Direct observation of dynamics of photonic bound states

Cavity-QED provides an effective route to modifying photon statistics and forming exotic quantum states of photons. The prerequisite to this is a very efficient coupling between photons and a quantum emitter. In this contribution, we use a quantum dot in a tunable microcavity to achieve pronounced interaction between optical photons [1]. We observe a strong nonlinearity in the transmission of the cavity with an on-set at an average of 0.25 photons per lifetime of the emitter. The nonlinearity means that the light-matter interaction depends strongly on the number of photons impinging on the two-level system [2]. We report the direct observation of a photon-number-dependent time delay in the transmission of the system. We observe that single photons, and two- and three-photon bound states incur different time delays of 144 ps, 66 ps and 45 ps, respectively. The reduced time delay of the two-photon bound state is a fingerprint of stimulated emission happening at the quantum level; the arrival of two photons within the lifetime of an emitter causes one photon to stimulate the emission of the other from the atom. We also determine the wave function of the two-photon bound state, which matches the theoretical predictions.



[1] N. Tomm, et al., arXiv:2205.03309 (2022).

[2] S. Mahmoodian, et al., Phys. Rev. X 10, 031011 (2020).