Background-Free Doppler Cooling of Trapped Ions Using Quadrupole Transition

We demonstrate experimentally the Doppler cooling of ⁴⁰Ca⁺ ions using the S_1/2 →D_3/2 electric quadrupole transition, in which the scattering light that is much different from the exciting light makes it possible to collect background-free fluorescence. The idea excluding 397 nm excitation has the advantage of high-sensitivity detection and only employs lasers in red and infrared wavelengths, which should be practical for trapped-ion quantum information processing.