A probe pulse (arrow on the top left) fills the cavity with photons. A control laser pulse (arrow on the left) takes the system from the ground state to its first excited state. The internal state of the atom changes to the tune of the light waves of the photons in the cavity, where it is in a strong coupling regime: it oscillates between the first and second excited states, while emitting and reabsorbing photons. These oscillations are called Rabi oscillations. The photons emitted (arrow on the top right) by the cavity behave like waves.
Picture credit: Alessandro Ridolfo and Rocco Vilardi, Messina University.
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