Effects of finite trapping on the decay, recoil, and decoherence of dark states of quantum emitter arrays

Abstract: The collective interaction of electronic excitations with the electromagnetic field in atomic arrays can lead to significantly reduced decay rates, resulting in the formation of subradiant states with promising applications in quantum information and quantum memories. In many previous studies, atoms are assumed to be perfectly positioned and fixed. In this work, we investigate the effects of finite trap strength on the long-lived subradiant states for two, three, and many atoms in a 1D waveguide or free space. We demonstrate that the decay rate is lower bounded by the spatial spread of the motional wavefunction, which is non-zero even for the vibrational ground state. Additionally, in relatively tight traps, the decay of a shared electronic excitation imparts recoil energy to the atoms, which is proportional to the lifetime of the excited state. In a 1D waveguide, this energy can reach a vibrational quantum, even in the Lamb-Dicke regime. Furthermore, for atoms in weaker traps, the induced dipole forces become significant, causing the atoms to undergo motion prior to decay. This motion results in a time-dependent decay rate, an additional energy transfer to the atoms following decay, and mixing between the different electronic eigenstates. Consequently, these effects lead to decoherence and a reduction in the fidelity of a stored electronic eigenstate. Infidelity can be mitigated by either decreasing the induced forces or increasing the trap strength. For quantum information storage, these considerations suggest optimal array configurations, both in terms of geometric arrangements and polarization directions. Our findings provide insights for the behavior of quantum memories and atom array experiments.