We give a necessary condition for photon state transformations in linear optical setups preserving the total number of photons. From an analysis of the algebra describing the quantum evolution, we find a conserved quantity that appears in all allowed optical transformations. We comment some examples and numerical applications, with example code, and give three general no-go results. These include (i) the impossibility of deterministic transformations which redistribute the photons from one to two different modes, (ii) a proof that it is impossible to generate a perfect Bell state in heralded schemes with a separable input for any number of ancillary photons and modes and a fixed herald and (iii) a restriction for the conversion between different types of entanglement (converting GHZ to W states).
@article{arxiv.2307.11478,
title = {No-go theorems for photon state transformations in quantum linear optics},
author = {Pablo V. Parellada and Vicent Gimeno i Garcia and Julio-José Moyano-Fernández and Juan Carlos Garcia-Escartin},
journal= {arXiv preprint arXiv:2307.11478},
year = {2025}
}