1The Unitary Flow
Unlike classical logic, every operation in a quantum circuit (except measurement) is reversible. This is a fundamental requirement of quantum mechanics, governed by unitary matrices. Information is never lost, just transformed.
2Noise in the Wire
Real circuits suffer from decoherence and gate errors. The longer the circuit (greater depth), the more likely errors will creep in and destroy the calculation. Optimizing circuit depth via transpilation is a key task in modern QML.
