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arXiv:1408.7027 · Physical Review B

Phonon-assisted robust and deterministic two-photon biexciton preparation in a quantum dot

S. Bounouar, Markus A. Müller, A. M. Barth, Martin Glässl, V. M. Axt, Peter Michler

Published Apr 9, 2015• 89 citations
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Abstract

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.

DOI
10.1103/physrevb.91.161302

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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 CXX=0.95\ifmmode±\else\textpm\fi0.02{C}_{XX}=0.95\ifmmode\pm\else\textpm\fi{}0.02CXX​=0.95\ifmmode±\else\textpm\fi0.02 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.
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.
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.
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.
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.
Quantum emitters driven by resonant two-photon excitation are a leading source for deterministically generated entangled photon pairs, essential for scalable photonic quantum technologies. However, conventional excitation schemes pose various constraints, limiting their scalability and practicability. Here, we demonstrate how biexciton preparation schemes with significantly improved robustness and reduced experimental demands can be identified using a novel design principle: ultrafast single-shot Floquet driving. This is achieved by employing two strongly and symmetrically detuned pulses, whose superposition generates a stroboscopic Hamiltonian that enables direct coupling between ground and biexciton states. Moreover, a pulse delay serves as a tuning knob, introducing an effective magnetic field making biexciton preparation particularly robust. Experimentally, we achieve a biexciton occupation exceeding 96% and preserve photon-pair entanglement with a fidelity of 93.4%. Our work opens avenues for the design of quantum control protocols with superior performance across diverse quantum emitters and qubit platforms.
The on-demand preparation of higher-order Fock states is of fundamental importance in quantum information sciences.We propose and compare different protocols to generate higher-order Fock states in solid state quantum-dot-cavity systems.The protocols make use of a series of laser pulses to excite the quantum dot exciton and off-resonant pulses to control the detuning between dot and cavity.Our theoretical studies include dot and cavity loss processes as well as the pure-dephasing type coupling to longitudinal acoustic phonons in a numerically complete fashion.By going beyond the two-level approximation for quantum dots, we study the impact of a finite exchange splitting, the impact of a higher energetic exciton state, and an excitation with linearly polarized laser pulses leading to detrimental occupations of the biexciton state.We predict that under realistic conditions, a protocol which keeps the cavity at resonance with the quantum dot until the desired target state is reached is able to deliver fidelities to the Fock state |5 well above 40%.