We now describe the quantum key-exchange algorithm due to Bennett and Brassard. The original paper also talks about a practical implementation of the algorithm—an implementation using polarization of photons. For this moment, we do not highlight such specific implementation issues, but describe the algorithm in terms of the conceptual computational units called qubits.
The usual actors Alice and Bob want to agree upon a shared secret using communication over an insecure channel. A third party who gave her name as Carol plans to eavesdrop during the transmission. Alice and Bob repeat the following steps. Here, H stands for the Hadamard transform.
Alice generates a random ...