量子加密技术有望击碎“棱镜”

    上周在有关于莫斯科的新闻中，最热门的话题依然是围绕着美国的“泄密者”斯诺登，他目前正寻求在俄罗斯临时避难。与此同时，在这座城市的另一边，尼古拉斯•吉新博士上周宣布，他找到了能够防止类似美国国家安全局（National Security Agency）的机构再次窥探公民隐私的办法。以后的数据加密法将无法攻破、无懈可击。

    吉新是一名瑞士量子物理学家，他在探索和研究原子和光子等微观物理问题上走在世界前列。（光子是光的基本粒子。）早在2001年，吉新就与其他人共同创办了一家名叫ID Quantique 的公司，希望借此把他在量子物理世界里发现的各种奇特现象应用到商业世界中。当年量子学界基本上还处理在纯理论阶段，在实践上基本还处于空白，更多地只是在实验室里进行研究，而不是应用于真实世界。但是在过去十年里，量子技术已经大大地成熟起来，而且已经可以给现实世界带来很多实际的好处，其中就包括ID Quantique公司目前提供给各大银行和政府的数据加密技术——这也是一种几乎不可能被攻破的加密技术。

    吉新说：“这种新技术听起来似乎像是一种量子物理学的魔法，不过它当然不是魔法，只是一种非常现代化的科学技术。”不过与经典的通讯和加密方法相比，它的确具有某种“魔力”。经典密码学通常依赖某种数学算法，随机生成解密密钥，使信息发送者最终发送出一条加密信息，然后接收者可以通过解密密钥将信息还原。如果有某个第三方（在数据安全界的术语里称之为“敌方”）获取了密钥的拷贝，那么它就可以获取这份信息的拷贝，然后破译它——或者也可以凭借足够的时间和强大的计算能力，利用强大的算法攻破解密密钥。（据说美国国家安全局和世界上其他情报机构都是这样做的。）但是吉新的量子加密技术利用了量子物理学的某些奇特现象，可以使传送中的密钥无法被人拷贝，也无法被偷走和破解，除非发送者主动丢弃了这个密钥。

    ID Quantique公司的量子加密法使用的最主要的量子物理学原理叫“量子纠缠”。它在这种情况下是指两颗单独的光子被置于一个相关联的状态下。根据量子物理规则，这两颗纠缠的光子会无可避免地产生相互影响，如果其中一颗光子的状态被改变了，就必然会影响另一颗的状态，不管这两颗光子的位置是远在天边还是近在眼前，甚至是分别在地球的两端。发送者将其中一颗光子发送给接收者，那么发送者和接收者就各拥有了一颗光子。这些光子没有编入任何有用的信息——需要传递的信息还是通过常见的经典加密术加密，但是解密密钥通过一个随机数字生成器生成。（真正的随机数字生成器代表了由量子物理学派生的另一项技术——这个稍后再说）。

    如果敌方要想截获这个含有密钥信息的光子，就必须在正确的时刻出现在发送者和接收者之间，但即便做到这一点，他也不能窃取到任何有用的信息。因为根据量子物理学，任何试图干预传输中的量子的行为，都会改变仍然在发送者手中的另一颗纠缠量子的状态，从而可以向发送者发出警报。这时发送者可以简单地丢弃掉被拦截的密钥，然后重新生成一个。

    The news out of Moscow of late has been dominated by Edward Snowden, the American leaker of secret state documents who is currently seeking temporary asylum in Russia. Meanwhile, across town and to much less fanfare, Dr. Nicolas Gisin found himself explaining last week the solution to the very problems of data security and privacy intrusion Snowden brought to light in exposing the vast reach of the National Security Agency's data collection tools: data encryption that is unbreakable now and will remain unbreakable in the future.

    Gisin is a Swiss quantum physicist and a pioneer in the exploration and manipulation of the very small -- that is, the various "quanta" of the micro world, things like individual atoms and photons. (Photons are the elementary particle of light.) In 2001, Gisin co-founded a company called ID Quantique with the aim of converting the strange phenomena found in the quantum world into commercial applications. At that time, the quantum world was still very much a theoretical place, one more suited for the laboratory than employed for practical application. But over the last decade quantum technologies have matured such that they can offer many practical benefits, including the kind of data encryption that ID Quantique now provides to various banks and governments -- data security that is virtually impossible to breach.

    "It sounds like there's some quantum magic in this new technology, but of course it's not magic, it's just very modern science," Gisin says. But next to classical communication and encryption methods, it might as well be magic. Classical cryptography generally relies on algorithms to randomly generate encryption and decryption keys enabling the sender to essentially scramble a message and a receiver to unscramble it at the other end. If a third-party (known as an "adversary" in data security lingo) obtains a copy of the key, that person can make a copy of the transmission and decipher it, or -- with enough time and computing power -- use powerful algorithms to break the decryption key. (This is what the NSA and other agencies around the world are allegedly up to.) But Gisin's quantum magic taps some of the stranger known phenomena of the quantum world to transmit encryption keys that cannot be copied, stolen, or broken without rendering the key useless.

    The primary quantum tool at work in ID Quantique's quantum communication scheme is known as "entanglement," a phenomena in which two particles -- in this case individual photons -- are placed in a correlated state. Under the rules of quantum mechanics, these two entangled photons are inextricably linked; a change to the state of one photon will affect the state of the other, regardless of whether they are right next to each other, in different rooms, or on opposite sides of the planet. One of these entangled photons is sent from sender to receiver, so each possesses a photon. These photons are not encoded with any useful information -- that information is encoded using normal classical encryption methods -- but with a decryption key created by a random number generator. (True random number generators represent another technology enabled by quantum physics -- more on that in a moment.)

    Any adversary would have to place herself in between sender and receiver at just the right moment in order to intercept this key-encoded photon, but even that would not enable her to steal any useful information. Thanks to the laws of quantum mechanics, any tampering with the photon in transit would change the state of the entangled photon still in the sender's possession, raising a red flag. The sender could then simply discard the intercepted key and generate another.