Creation of an arbitrary coherent superposition state with chirped delayed pulses

We demonstrate numerically that in a three-level Lambda-type atomic system, by using two laser pulses, it is able to create arbitrary coherent superposition of the two lower levels via varying the linear chirp rate of the pump pulse in the case of two-photon detunings. The underlying physics of the phenomenon is given by analysing the Hamiltonian of the system on the basis of dressed states. We find that the phenomenon is due to the existence of two anticrossings of the closely spaced dressed states.