We show that the technique of Stark-chirped rapid adiabatic passage (SCRAP), hitherto used for complete population transfer between two quantum states, offers a simple and robust method for complete population transfer amongst three states in atoms and molecules. In this case SCRAP uses three laser pulses: a strong far-off-resonant pulse modifies the transition frequencies by inducing dynamic Stark shifts and thereby creating time-dependent level crossings amongst the three diabatic states, while near-resonant and moderately strong pump and Stokes pulses, appropriately offset in time, drive the population between the initial and final states via adiabatic passage. The population transfer efficiency is robust to variations in the intensities of the lasers, as long as these intensities are sufficiently large to enforce adiabatic evolution. With suitable pulse timings the population in the (possibly decaying) intermediate state can be minimized, as with stimulated Raman adiabatic passage (STIRAP). This technique applies to one-photon as well as multiphoton transitions and it is also applicable to media exhibiting inhomogeneous broadening; these features represent clear advantages over STIRAP by overcoming the inevitable dynamical Stark shifts that accompany multiphoton transitions as well as unwanted detunings, e.g., induced by Doppler shifts.
We study the dynamics of chirped adiabatic passage (CHIRAP) with temporally delayed pulses to selectively and fully transfer population in atoms and molecules. We show that the transfer of population via stimulated Raman adiabatic passage (STIRAP) and stimulated two-photon adiabatic absorption with temporally delayed laser pulses is not adversely affected by applying a chirp to the pulses, provided the intensity of the laser is sufficiently large to allow adiabatic passage. We then proceed to consider systems in which a near degeneracy of terminal state levels results in nonselective population of the terminal levels using nonchirped STIRAP or two-photon absorption stimulated passage, because the dynamics within the nearly degenerate pair of levels is not adiabatic. However, the CHIRAP process selectively populates only one of the terminal levels.
We review and compare recent experiments and theoretical models demonstrating complete population transfer to selected states in atoms and molecules using (1) Stimulated Raman Adiabatic Rapid Passage (STIRAP) with temporally delayed but partially overlapping time dependent fundamental and Stokes fields and (2) Chirped Adiabatic Passage (CHIRAP) using frequency chirped field with the frequency swept through the resonance frequency of transitions. The transfer efficiency in these processes is unity when the adiabatic criteria are satisfied. Alignment and orientation of the initial and terminals levels of the transitions can be obtained using these methods. The dynamics involve the atomic or molecular internal angular momentum states because the Rabi frequencies for the transitions depend on the magnetic quantum numbers of the states. Full wavefunction and density matrix (necessary when spontaneous emission of the excited states plays a role) calculations are presented and adiabatic-following wavefunction and density matrix descriptions are advanced to explain unexpected experimental results.
Population transfer in a three-level A system is simulated numerically and optimized. Almost complete population transfer from |1) to |3) is achieved by a genetic algorithm while the population in state |2) reached minimum over the entire evolution at the same time. The result shows that the optimal pulse sequence is the well-known stimulated Raman adiabatic passage (STIRAP) scheme. The detuning of pump pulse and Stokes pulse Δp and Δs with the opposite sign and the chirps χp and χs with the same sign are in favour of the complete and robust population transfer for few-cycle laser pulse. Rabi frequencies Ωp and Ωs have insensitive effects on the complete population transfer during a large scope of their ratio when they are large enough.
We examine the conditions needed to accomplish stimulated Raman adiabatic passage (STIRAP) when the three levels (ground
g, excited
e, and final
f) are degenerate, with arbitrary couplings contributing to the pump-pulse interaction $(g\text{\ensuremath{-}}e)$ and to the Stokes-pulse interaction $(e\text{\ensuremath{-}}f)$. We show that in general a sufficient condition for complete population removal from the
g set of degenerate states for arbitrary, pure or mixed, initial state is that the degeneracies should not decrease along the sequence
g,
e, and
f. Using a sequence of unitary transformations we demonstrate that when this condition holds it is possible to achieve the degenerate counterpart of conventional STIRAP, whereby adiabatic passage produces complete population transfer. Indeed, the system is equivalent to a set of independent three-state systems, in each of which a STIRAP procedure can be implemented. We also examine the cases when this degeneracy constraint does not hold and show what can be accomplished in those cases. For example, for angular momentum states when the degeneracy of the
g and
f levels is less than that of the
e level we show how a special choice for the pulse polarizations and phases can produce complete removal of population from the
g set. Our scheme can be a powerful tool for coherent control in degenerate systems, because of its robustness, when selective addressing of the states is not required or impossible. We illustrate the analysis with several analytically solvable examples, in which the degeneracies originate from angular momentum orientation, as expressed by magnetic sublevels.