As a person working on the physics perspective of quantum communication, I understand the process upto which the quantum key is shared securely. What happens after this process to send message bits? The literature that I've come across talk about encryption and decryption algorithms and not anything further in detail. Shouldn't the length of the key be equal to the number of bits to be ultimately sent?
The actual "process upto which the quantum key is shared securely" is nontrivial from an information theoretic perspective, to the point that few people understand it fully (I don't), and some do not even realize that prerequisites include a shared secret. See sections A and B in this for an overview, and section E.1 for why we should take the security claims with more than a grain of salt.
What occurs after that Quantum Privacy Amplification (often called Quantum Key Distribution) step depends on the objective:
- If we want confidentiality without unproven mathematical hypothesis (as in the previous step), we need as many key bits as data bits enciphered; and some more for integrity. See C.1 in above link.
- Alternatively, if we want a fast link, we use classical cryptography with the key established (perhaps, periodically) by Quantum Privacy Amplification. But we must trust the (symmetric) crypto used, despite lack of mathematical proof that it is safe. See C.2 in above link.