Merging "nano", rare-earths, and nonlinear crystals: key technology for large-scale quantum networks

加拿大的经济、政治和社会福祉在很大程度上取决于安全的电子通信，例如电子银行、电子保健、电子商务和电子政务。然而，目前的公钥加密系统依赖于关于计算复杂性的未经证实的假设，容易受到算法进步和更好的经典计算机技术的影响，并且随着量子计算机的出现而过时。因此，过去或现在编码和发送的信息的保密性，即使在今天仍然得到保证，也很容易受到未来密码破译技术的改进的影响，这可能导致在保护不足的情况下对记录的信息进行解码，然后它们就失去了重要性。量子密钥分发（QKD）利用在特定量子状态下制备的单个光子作为信息载体，有望最终解决这些问题：新技术无法破坏的安全性。然而，由于光子在传输过程中会丢失，而且由于量子力学的基本限制——不可克隆定理，放大器无法使用，量子密钥分配目前面临着200公里左右的距离限制。有趣的是，这个限制可以通过尚不存在的量子中继器来消除。

The economic, political, and social well-being of Canada depends crucially on secure electronic communications, e.g. for e-banking, e-health, e-commerce, and e-government. Yet, current public key crypto systems rely on unproven assumptions about computational complexity, are susceptible to algorithmic advances and better classical computer technology, and will become obsolete with the advent of the quantum computer. Hence, the secrecy of messages encoded and sent in the past or present, even if still ensured today, is vulnerable to future improvements in code breaking, which may lead to decoding of recorded messages with insufficient protection before they lose importance. Quantum key distribution (QKD), which employs individual photons prepared in specific quantum states as carriers of information, promises the ultimate solution to these problems: security that cannot be compromised by new technology. Yet, as photons get lost during transmission, and amplifiers cannot be used due to a fundamental restriction of quantum mechanics known as the no-cloning theorem, QKD currently faces a distance limit of around 200 km. Interestingly, this limit can be removed through not-yet-existing quantum repeaters.