Vacuum-field level shifts in a single atom mediated by a single distant mirror

A single Ba⁺ ion is confined in a miniature Paul trap and Doppler cooled with green laser light at 493 nm and red light at 650 nm. A theoretical description of the experiment predicts that a level shift of the upper (P₁2/) state is associated with the modified decay. This level shift is observed in the following way: as the frequency detuning of the red laser is scanned across resonance, the phase between green and red fringes is recorded. Varying the mirror-ion distance leads to high-visibility interference fringes in the green fluorescence (∼60%) whilst the red fluorescence, that is transmitted by the mirror and detected by photomultiplier, is seen to be modulated with the same period but with lower visibility (∼1-2%). The modulation of the red light indicates a modified decay rate on the green transition, showing that the mirror changes the vacuum field at the green wavelength and leads to enhancement and inhibition of spontaneous decay from the upper state.