Also important to ask WHY QKD and WHERE / WHEN it is needed..
For context, to keep our information safe we rely on Symmetric Cryptography algorithms such as the Advanced Encryption Standard (AES).
Here, the sender and receiver use the same secret key and cryptographic algorithm to encrypt and decrypt data. However, the shared key needs to be kept secret and this led to Public Key Cryptography (PKC) to distribute keys safely over public networks.
Now the security strength of these PKC aka asymmetric cryptosystems is based on the difficulty in tackling incredibly complex math problems, such as prime number factorization (RSA) and the discrete logarithm problem.
Even with the most powerful supercomputer today, deciphering classically encrypted information is near impossible in feasible time. However, we are only safe because of classical computing’s limited power and capabilities to tackle these calculations.
Fault Tolerant Quantum Systems have a real threat to PKC.
For example, Elliptic Curve Cryptography (ECC) is considered the most secure and efficient asymmetric cryptosystems. It provides the same level of security as RSA and DLP systems with shorter key operands which makes it convenient to be used by systems of low computational resources. 160-bit elliptic curves could be broken by a 1000-qubit quantum computer