Observation of an Exceptional Point Associated With Coupled Rb Atomic Oscillators*

Exceptional points (EPs) arise in open quantum systems for which the Hamiltonian is non-Hermitian and have been observed previously in photonic resonators, plasmonics, and Feshbach resonances. Theory has predicted that a phase shift occurs in the proximity of an EP [1] and the present work confirms this prediction for coupled atomic Rb oscillators. Quantum beating in the vicinity of the Rb (5d_5/2-5p_3/2)-(5p_3/2-5s_1/2) resonance at 70.4 cm^-1 (~2.1 THz) is observed through one- and two-photon excitation of the atom with 50-200 fs laser pulses, and parametric four-wave mixing. The Fano lineshape for this oscillator is tuned by varying the mean Rb-Rb distance (<R>) and a phase shift of ~π/4 is observed in the <R>=80-90 nm interval. The transformation of the Fano profile and the phase shift are signatures of an EP, and the ability to tune both is attributed to the impact of the dipole-dipole interaction (C₃R^-3) on the interatomic potential. Data has also been obtained for Rb-Ar and Rb-Cs mixtures.

[1] U. Günther, I. Rotter, and B. Samsonov, J. Phys. A 40, 8815 (2007)