Quantum cryptography utilizes the laws of physics, as opposed to mathematical assumptions, to enable the secure exchange of a secret key between two parties. It is considered more robust because mathematical assumptions can unravel with the advent of stronger computing power, whereas physics laws cannot be broken.
For two parties to exchange a secret key, they require a transmission channel on which quantum bits or qubits can be transmitted. In practice, the qubits are usually photons, the elementary particle of light, and the channel is an optical fiber for telecommunication networks or the open air for things like satellite communications.
What makes quantum cryptography powerful is the fact that mere act of reading a qubit, changes its value, which makes any attempt to intercept qubits immediately apparent.
In order for two parties to exchange the qubits that make up the secret key, they engage in an elaborate protocol (e.g. BB84) at the end of which both parties have the key.
Quantum cryptography is not used directly to transmit the secret information – it is used to distribute the random secret key used to encrypt the secret information. Once a key has successfully been transmitted, classical symmetric ciphers such as the one-time pad or AES are used to encrypt and decrypt information.
