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...

We study theoretically phonon-assisted relaxation processes in a system consisting of one or two electrons confined in two vertically stacked self-assembled quantum dots. The calculation is based on a k·p approximation for single particle wave functions in a strained self-assembled structure. From these, two-particle states are calculated by including the Coulomb interaction and the transition rates between the lowest energy eigenstates are derived. We take into account phonon couplings via deformation potential and piezoelectric interaction and show that they both can play a dominant role in different parameter regimes. Within the Fermi golden rule approximation, we calculate the relaxation rates between the lowest energy eigenstates which lead to thermalization on a picosecond time scale in a narrow range of dot sizes.

We investigate theoretically under which conditions a stable and high-fidelity preparation of the biexciton state in a quantum dot can be realized by means of adiabatic rapid passage in the presence of acoustic phonon coupling. Our analysis is based on a numerically complete real-time path integral approach and comprises different schemes of optical driving using frequency-swept (chirped) pulses. We show that depending on the size of the biexciton binding energy, resonant two-photon excitations or two-color schemes can be favorable. It is demonstrated that the carrier-phonon interaction strongly affects the efficiency of the protocols and that a robust preparation of the biexciton is restricted to positive chirps and low temperatures. A considerable increase of the biexciton yield can be achieved realizing temperatures below 4 K.