Use this tag on questions about attacks that break cryptographic guarantees / security properties of primitives and protocols and utilize quantum computers as an essential part.
En example of quantum cryptanalysis is the use of Shor's algorithm to attack classical ciphers such as RSA and Elliptic curves or Grover's algorithm to attack symmetric ciphers such as AES. Questions about the number of qubits, gates or order of complexity to break such an algorithm should use this tag.
The tag quantum-cryptography is for cryptographic constructions that use quantum effects to achieve security. These tags should generally not be used together. Questions that are specific to the algorithms used for quantum cryptanalysis are probably better asked at our sister site, Quantum Computing.