Radiative lifetime measurements of high-$n$ Rb Rydberg states

We have measured the radiative lifetimes of $S$, $P$, and $D$ states of rubidium in the range $30 \le n \le 50$ using cold atoms in a MOT. Two experimental techniques have been adopted to reduce random and systematic errors. First, a frequency doubled, pulse amplified diode laser is used to excite the target $n\ell$ Rydberg state. The output from this laser has a Fourier-transform linewidth of $\approx 100$ MHz at 480 nm, and results in minimal shot-to-shot variation in the Rydberg state population when it is used to drive the $5p_{3/2}$ $\rightarrow$ $n\ell$ transition. Second, we monitor the target state population as a function of time delay from the 480 nm laser pulse using a short mm-wave pulse that is resonant with the two-photon transition $n\ell$ $\rightarrow$ $(n+1)\ell$. We then selectively field ionize the $(n+1)\ell$ state, and detect the resulting electrons with a microchannel plate (MCP). We step the time delay between the laser pulse and the mm-wave pulse and acquire the MCP signal as a function of the delay. This signal is an accurate mirror of the $n\ell$ population, which we fit to an exponential decay to recover the $n\ell$ state lifetime.