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arXiv:1303.3435 · Physical Review Letters

Proposed Robust and High-Fidelity Preparation of Excitons and Biexcitons in Semiconductor Quantum Dots Making Active Use of Phonons

Martin Glässl, A. M. Barth, V. M. Axt

Published Apr 2, 2013• 100 citations
arXivHTMLPDF

Abstract

It is demonstrated how the exciton and the biexciton state of a quantum dot can be prepared with high fidelity on a picosecond time scale by driving the dot with a strong laser pulse that is tuned above the exciton resonance for exciton preparation and in resonance with the exciton transition for biexciton preparation. The proposed protocols make use of the phonon-induced relaxation towards photon dressed states in optically driven quantum dots and combine the simplicity of traditional Rabi oscillation schemes with the robustness of adiabatic rapid passage schemes. Our protocols allow for an on-demand, fast, and almost perfect state preparation even at strong carrier-phonon interaction where other schemes fail. In fact, the performance of the presented protocols is shown to be better the stronger the carrier-phonon interaction is.

DOI
10.1103/physrevlett.110.147401

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It is demonstrated how the exciton and the biexciton state of a quantum dot can be prepared with high fidelity on a picosecond time scale by driving the dot with a strong laser pulse that is tuned above the exciton resonance for exciton preparation and in resonance with the exciton transition for biexciton preparation. The proposed protocols make use of the phonon-induced relaxation towards photon dressed states in optically driven quantum dots and combine the simplicity of traditional Rabi oscillation schemes with the robustness of adiabatic rapid passage schemes. Our protocols allow for an on-demand, fast, and almost perfect state preparation even at strong carrier-phonon interaction where other schemes fail. In fact, the performance of the presented protocols is shown to be better the stronger the carrier-phonon interaction is.
We investigate both experimentally and theoretically a simple yet more robust and flexible alternative to Rabi oscillation-type biexciton preparation protocols traditionally used for semiconductor quantum dots. The quantum dot is excited by a strong laser pulse positively detuned from the two-photon resonance yielding an on demand initialization of the biexciton state by making use of the phonon-induced thermalization of the photon dressed states. It is shown that for excitation pulses in the picosecond range, a stable and high occupation of up to ${C}_{XX}=0.95\ifmmode\pm\else\textpm\fi{}0.02$ is reached. Notably, the generated photons show similar coherence properties as measured in the resonant two-photon scheme. This protocol is a powerful tool for the control of complex solid state systems combining radiative cascades, entanglement, and resonant cavity modes.
Pulsed resonant fluorescence is used to probe ultrafast phonon-assisted exciton and biexciton preparation in individual self-assembled InGaAs quantum dots. By driving the system using large area $(\ensuremath{\ge}10\ensuremath{\pi})$ near resonant optical pulses, we experimentally demonstrate how phonon-mediated dissipation within the manifold of dressed excitonic states can be used to prepare the neutral exciton with a fidelity $\ensuremath{\ge}70%$. By comparing the phonon-assisted preparation with resonant Rabi oscillations we show that the phonon-mediated process provides the higher-fidelity preparation for large pulse areas and is less sensitive to pulse area variations. Moreover, by detuning the laser with respect to the exciton transition, we map out the spectral density for exciton coupling to the bulk LA-phonon continuum. Similar phonon-mediated processes are shown to facilitate direct biexciton preparation via two-photon biexciton absorption, with fidelities >80>80%>80. Our results are found to be in very good quantitative agreement with simulations that model the quantum dot--phonon bath interactions with Bloch-Redfield theory.
A biexciton in a semiconductor quantum dot is a source of polarization-entangled photons with high potential for implementation in scalable systems. Several approaches for nonresonant, resonant, and quasiresonant biexciton preparation exist, but all have their own disadvantages; for instance, low fidelity, timing jitter, incoherence, or sensitivity to experimental parameters. We demonstrate a coherent and robust technique to generate a biexciton in an InGaAs quantum dot with a fidelity close to 1. The main concept is the application of rapid adiabatic passage to the ground-state--exciton--biexciton system. We reinforce our experimental results with simulations which include a microscopic coupling to phonons.
