Coulomb bound states of strongly interacting photons

TL;DR

This paper demonstrates that photons coupled to Rydberg states can form Coulomb-like bound states, creating a photonic analogue of the hydrogen atom with potential applications in quantum information and nonlinear optics.

Contribution

It introduces a novel mechanism for photon-photon interactions via Coulomb potential, leading to bound states analogous to atomic systems, which was not previously realized.

Findings

Discovery of Coulomb-like bound states of photons

Identification of metastable states with hydrogen-like spectra

Propagation of these states with negative group velocity

Abstract

We show that two photons coupled to Rydberg states via electromagnetically induced transparency can interact via an effective Coulomb potential. This interaction gives rise to a continuum of two-body bound states. Within the continuum, metastable bound states are distinguished in analogy with quasi-bound states tunneling through a potential barrier. We find multiple branches of metastable bound states whose energy spectrum is governed by the Coulomb potential, thus obtaining a photonic analogue of the hydrogen atom. Under certain conditions, the wavefunction resembles that of a diatomic molecule in which the two polaritons are separated by a finite "bond length." These states propagate with a negative group velocity in the medium, allowing for a simple preparation and detection scheme, before they slowly decay to pairs of bound Rydberg atoms.