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.
Thermal radiation played a pivotal role in the preliminary development of quantum physics where it helped resolve the apparent incongruity of the ultraviolet catastrophe. In contemporary physics, thermal state generation and manipulation finds new application in fields such as quantum imaging and quantum illumination and as a practical realisation of Maxwell's demon.
Thermal states are also used in conjuction with novel imaging techniques aimed to reduce the noise of an acquired images, to improve its resolution or even reconstruct the image of an object using radiation that has not interacted with the object itself.
Another application, where thermal states are employed, is hypothesis testing and parameter estimation. Such technique allows to beat the diffraction limit by unlocking all the information about amplitude and phase in the collected light.