Observation of motional sidebands in single 40Ca+ ions with improved detection efficiency

A method for effectively removing background photons and improving the signal-to-background ratio in detection of fluorescence from a single cooled ion is described. In the method, an additional spatial filter placed at an off-focal position is used to remove scattered photons from trap-electrode surfaces. A signal-to-background ratio of more than 200 is obtained. By using this setup, a resolved carrier and motional sidebands on the 4²S_1/2–3²D_5/2 electric quadrupole transition in ⁴⁰ Ca⁺ are observed.