We investigate theoretically the temporal behavior of a quantum dot under off-resonant optical excitation targeted at fast acoustic phonon-assisted state preparation. We demonstrate that in a preparation process driven by short laser pulses, three processes can be identified: a dressing of the states during the switch-on of the laser pulse, a subsequent phonon-induced relaxation, and an undressing at the end of the pulse. By analyzing excitation scenarios with different pulse shapes we highlight the decisive impact of an adiabatic undressing on the final state in short-pulse protocols. Furthermore, we show that in exciton-biexciton systems the laser characteristics such as the pulse detuning and the pulse length as well as the biexciton binding energy can be used to select the targeted quantum dot state.
Self-assembled semiconductor quantum dots are often referred to as artificial atoms. They are bright single photon sources implemented in an environment easier scalable then implementations with single atoms. These properties promote quantum dots as promising candidates for quantum information technology. A high fidelity state preparation reduces the error rate in quantum information protocols and is hence an essential requirement. \n \nIn this thesis the technique of rapid adiabatic passage is implemented with ultra-fast pulse parameters and is used to study single self-assembled quantum dots by detecting the resonance fluorescence signal following an optical excitation. Different experimental schemes provide insights into different aspects of a quantum dot and its environment. \n \nThe negative trion transition in a single quantum dot represents a good approach to creating an ideal two-level system. With this two-level system, the interaction of the electron trapped in the QD and phonons in the host material is studied with a single chirped pulse. Measuring the resonance fluorescence response of the quantum dot as a function of the pulse area for a set of chirp parameters reveals the electron-phonon interaction. Phonons in the semiconductor environment are a source of decoherence. The non-monotonic behaviour of the coupling to phonons is experimentally demonstrated. Furthermore, a decoupling regime where the electron oscillation is too fast for phonons to follow is reached. A high fidelity state preparation with a vanishing coupling to phonons is demonstrated. Experimental results are affirmed with an excellent agreement with simulations. \n \nIn another scheme, a three-level system consisting of the crystal ground state of the quantum dot, the neutral exciton and the biexciton is investigated. The goal in this experiment is a coherent, resonant and high fidelity preparation of a biexciton state robust against system parameter fluctuations. A biexciton in a semiconductor quantum dot is a source of polarization-entangled photons with high potential for implementation in scalable systems. An excitation with chirped pulses applying the technique of rapid adiabatic passage is the key for the biexciton preparation scheme. In contrast to other state of the art techniques, an interaction with phonons is here intentionally minimized reducing the dephasing and maintaining at the same time a robustness with respect to pulse area and detuning. A fidelity close to one is reached over a pulse area range of more than π. Also in this case, the interpretation of the experiment is confirmed by an excellent agreement with simulations which include a microscopic coupling to phonons. \n \n \nIn a third experiment, a sequence of two chirped pulses with different point spread functions is used to overcome the diffraction limit in spectroscopy. A universal technique - optical nanoscopy via quantum control - to perform diffraction-unlimited spectroscopy on two-level systems is presented. A model for simulating the system is developed and gives a prediction for the expected spatial resolution as a function of the pulse parameters. The concept is demonstrated and the prediction is fulfilled on self-assembled semiconductor quantum dots. A resolution down to 30 nm with an excitation wavelength of 950 nm is reached.
We demonstrate theoretically that the single-photon purity of photons emitted from a quantum dot exciton prepared by phonon-assisted off-resonant excitation can be significantly higher in a wide range of parameters than that obtained by resonant preparation for otherwise identical conditions. Despite the off-resonant excitation, the brightness stays on a high level. These surprising findings exploit the fact that the phonon-assisted preparation is a two-step process where phonons first lead to a relaxation between laser-dressed states while high exciton occupations are reached only with a delay to the laser pulse maximum by adiabatically undressing the dot states. Due to this delay, possible subsequent processes, in particular multiphoton excitations, appear at a time when the laser pulse is almost gone. The resulting suppression of reexcitation processes increases the single-photon purity. Due to the spectral separation of the signal photons from the laser frequencies this enables the emission of high quality single photons not disturbed by a laser background while taking advantage of the robustness of the phonon assisted scheme.