Strongly interacting photons via Rydberg-Rydberg interactions

Abstract

A quantum nonlinear optical medium, i.e. a medium where the light propagation depends on photon number, has been a long-standing goal due to its applications in quantum information, communication and metrology. When the medium is nonlinear at single photon level, it can be viewed as strong interactions between individual photons mediated by the medium. Here, we achieve such strong interactions by coupling the photons to highly polarizable Rydberg states with a phenomena called electromagnetically induced transparency (EIT). The strong van der Waals or dipole-dipole interactions between Rydberg excitations map to the photons under EIT conditions. The photons are incident on a cigar-shaped laser-cooled rubidium cloud in free space. After the photons emerge out of the cloud, we measure the photon correlations from time-resolved single photon detections, which reveal crucial information about the quantum states of strongly interacting two or three photons. In this thesis, I will present four experiments. The first two experiments demonstrate quantum nonlinearities with a propagating continuous wave (cw) light field via Rydberg-Rydberg interactions in the dissipative and dispersive regimes, respectively. In the dissipative regime, strong photon anti-bunching is observed. In the dispersive regime, we achieve a conditional phase shift ~ [pi]/4, together with photon-bunching driven by attractive force. Moreover, the photons acquire a finite mass and we see evidence for a diphoton molecule. In the third experiment, by measuring higher-order correlation functions, we observe a three-photon bound state evidenced by tighter binding in addition to a larger conditional phase shift than the two-photon states. By comparing with an effective field theory, our results suggest that there might be a three-photon force on top of the pairwise interactions owing to the saturation of the interaction. Namely, only one Rydberg excitation can be created within a characteristic length scale called blockade radius. Finally, we explore the exchange interaction instead of the widely studied blockade shifts. Under the exchange interactions, a propagating photon and a stored one experience coherent collisions protected by a symmetry of the Hamiltonian and pick up a robust [pi]/2 phase shift.