Stimulated Raman adiabatic passage

The Stimulated Raman adiabatic passage (STIRAP) process permits transfer of a population between two applicable quantum states via at least two coherent electromagnetic (light) pulses. These light pulses drive the transitions of the three level Ʌ atom or multilevel system. The process is a form of state-to-state coherent control.

Population transfer in three level Ʌ atom

Consider the description of three level Ʌ atom having ground states $|g_{1}\rangle$ and $|g_{2}\rangle$ (for simplicity suppose that the energies of the ground states are the same) and excited state $|e\rangle$. Suppose in the beginning the total population is in the ground state $|g_{1}\rangle$. Here the logic for transformation of the population from ground state $|g_{1}\rangle$ to $|g_{2}\rangle$ is that initially the unpopulated states $|g_{2}\rangle$ and $|e\rangle$ couple, afterward superposition of state $|g_{2}\rangle$ and $|e\rangle$ couple to the state $|g_{1}\rangle$. Thereby a state is formed that permits the transformation of the population into state $|g_{2}\rangle$ without populating the excited state $|e\rangle$. This process of transforming the population without populating the excited state is called the stimulated Raman adiabatic passage.