Grover’s algorithm confers on quantum computers a quadratic advantage over classical computers for searching in an arbitrary data set, a scenario that describes Bitcoin mining. It has previously been argued that the only side effect of quantum mining would be an increased difficulty. In this work, we argue that a crucial argument in the analysis of Bitcoin security breaks down when quantum mining is performed. Classically, a Bitcoin fork occurs rarely, i.e., when two miners find a block almost simultaneously, due to propagation time effects. The situation differs dramatically when quantum miners use Grover’s algorithm, which repeatedly applies a procedure called a Grover iteration. The chances of finding a block grow quadratically with the number of Grover iterations applied. Crucially, a miner does not have to choose how many iterations to apply in advance. Suppose Alice receives Bob’s new block. To maximize her revenue, she should stop and measure her state immediately in the hopes that her block (rather than Bob’s) will become part of the longest chain. The strong correlation between the miners’ actions and the fact that they all measure their states at the same time may lead to more forks—which is known to be a security risk for Bitcoin. We propose a mechanism that, we conjecture, will prevent this form of quantum mining, thereby circumventing the high rate of forks.

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关于量子比特币采矿的不安全性

Grover的算法在量子计算机上具有比传统计算机更多的二次优势，可以在任意数据集中进行搜索，这种情况描述了比特币的挖掘。以前曾有人争论说，量子挖掘的唯一副作用将是增加难度。在这项工作中，我们认为执行量子挖掘时，对比特币安全性分析中的一个关键论点会被打破。通常，由于传播时间的影响，很少发生比特币分叉，即当两个矿工几乎同时找到一个区块时。当量子矿工使用格罗弗算法时，情况大不相同，该算法反复应用称为格罗弗迭代的过程。找到块的机会随着应用的Grover迭代次数成倍增加。至关重要的是 矿工不必预先选择要应用的迭代次数。假设爱丽丝收到鲍勃的新块。为了最大程度地提高收入，她应该立即停止并评估自己的状态，希望自己的街区（而不是鲍勃的街区）成为最长的连锁店的一部分。矿工的行为与他们都同时测量其状态的事实之间的密切相关性可能导致更多的分叉，这被认为是比特币的安全风险。我们推测，我们提出了一种机制来防止这种形式的量子挖掘，从而规避了高分叉率。矿工的行为与他们都同时测量其状态的事实之间的密切相关性可能导致更多的分叉，这被认为是比特币的安全风险。我们推测，我们提出了一种机制来防止这种形式的量子挖掘，从而规避了高分叉率。矿工的行为与他们都同时测量其状态的事实之间的密切相关性可能导致更多的分叉，这被认为是比特币的安全风险。我们推测，我们提出了一种机制来防止这种形式的量子挖掘，从而规避了高分叉率。