Rotation-time symmetry in bosonic systems and the existence of exceptional points in the absence of mathcal{PT} symmetry

We study symmetries of open bosonic systems in the presence of laser pumping. Non-Hermitian Hamiltonians describing these systems can be parity-time ${cal{PT}}$ symmetric in special cases only. Systems exhibiting this symmetry are characterised by real-valued energy spectra and can display exceptional points, where a symmetry-breaking transition occurs. We demonstrate that there is a more general type of symmetry, i.e., rotation-time ${cal{RT}}$ symmetry. We observe that {cal{RT}}-symmetric non-Hermitian Hamiltonians exhibit real-valued energy spectra which can be made singular by symmetry breaking. To calculate the spectra of the studied bosonic non-diagonalisable Hamiltonians we apply diagonalisation methods based on bosonic algebra. Finally, we list a versatile set rules allowing to immediately identifying or constructing {cal{RT}}-symmetric Hamiltonians. We believe that our results on the {cal{RT}}-symmetric class of bosonic systems and their spectral singularities can lead to new applications inspired by those of the {cal{PT}}-symmetric systems.