# Can a quantum computer break quantum cryptography?

I’m not sure if this makes sense, but I know that there is quantum and post-quantum encryption, and I’m curious whether quantum computing can break a quantum encryption.

In more detail, in many of the quantum key distribution schemes, such as the BB84 protocol and the E91 protocol, qubits are exchanged between parties that wish to communicate. These protocols are based on the no-cloning theorem, and are information-theoretically secure. This implies that no amount of computational power would be sufficient to crack ciphers based on these protocols. However, practical implementations of such schemes might indeed have other serious vulnerabilities - the machine that sent photons in the very first implementation of BB84 made different sounds, depending on whether it was sending qubits in the $$\{|0\rangle,|1\rangle\}$$ basis or the $$\{|+\rangle,|-\rangle\}$$ basis!