Photon Detection as a Process of Information Gain

Making use of the equivalence between information and entropy, we have shown in a recent paper that particles moving with a kinetic energy [Formula: see text] carry potential information [Formula: see text] relative to a heat reservoir of temperature [Formula: see text]. In this paper we build on this result and consider in more detail the process of information gain in photon detection. Considering photons of energy [Formula: see text] and a photo-ionization detector operated at a temperature [Formula: see text] , we evaluate the signal-to-noise ratio [Formula: see text] for different detector designs and detector operation conditions and show that the information gain realized upon detection, [Formula: see text] , always remains smaller than the potential information [Formula: see text] carried with the photons themselves, i.e.,: [Formula: see text]. This result is shown to be generally valid for all kinds of technical photon detectors, which shows that [Formula: see text] can indeed be regarded as an intrinsic information content that is carried with the photons themselves. Overall, our results suggest that photon detectors perform as thermodynamic engines that incompletely convert potential information into realized information with an efficiency that is limited by the second law of thermodynamics and the Landauer energy bounds on information gain and information erasure.