The security of a cryptosystem in most cases relies on the key being kept secret. Quan- tum key distribution (QKD) enables two authenticated parties without other prior information to share a perfectly secure key. A trivial method to obtain a secret key is to use QKD to reestablish a new key for each message. In this book we study method to update the keys. We call this method quantum key evolution (QKE). The QKE provides a new secret key in each round of the protocol. Therefore, a new secret key is established for next round of protocol execution.