We report precision measurements of the excited state lifetime of the 5p 2 P 1/2 and 5p 2 P 3/2 levels of a single trapped Cd + ion. The ion is excited with picosecond laser pulses from a mode-locked laser and the distribution of arrival times of spontaneously emitted photons is recorded. The resulting lifetimes are 3.148 ± 0.011 ns and 2.647 ± 0.010 ns for 2 P 1/2 and 2 P 3/2 respectively. With a total uncertainty of under 0.4%, these are among the most precise measurements of any atomic state lifetimes to date. Here we report excited state lifetime measurements using a time-correlated single photon counting technique on a single atom. This method, designed especially to eliminate common systematic errors, involves selective excitation of a single trapped ion to a particular excited state (lifetime of order nanoseconds) by an ultrafast laser pulse (duration of order picoseconds). Arrival of the spontaneously-emitted photon from the ion is correlated in time with the excitation pulse, and the excited state lifetime is extracted from the distribution of time delays from many such events.By performing the experiment on a single trapped ion, we are able to eliminate prevalent systematic errors, such as: pulse pileup that causes multiple photons to be collected within the time resolution of the detector, radiation trapping or the absorption and re-emission of radiation by neighboring atoms, atoms disappearing from view before decaying, and subradiance or superradiance arising from coherent interactions with nearby atoms. By using ultrafast laser pulses, we eliminate potential effects from applied light during the measurement interval.With this setup, at most one photon can be emitted following an excitation pulse. While this feature is instrumental in eliminating the above systematic errors, it would appear that this signal would require large integration times for reasonable statistical uncertainties. However, with a lifetime of only a few nanoseconds, millions FIG. 1: The experimental apparatus. (a) A picosecond mode locked Ti:Saph laser is tuned to four times the resonant wavelength for either the 5p 2 P 1/2 or the 5p 2 P 3/2 level of Cd + . Each pulse is then frequency-quadrupled through non-linear crystals, filtered from the fundamental and second harmonic, and directed to the ion. An amplified cw diode laser is also frequency quadrupled and tuned just red of the 2 P 3/2 transition for Doppler cooling of the ion within the trap. Acoustooptic modulators (AOM) are used to switch on and off the lasers as described in the text. Photons emitted from the ion are collected by an f /2.1 imaging lens and directed toward a photon-counting photo multiplier tube (PMT). The output of the PMT provides the start pulse for the time to digital converter (TDC), whereas the stop pulse is provided by the reference clock of the mode-locked laser. of such excitations can be performed each second, thus potentially allowing sufficient data for a statistical error of under 0.1% to be collected in a matter of minutes [6].A diagram of th...