Single-photon emitters in monolayer WSe2 are created at the nanoscale gap between two single-crystalline gold nanorods. The atomically thin semiconductor conforms to the metal nanostructure and is bent at the position of the gap. The induced strain leads to the formation of a localized potential well inside the gap. Single-photon emitters are localized there with a precision better than 140 nm.

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...

For a CdTe quantum dot doped with a single Mn atom we analyze the dynamics of the Mn spin when the dot is excited by ultrashort laser pulses. Because of the exchange interaction with the Mn atom, electron and hole spins can flip and induce a change of the Mn spin. Including both heavy and light-hole excitons and using suitable pulse sequences, angular momentum can be transferred from the light to the Mn system while the exciton system returns to its ground state. We show that by a series of ultrashort laser pulses the Mn spin can be selectively driven into each of its six possible orientations on a picosecond timescale. By applying a magnetic field the total switching time and the required number of pulses can be strongly reduced.

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Abstract. For many applications of semiconductor quantum dots in quantum technology a well controlled state preparation of the quantum dot states is mandatory. Since quantum dots are embedded in the semiconductor matrix, the interaction with phonons plays often a major role in the preparation process. In this review, we discuss the influence of phonons on three basically different optical excitation schemes which can be used for the preparation of exciton, biexciton, and superposition states: a resonant excitation leading to Rabi rotations in the excitonic system, an excitation with chirped pulses exploiting the effect of adiabatic rapid passage, and an off-resonant excitation giving rise to a phononassisted state preparation. We give an overview over experimental and theoretical results showing the role of the phonons and compare the performance of the schemes for state preparation.