Progress on measuring the |α/β| transition polarizability ratio in the one photon Rb 5S - 6S transition

We continue to make progress on measuring the ratio of the α and β transition polarizabilities of the highly forbidden one photon Rb 5S - 6S transition, in a strong dc electric field. With a laser excitation wavelength of 497 nm, we excite the 6S state with one photon from the 5S ground state, and we count the 780 nm fluorescent photons in a PMT from the decay chain 6S - 5P - 5S. With an excitation in a dc electric field of about 15 kV/cm, we have obtained a strong signal for the ΔF=0 α transition with E direction parallel to photon polarization. However with the E direction perpendicular to photon polarization, and ΔF=±1, there is in theory no α transition strength, but only β. The β transition strength is predicted to be 625 times weaker than the already weak α transition strength. We have looked carefully for the β transition, and see a very slight signal, smaller than the shot noise in the photon counting detector, and consistent with zero. This allows us to set an upper limit of |β/α| ≤ 1/15, which is about double the theoretically predicted value of |β/α| = 1/25. We are working to improve our signal/noise with a new hyperfine pumped Rb beam, and a cycling transition detection scheme, which should improve our signal/noise by a factor of 1000, allowing the β signal and the zero electric field M1 signal to be seen.