为OKarD的QKD发报机是如何捕捉发送到公共数据信息网的信号，来控制智能电网的。

Smart grids balance demand and supply so that electricity can be distributed more efficiently.
智能电网可以平衡电力供应与需求，从而让电力分配更有效。

This requires constant monitoring of the voltage, current and frequency of the grid in lots of different places—and the rapid transmission of the results to control centres.
这需要对不同地区电网的电压，电流和频率进行不间断地监测，同时监测结果要快速地传回到控制中心。

That transmission, however, also needs to be secure in case someone malicious wants to bring the system down.
然而，也需要确保这种传输的安全，以防某些人恶意地破坏这个系统。

In their different ways, all these projects are ambitious.
虽然他们防止窃听的方式不一样，但是这些项目都充满着远大的抱负。

All, though, rely on local fixed lines to carry the photons.
但是，它们都要依靠当地固定的线路来传送光子。

Other groups of researchers are thinking more globally.
另外一些研究者的思维则更加全球化。

To do that means sending quantum-secured data to and from satellites.
要想达到这些研究者的要求，就必须通过卫星来传送量子安全信息。

At least three groups are working on this: Thomas Jennewein and his team at the Institute for Quantum Computing in Waterloo, Canada; a collaboration led by Anton Zeilinger at the University of Vienna and Jian-Wei Pan at the University of Science and Technology of China; and Alex Ling and Artur Ekert at the Centre for Quantum Technologies in Singapore.
现在至少有三个研究团体都在进行着这方面的研究。分别是加拿大滑铁卢的量子研究所托的马斯-詹内怀恩和他的团队，维也纳大学的安东-塞林格和中国科技大学的潘建国为首的合作团队，还有新加坡量子技术中心的阿莱克斯-林和阿图-恩科特团队。

Dr Jennewein's proposal is for Alice to beam polarisation-encoded photons to a satellite.
詹内怀恩博士的计划建立在以艾莉斯为基础上的，她计划发送偏振编码的光子给卫星。

Once she has established a key, Bob, on another continent, will wait until the satellite passes over him so he can send some more photons to it to create a second key.
一旦她建立起一个密钥，那么当卫星运行到位于另一个大陆的鲍伯附近时，鲍伯就可以发送更多的光子给它，就建立起了第二个密钥。

The satellite will then mix the keys together and transmit the result to Bob, who can work out the first key because he has the second.
卫星届时会把这两个密钥混合起来，再把结果传输给鲍伯，拥有第二个密钥的鲍伯就可以解开第一个密钥。

Alice and Bob now possess a shared key, so they can communicate securely by normal terrestrial networks.
艾莉斯和鲍伯则会拥有一个共享密钥，所以他们能够通过正常的地面网络进行安全地联系。

Dr Jennewein plans to test the idea, using an aircraft rather than a satellite, at some point during the next 12 months.
詹内怀恩博士计划在未来一年内的某个时候，用飞机，而不是卫星测试一下这个想法。

An alternative, but more involved, satellite method is to use entangled photon pairs. Both Dr Zeilinger's and Dr Ling's teams have been trying this.
一个更复杂的卫星方法是运用混杂在一起的光子对。塞林格博士和林博士所领导的两个团队一直专攻于这个方向。

Entanglement is a quantum effect that connects photons intimately, even when they are separated by a large distance.
量子缠结是一种即使光子相隔很远也能快速连接的量子效应。

Measure one particle and you know the state of its partner.
如果能测到一个粒子，你就能知道它同伴的状态。

In this way Alice and Bob can share a key made of entangled photon pairs generated on a satellite.
通过这种方式，艾莉斯和鲍伯能够共享一个在卫星上产生的，缠结光子对组成的密钥。

Dr Zeilinger hopes to try this with a QKD transmitter based on the International Space Station.
寒林格博士希望能够用国际空间站的QKD发射器进行这项试验。

He and his team have been experimenting with entanglement at ground level for several years.
他和他的团队在地面上进行这种缠结试验已经几年了。

In 2007 they sent entangled photon pairs 144km through the air across the Canary Islands.
2007年，他们利用空气让光子对穿过了加那利群岛，传输了144公里。

Dr Ling's device will test entanglement in orbit, but not send photons down to Earth.
林博士的设备将会在轨道上测试缠结效应，但是不会把光子传回到地球。

If this sort of thing works at scale, it should keep Alice and Bob ahead for years.
如果这方面工作的规模不断扩大，那么艾莉斯和鲍伯将会保持领先很多年。

As for poor Eve, she will find herself entangled in an unbreakable quantum web.
对于可怜的伊芙来说，她会发现自己身陷于一个牢不可破的量子网中无法解脱。