Abstract
Electromagnetically induced transparency (EIT) is a well-known phenomenon due in part to its applicability to quantum devices such as quantum memories and quantum gates. EIT is commonly modeled with a three-level lambda system; however, this simplified model does not capture all the physics of EIT experiments with real systems. We present a theoretical study of the effect of two closely-spaced excited states on EIT and off-resonance Raman transitions. In addition to the expected broadening of the EIT resonance and reduction in EIT transparency due to multiple excited states, we find some unexpected and interesting results. A comparison with a model that does not allow for coupling of the excited states shows that the coherent interaction of the fields with two excited states whose separation is smaller than their Doppler broadened linewidth can enhance the EIT transparency under some conditions and can enhance and shift the resonance frequency of off-resonance Raman absorption. Furthermore, through a dressed state analysis of the full four-level system, we find that the underlying mechanism for the reduction in EIT transparency is a shift in the two-photon resonance due to unequal dipole moments of the transitions coupled by the control field. Complete transparency can only be recovered for systems with equal dipole moments or when the separation between the two excited states is at least of the order of the Doppler width. To support our theoretical study, we present experimental EIT measurements in the D1 lines of 85Rb and 87Rb that agree with our predictions of an enhancement or reduction of EIT transparency under specific conditions. The experimental results also verify the roles that the dipole moments and the separation between the two excited states have on recovering the EIT transparency.
Highlights
Induced transparency (EIT) and off-resonant Raman transitions are established techniques to implement optical quantum memories [1,2,3,4,5,6] and quantum gates [7,8,9]
We present a study of the effects of two closely-spaced excited states on Electromagnetically induced transparency (EIT) and off-resonance Raman transitions
We find that for high decoherence rates the presence of two closely-spaced excited states whose frequency separation is less than their Doppler broadened linewidths enhances the EIT transmission and broadens the width of the EIT resonance
Summary
Induced transparency (EIT) and off-resonant Raman transitions are established techniques to implement optical quantum memories [1,2,3,4,5,6] and quantum gates [7,8,9]. We derive analytical expressions for the atomic susceptibilities with Doppler broadening using the density matrix formalism under the assumption From this comparison, we find several interesting results that are due to having two excited levels that coherently interact with the optical fields. We further investigate the underlying physical process through a dressed state model and find that this reduction does not occur when the separation between the excited states is larger than the Doppler broadening or when the dipole moments of the transitions are equal We use this model to study the effect of the spacing between the two excited states, decoherence rate, and Rabi frequency on the EIT transmission. We present experiments done in the D1 lines of 85Rb and 87Rb and use our dressed state model to explain how the two closely-spaced excited state leads to the observed behaviors in the different transitions of the D1 lines
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