A blockchain is a mathematical structure that stores data securely over time. The idea has risen to fame on the back of the Bitcoin boom. Bitcoin relies on blockchains to securely store its related currency transactions.
But the same technology can store any kind of data—shipping data, the progress of computer programs, smart contracts, and so on. Indeed, blockchains look set to become one of the enabling technologies of the 21st century.
And yet they have an Achilles’ heel. The security of a blockchain is guaranteed by standard cryptographic functions. These are relatively secure because breaking them requires huge computing resources, which are not generally available.
That looks set to change with the emergence of powerful quantum computers. It will be child’s play for such devices to break this kind cryptographic protection. But quantum computers cannot break quantum cryptographic codes, so various groups have suggested adding quantum cryptography to blockchains to guarantee their security.
There is a better, more fundamental solution, say Del Rajan and Matt Visser at the Victoria University of Wellington in New Zealand. Quantum cryptography merely adds a quantum layer to the standard blockchain protocol. Instead, they suggest making the entire blockchain a quantum phenomenon.
Their idea is to create a blockchain using quantum particles that are entangled in time. That would allow a single quantum particle to encode the history of all its predecessors in a way that cannot be hacked without destroying it. Such a protocol relies on the laws of physics to guarantee security. However, it also leads to somebody unusual side effects. “This decentralized quantum blockchain can be viewed as a quantum networked time machine,” say Rajan and Visser.
First some background. A blockchain is simply a ledger that records information of a certain type—currency transactions, for instance. The transactions are continually added to a database called a block, but at the end of a given time period, the block is encrypted using a mathematical device called a hashing function. This produces a unique number that can be used to represent the data exactly.
This unique number is then included in the next block with the next set of transactions. After a time, it is all encrypted using the hashing function to produce a new unique number. This is added to the next block. And so on, creating a chain of blocks that are all nested inside the latest one—hence the name blockchain.
Anybody attempting to falsify the historical record would need to find a way to alter the data in a way that does not change the outcome of the hashing function. And that is so computationally challenging that it is considered impossible with a classical computer. But it is possible with the kind of quantum computers that will soon be available.
So Rajan and Visser have come up with a different approach that relies on a fully quantum version of a blockchain. The phenomenon at the heart of their approach is called entanglement. When two quantum particles are entangled, they share the same existence. This happens when they interact at the same point in space and time. After that, a measurement on one immediately influences the other, no matter how far apart they may be.
What guarantees security is that entanglement is extraordinarily fragile. A measurement on one of a pair of entangled particles immediately destroys the link. So if a malicious user attempts to interfere with one of the pair, it is immediately obvious to the other.
Just as particles can become entangled across space, they can also become entangled over time. So a particle existing in the present can be entangled with one that existed in the past. And a measurement on it immediately influences its predecessor.
That leads to some subtle and counterintuitive phenomena. For example, there is a special quantum sense in which it becomes possible to influence the past. Of course, there are strict limits on what this makes possible. It’s not possible, for example, to set in train a series of events that will kill your grandparents, thus ensuring you never existed. That kind of paradox isn’t allowed.
But it does become harder to distinguish between cause and effect. Another effect is that it becomes possible to increase the amount of information that can be transmitted through time.
It is this type of temporal entanglement that Rajan and Visser exploit to produce a quantum blockchain. The basic idea is to encode data on a quantum particle. This becomes the first quantum block.
When more data is available, this is combined with the data from the first particle in a quantum operation that entangles it with a second particle. The former is then discarded, and the record of the first block of transactions is combined with the second block. The data from a third block can be added in the same way, creating a chain.
This chain is secure because anybody attempting to tamper with it immediately invalidates it. That’s the advantage of quantum entanglement.
This quantum blockchain has another advantage: the earlier blocks are completely tamper-proof. “The attacker cannot even attempt to access the previous photons since they no longer exist,” say Rajan and Visser. “Entanglement in time provides a far greater security benefit than an entanglement in space.”
What’s more, most of the technology to make this work already exists, at least in proof-of-principle form. “All the subsystems of this design have already been shown to be experimentally realized,” say Rajan and Visser.
That’s interesting work that is likely to become more relevant as powerful quantum computers begin to emerge. IBM already has a 50-qubit quantum computer, and more powerful machines are in the pipeline. It’s only a matter of time before they become capable of undermining trust in blockchains.
But a key part of the infrastructure necessary to make this kind of quantum blockchain work is not yet available: a quantum web. This is a network that can transmit quantum information via quantum routers without destroying its quantum properties. This kind of system is currently being designed and expected to be rolled out in Europe, the US, and China in the coming months or years.
Indeed, the job of building such a system is essentially an engineering task rather than one of fundamental physics. So it’s just a matter of time before a quantum blockchain becomes possible. Whether it will be this protocol that emerges as the best is another question, of course.
Perhaps Rajan and Visser could put their quantum time machine to good use by finding out what technology eventually triumphs in the future!
