Life with Physics

Background Quantum computing can bring exponential speedup over some classical algorithms. Most of the early-age algorithms are designed as proof of concept and thus lack of practical interests. Shor's algorithm for integer factorization is the first quantum algorithm that will bring practical applications: breaking the current RSA encryption  on the internet. However, implementing Shor's algorithm requires the manipulation of thousands of qubits, which is far beyond today's engineering capability. The breakthrough starts with the appearance of Harrow-Hassidim-Lloyd (HHL) algorithm in 2009, which solves linear equations in logarithmic time. Solving linear equations is of great importance in almost all the scientific and engineering disciplines. It is also an indispensable subroute in many machine learning algorithms. Besides its wide applications, implementing HHL algorithm only requires tens of qubits and thus seems feasible in the near future. Along with the rise of deep

原文是对知乎上“ 当前量子计算技术前沿是什么水平 ”问题的回答，现已被删除，特此转载留档。原文中还有一些图片，网上只有纯文字版残留。非本人原创，完全转载，如有侵权，即刻删除。 王之鑫 耶鲁大学 应用物理系博士在读 更新（03-19-2018）：感谢大家三天来的关注和反馈。量子通信部分略有修改，一些细节语言更严谨了些。需要强调的是，实际条件下量子通信的安全性分析是一个复杂的研究方向，科学家们也一直在为减少实用量子通信的安全漏洞不懈努力。例如，诱骗态 (decoy state) 和测量装置无关量子密钥分发 (measurement-device-independent quantum key distribution) 弥补了第一代商用量子通信技术的两个重要安全漏洞。但是，绝对安全的量子通信在现实中是不存在的，使用了新技术的量子密码仍然存在其它安全漏洞。对此感兴趣的童鞋可以参考下面三篇综述长文： V. Scarani, et al. The security of practical quantum key distribution. Rev. Mod. Phys. 81, 1301 (2009) R. Alléaume, et al. Using quantum key distribution for cryptographic purposes: A survey. Theor. Comput. Sci. 560, 62 (2014) E. Diamanti, et al. Practical challenges in quantum key distribution. npj Quantum Information 2, 16025 (2016) 另外，为了避免误解，第八部分中关于中国互联网巨头“量子战略”的评论更具体了些，写清了“亩产十万斤”到底指什么。此段观点没有变化。 最后，出于自我保护，我将在知乎永久封笔。Addio! 本人坐标耶鲁大学，是 Devoret-Schoelkopf 超导量子计算实验室迄今唯一本科来自中国的博士生。 文章很长，分为九个独立的问题，可分别阅读： （一）量子是个啥？ （二）各种量子技术都是啥？ （三）量子计算机有啥用？ （四）量子计算机怎么做？ （五）当前量子计算实验研究的各路高手都是谁？ （六）量子计算到底难在哪？进展到