量子密码学，又称量子密钥分发，是利用量子力学特性来保证通信安全性。它使通信的双方能够产生并分享一个随机的、安全的密钥，来加密和解密消息。

量子密码的一个最重要的，也是最独特的性质是：如果有第三方试图窃听密码，则通信的双方便会察觉。这种性质基于量子力学的基本原理：任何对量子系统的测量都会对系统产生干扰。第三方试图窃听密码，必须用某种方式测量它，而这些测量就会带来可察觉的异常。通过量子叠加态或量子纠缠态来传输信息，通信系统便可以检测是否存在窃听。当窃听低于一定标准，一个有安全保障的密钥就可以产生了。

量子密码的安全性基于量子力学的基本原理，而传统密码学是基于某些数学算法的计算复杂度。传统密码学无法察觉窃听，也就无法保证密钥的安全性。

Quantum cryptography is the science of exploiting quantum mechanical properties to perform cryptographic tasks. The best known example of quantum cryptography is quantum key distribution which offers an information-theoretically secure solution to the key exchange problem. Currently used popular public-key encryption and signature schemes (e.g., RSA and ElGamal) can be broken by quantum adversaries. The advantage of quantum cryptography lies in the fact that it allows the completion of various cryptographic tasks that are proven or conjectured to be impossible using only classical (i.e. non-quantum) communication (see below for examples). For example, it is impossible to copy data encoded in a quantum state and the very act of reading data encoded in a quantum state changes the state. This is used to detect eavesdropping in quantum key distribution.