# Can a quantum computer implement a classical one?

The normal question about this topic would be: Can a classical computer implement a quantum computer? But now I wonder the exact opposite: can a quantum computer implement a classical one? Can we say that classical computation is a subset of quantum computing?

• Isn't the question whether one class of computer can tractably implement the other? I believe that a classical computer can simulate a quantum computer, but the complexity just gets bad very rapidly in the worst cases. – Ben Crowell Feb 16 '18 at 16:01
• Your current computer is using tons of qubits without taking advantage of any of the special quantum features. – Erik Eidt Feb 24 '18 at 21:37

• @knzhou Reversible computing is Turing complete and can be happily implemented in quantum computers. (In more detail: quantum computers cannot implement 'standard' (lossy) AND gates, but they have no problem with the reversible-computing equivalent $(x,y,z)\mapsto (x,y,z\oplus(x\otimes y))$.) – Emilio Pisanty Feb 16 '18 at 16:25