Adiabatic population transfer in multistate chains via dressed intermediate states

The well-known process of stimulated Raman adiabatic passage (STIRAP) provides a robust technique for achieving complete population transfer between the first and last state of a three-state chain, with little population, even transiently, in the intermediate state. The extension of STIRAP to general N-state chainwise-linked systems continues to generate interest. Recently Malinovsky and Tanner (Phys. Rev. A 56, 4929 (1997)) have shown with numerical simulation that a resonant pulse sequence, which they term "straddle STIRAP", can produce (under appropriate conditions, including specific pulse areas) complete population transfer with very little population in intermediate states. Their proposal supplements a pair of counterintuitively ordered delayed laser pulses, driving the first and last transition of the chain and corresponding to the pump and Stokes pulses in STIRAP, with one or more additional strong pulses of longer duration which couple the intermediate transition(s) and overlap both the pump and the Stokes pulses. In this paper, we modify the "straddling" Malinovsky-Tannor pulse sequence so that the intermediate couplings are constant (and strong), at least during the times when the pump and Stokes pulses are present, and the intermediate states therefore act as a strongly coupled subsystem with constant eigenvalues. Under this condition, we show that the original N-state chain is mathematically equivalent to a system comprising N - 2 parallel Lambda-transitions, in which the initial state is coupled simultaneously to N - 2 dressed intermediate states, which in turn are coupled to the final state. The population transfer is optimized by suitably tuning the pump and Stokes frequencies to resonance with one of these dressed intermediate states, which effectively acts as the single intermediate state in a three-state STIRAP-like process. We show that tuning to a dressed intermediate state turns the system (for both odd N and even N) into a three-state system - with all of the properties of conventional STIRAP (complete population transfer: little transient population in the intermediate states, insensitivity to variations in the laser parameters, such as pulse area). The success of the tuning-to-dressed-state idea is explained by using simple analytic approaches and illustrated with numerical simulations for four-, five-, six- and seven-state systems.