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

We propose a new spectroscopic method fur measuring weak transitions in col d and trapped atoms, which exploits the long spin relaxation times and tigh t spatial confinement offered by dark optical traps to achieve extremely hi gh 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 mu W. We mere able to measure transitions with as small excitation rate as < 0.1 s(-1), using 10(7) fold quantum amplification of the transition rate due to spin s helving anti normalized detection with a strong cycling transition.