Signature schemes are widely used in electronic communication to guarantee the authenticity and transferability of messages mainly via public‑key protocols.
Since the security of public‑key schemes is not information theoretic but relies on computational assumptions, it can be retrospectively affected by future advances in technology or the discovery of efficient algorithms.
The security of quantum digital signatures (QDS), on the other hand, is information theoretic, guaranteed by the laws of quantum mechanics to be secure against an adversary with unrestricted computational capabilities.
Optical signals are subject to a distance‑dependent loss as they propagate through transmission media.
High‑intensity, classical, optical signals can routinely be amplified to overcome the degradation caused by this loss.
However, quantum optical states cannot be deterministically amplified and any attempt to do so will introduce intrinsic noise that spoils the desired quantum properties.
Non‑deterministic optical amplification, based on postselection of the output depending on certain conditioning detection outcomes, is an emerging enabling technology in quantum measurement and quantum communications.