Ultrasensitive two-photon spectroscopy based on long spin-relaxation time in a dark optical trap

We propose a new spectroscopic method for measuring weak transitions in cold and trapped atoms, which exploits the long spin relaxation times and tight spatial confinement offered by dark optical traps to achieve extremely high sensitivity. We demonstrate our scheme by measuring a 5S_1/2 → 5D_5/2 two-photon transition in cold Rb atoms trapped in a new single-beam dark optical trap, using an extremely weak probe laser power of 25 μW. We were able to measure transitions with as small excitation rate as < 0.1s^-1, using 10⁷ fold quantum amplification of the transition rate due to spin shelving and normalized detection with a strong cycling transition. * Present address: Institute of Applied Physics, Technical University of Sofia, 1780 Sofia, Bulgaria.