Centrum Fizyki Teoretycznej PAS
The treatment of photons as relativistic elementary particles enables us to delve deeper into the notion of fidelity. The most striking result is the appearance of various measures of fidelity for quantum states of photons. These different measures underscore various properties of photons. The extension of the concept of fidelity to coherent states of the electromagnetic field reveals an unexpected feature: The emergence of a classical phase on top of the quantum specification of photos that form the coherent state.