请参照译文：如果量子计算机威胁区块链，量子链可能是防御
区块链是一种随时间安全存储数据的数学结构。这个想法在比特币繁荣时期已经上升到了名声。比特币依靠区块链来安全地存储其相关的货币交易。
但同样的技术可以存储任何种类的数据传输数据，计算机程序的进展情况，智能合约等等。确实，区块链看起来将成为21世纪的有利技术之一。
然而他们有一个致命弱点。标准密码功能保证了区块链的安全性。这些是相对安全的，因为打破它们需要巨大的计算资源，而这些资源并不普遍。
随着强大的量子计算机的出现，这看起来会发生改变。这种设备打破这种密码保护将是孩子们的玩耍。但是量子计算机不能破解量子密码，所以各个团体都建议将量子密码学加入区块链以保证其安全性。
新西兰惠灵顿维多利亚大学的Del Rajan和Matt Visser说，有一个更好，更根本的解决方案。量子密码学仅仅为量子层添加标准区块链协议。相反，他们建议让整个区块链成为一个量子现象。
他们的想法是使用时间缠绕的量子粒子来创建区块链。这将允许单个量子粒子以不破坏它的方式进行破解的方式对所有前辈的历史进行编码。这样的协议依靠物理定律来保证安全。但是，这也会导致一些不寻常的副作用。“这种分散的量子链可以被看作是一个量子网络化的时间机器，”Rajan和Visser说。
首先是一些背景。区块链只是一个记录某种类型货币交易信息的分类帐，例如。事务不断被添加到一个称为块的数据库中，但在给定时间段结束时，块将使用称为散列函数的数学设备进行加密。这产生一个唯一的数字，可以用来精确地表示数据。
然后这个唯一的号码被包含在下一个块中，并包含下一组事务。一段时间后，它都使用哈希函数进行加密以产生一个新的唯一编号。这被添加到下一个块。依此类推，创建了一系列全部嵌套在最新版块中的块，因此可以创建名称区块链。
任何试图伪造历史记录的人都需要找到一种方法来改变数据，而不会改变哈希函数的结果。这在计算上是如此具有挑战性，以至于在经典计算机中被认为是不可能的。但是即将推出的这种量子计算机是可能的。
所以Rajan和Visser提出了一种不同的方法，它依赖于区块链的完全量子版本。他们的方法核心的现象称为纠缠。当两个量子粒子纠缠在一起时，它们共享相同的存在。当他们在空间和时间的相同点进行交互时会发生这种情况。之后，无论距离有多远，对其中一个的测量立即影响另一个。
保证安全的纠缠是非常脆弱的。对一对纠缠粒子中的一个进行测量会立即破坏链路。因此，如果恶意用户试图干扰其中的一个，那么对另一个人来说，这一点是显而易见的。
正如粒子可能跨空间缠绕一样，它们也会随着时间的推移而纠缠。所以现在存在的一个粒子可以与过去存在的粒子纠缠在一起。它的测量立即影响其前身。
这会导致一些微妙的和违反直觉的现象。例如，有一种特殊的量子感觉，可以影响过去。当然，这是有可能的严格限制。例如，不可能设置一系列杀死你的祖父母的事件，从而确保你永远不会存在。这种悖论是不允许的。
但是，区分原因和结果确实变得更加困难。另一个影响是可以增加可以通过时间传输的信息量。
Rajan和Visser利用这种时间纠缠来产生量子区块链。基本思想是在量子粒子上编码数据。这成为第一个量子块。
当有更多的数据可用时，这与来自量子操作中第一个粒子的数据结合起来，使第二个粒子缠绕它。然后丢弃前者，将第一块交易的记录与第二块相结合。来自第三个块的数据可以以相同的方式添加，创建一个链。
这条链是安全的，因为任何试图篡改它的人立即使其无效。这是量子纠缠的优势。
这个量子块链具有另一个优点：早期的块是完全防篡改的。“攻击者甚至无法尝试访问之前存在的光子，”Rajan和Visser说。“时间缠绕提供了比空间纠缠更大的安全利益。”
更重要的是，使这项工作的大部分技术已经存在，至少在原理形式上是这样。“这个设计的所有子系统已经被证明是通过实验实现的，”Rajan和Visser说。
这是一项有趣的工作，随着强大的量子计算机开始出现，这项工作可能会变得更加重要。IBM已经有了一个50-qubit的量子计算机，而更强大的机器正在筹备中。他们有能力破坏对区块链的信任只是时间问题。
但是，使这种量子区块链工作所需的基础设施的关键部分还不可用：量子网络。这是一个可以通过量子路由器传输量子信息而不破坏其量子特性的网络。这种系统目前正在设计和预计将在未来几个月或几年内在欧洲，美国和中国推出。
事实上，建立这样一个系统的工作本质上是一项工程任务，而不是基础物理学的任务之一。所以，量子区块链成为可能之前只是时间问题。当然，这个协议是否会成为另一个问题呢？
也许Rajan和Visser可以通过发现哪种技术最终会在未来取得胜利而使他们的量子时间机器得到很好的利用！
