The path to achieving a reliable Rydberg-blockade effect, which could potentially be used in a fast phase gate, requires studies of driving ground-state atoms into Rydberg states. We highlight here work done on the non-linear Autler-Townes effect with 85Rb on the two-step excitation 5S-->5P-->44D. The Autler-Townes effect is observed spectroscopically as a splitting of the 5S-->5P-->44D resonance line, proportional to the Rabi Frequency of the pump laser. In order to observe the Autler-Townes effect the pump laser's Rabi frequency must exceed the natural linewidth of the transition. It is thus advantageous to do Autler-Townes experiments where the probe transition has negligible linewidth. In the following data shown, the 44D Rydberg state has a lifetime of 60microseconds and thus a 16kHz linewidth, which is on order 100 times less than the linewidth of the 5S-->5P transition studied. The Autler-Townes effect is most illusive in the dressed-state picture, appropriate for the strong field of the pump (5S-->5P) laser. In the dressed state picture the 5S and 5P states are no longer pure states, but rather mixtures of 5S and 5P. There is a |5S+5P> and a |5S-5P> state. There is thus some probability that the weak probe laser will create Rydberg excitations from both of these two states separated by the Rabi frequency of the 5S-->5P transition.
A definition of the Rabi frequency between states |1> and |2> is given here in terms of electric field (E):
A definition of the Rabi frequency in terms of intensity on a transition with linewidth, gamma, is given here, where Isat is given by 1.64mW/cm2 for Rb:
The Rabi Frequency therefore depends on the Clebsch-Gordon coefficients of the transitions, revealing that the Autler-Townes splitting with sigma polarization will differ from the splitting with linear polarization.
Typical Autler-Townes measurements for 5S-->5P sigma + polarization are shown on the left. At right the graph shows clearly the difference between the Autler-Townes splitting of sigma and linear pump laser polarization. As expected by the Clebsch-Gordon coefficients the splitting of the linear polarized light was 67% less than that of sigma polarized light. :