stirap

STIRAP, or Stimulated Raman Adiabatic Passage, is a quantum optical technique used to transfer population between two quantum states in a three-level system. It uses two phase-coherent laser pulses, arranged in a counterintuitive sequence, to adiabatically steer the system through a dark state that avoids significant population of a lossy intermediate state.

In a Lambda configuration, the lower states |1> and |2> are connected to a common excited state |3> by the Pump (|1>↔|3>) and Stokes (|2>↔|3>) fields. The Stokes pulse precedes the Pump pulse, and as they temporally overlap the system follows the instantaneous eigenstate with zero projection onto |3>. As the pulses evolve, the dark state changes from |1> to |2>, resulting in nearly complete transfer of population without populating |3>.

Advantages include high efficiency and robustness to moderate fluctuations in pulse area and detuning, and reduced spontaneous emission because the intermediate state remains sparsely populated. Limitations involve the need for a well-isolated three-level system, sensitivity to decoherence, and the requirement of stable relative phase and frequency between the pulses. Imperfect adiabaticity or off-resonant couplings can degrade transfer.

STIRAP has been demonstrated in atomic, molecular, and solid-state systems and is used in quantum information protocols, precision control of chemical reactions, and preparation of ultracold molecules. Variants include fractional STIRAP, which creates superpositions, and multi-level extensions that address more complex ladder systems.