Control of Quantum Coherence via Chirped Fractional Stimulated Raman Adiabatic Passage

Stimulated Raman Adiabatic Passage (STIRAP) is a robust and efficient method for population transfer without any loss due to spontaneous decay. Two-photon resonance is an important condition in STIRAP for the process to be perfectly adiabatic. However, by chirping both pump and Stokes pulses with carefully chosen chirp rates and time delay between them, it is possible to transfer the population adiabatically in the case of non-zero two-photon detuning. It has been found that the population can be flown exclusively to one of the desired levels in a nearly degenerate 4-level system by controlling the sign of chirp rates. Fractional-STIRAP, a variation of STIRAP in which the Stokes pulse overlaps with the pump pulse as they both drop to zero, has been used to create a coherent superposition of the initial and final states adiabatically. This is based on the idea of elongating the Stokes pulse to preserve the populations at half levels and the coherence at maximum. In this work, we apply chirped pulses of pump and Stokes in fractional-STIRAP to create maximally coherent superposition selectively in a nearly degenerate 4-level system. This method provides a way to control the dynamics in systems with close energy levels and allow generation of output signals with optimal intensity in detection and sensing techniques.