When Abandoned Bits Bite Back – Reversibility and Quantum Computing

Reversible logic was originally proposed as a way to build classical computers with very low energy requirements. Its greatest impact so far has in fact been on the field of quantum computing, where it is used pervasively – but for an entirely different purpose. Quantum interference effects offer the potential for speeding up certain classical calculations. But if state information is expelled from a quantum computer into the unmodeled environment – as happens whenever an irreversible operation is performed – then it becomes impossible to predict or control when interference will occur. Reversibility is necessary because it allows bits to be erased by uncomputing them rather than simply expelling them, preserving control over interference effects.