Influence of acoustic phonons on the optical control of quantum dots driven by adiabatic rapid passage

Abstract

The role of phonons for adiabatic rapid passage in semiconductor quantum dots is studied theoretically. While in an ideal system adiabatic rapid passage results in a full inversion of the quantum dot occupation, phonons hamper this behavior drastically. We show that the transitions between the adiabatic states lead to a temperature-dependent decrease of the final exciton occupation. In contrast to the ideal evolution, the phonon-related perturbation induces dependencies on the pulse power and on the sign of the chirp.Recent experiments 1,2 have confirmed the theoretical predictions 3 that preparation of the state of an exciton confined in a quantum dot (QD) by a chirped (frequency-swept) laser pulse, referred to as adiabatic rapid passage (ARP), leads to high-fidelity control of the QD occupation. Unlike the resonant Rabi rotation method 4 or its generalizations based on voltage control, 5,6 where the final state critically depends on the excitation or control conditions, the ARP technique guarantees that the dot will be occupied by an exciton as soon as the pulse intensity exceeds the threshold for the adiabatic passage. In the ideal case, the system state during the pulsed excitation follows the adiabatic eigenstates across the anticrossing, which leads to occupation swapping between the empty dot and exciton states.On the other hand, theoretical analysis of charge dynamics in QDs has led to the conclusion that the fidelity of quantum control is limited by phonon-induced dephasing. 7-13 The essential role of phonons in the dephasing of optically driven confined excitons has indeed been confirmed experimentally. 14,15 One can, therefore, expect that the carrier-phonon coupling will also limit the fidelity of control achievable in the ARP control scheme. One decoherence channel that may play a role is related to transitions between the adiabatic branches, 16 which can be expected to introduce the temperature-dependent difference between the two directions of the frequency sweeping, as they correspond to the evolution along the energetically higher or lower branch of the adiabatic spectrum. Recently, effects induced by coupling to longitudinal optical phonons have been analyzed in the range of very short and strongly chirped pulses. 17 However, in the parameter range relevant to the experiments, 1,2 where the spectral characteristics of the pulses exclude excitation of high-energy optical modes, the coupling to acoustic phonons can be expected to be the main source of decoherence.In this Rapid Communication, we study the evolution of the exciton system in a QD driven by a chirped pulse in the regime of ARP in the presence of the coupling to acoustic phonons. We show that at low temperatures the phonon-induced decoherence leads to a strong asymmetry in the final exciton occupation depending on the sign of the chirp and to a dependence on the pulse power, resulting in an optimum for pulse areas in the range between about π and 2π , as indeed observed in the experiment. 1 We model the QD in the strong confinement lim...

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