While both quantum and classical physics are indeed reversible at a fundamental level, the requirement for quantum gates to be reversible is more stringent than for classical gates. The distinction arises from the fundamental differences between classical and quantum information processing.
In classical computing, reversible operations are not strictly required because classical bits, the fundamental units of classical information, are easily copied and discarded without violating any fundamental physical principles. Classical logic gates can introduce information loss or irreversibility without any inherent consequences since classical information can be replicated or recovered from other sources.
On the other hand, in quantum computing, the fundamental units of information are qubits, which possess unique properties of superposition and entanglement. These properties make quantum information fundamentally different from classical information. Quantum information cannot be freely copied or discarded due to the no-cloning theorem, which states that an arbitrary quantum state cannot be perfectly duplicated.
To preserve the coherence and integrity of quantum information, quantum gates must be reversible. This requirement ensures that the quantum state can be recovered from the output of a gate, and no information is lost or destroyed during the computation. Reversible quantum gates maintain the unitarity and reversibility of the overall quantum computation, allowing the quantum system to be evolved backward in time if needed.
Furthermore, the reversibility of quantum gates is crucial for implementing quantum error correction and fault-tolerant quantum computing. Error correction relies on the ability to undo and correct errors that occur during quantum operations. Reversibility enables the recovery of the original state before the error occurred, facilitating error correction procedures.
In summary, while both classical and quantum physics are reversible, the requirement for quantum gates to be reversible in quantum computing arises from the unique properties of quantum information, such as superposition and the no-cloning theorem. Reversible quantum gates are necessary to preserve quantum coherence, information integrity, and enable fault-tolerant quantum computation.