Why people believe this
Classical computing uses irreversible gates like AND and OR — information is destroyed. Quantum gates seem similar so they might also be irreversible.
The correction
All quantum gates — without measurement — must be reversible. This is a consequence of unitarity: quantum evolution is described by unitary matrices, and every unitary matrix has an inverse. AND and OR are irreversible classically because they map 2 bits to 1 bit, losing information. Quantum gates always map n qubits to n qubits. The Toffoli gate (CCX) proves that any classical computation can be made reversible with at most a constant overhead. Irreversibility only enters quantum computing through measurement.
Try it in the simulator
What to do
Build any circuit, then add the exact reverse — each gate applied in reverse order. Run it and observe that you always recover |00...0>. Quantum gates are reversible by construction. Try H then H (equals identity), or CNOT then CNOT (equals identity). Every gate undoes itself.
Research notes
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