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Physical Review B

High-fidelity biexciton generation in quantum dots by chirped laser pulses

A. Debnath, C. Meier, B. Chatel, T. Amand

Published Nov 19, 2013• 37 citations
Publisher

Abstract

We present a detailed theoretical analysis of biexciton state generation in InAs-GaAs quantum dots by strong, chirped laser pulses. Specifically, we derive an accurate analytical expression, which not only provides a clear physical picture of the process, but also allows identifications of laser parameter regimes where efficient biexciton generation should be possible, even at temperatures up to 80 K. The results are confirmed by numerical simulations, in very good agreement with the model proposed. A clear choice of parameters is proposed, which might pave the way towards the optimal design of high-fidelity sources of entangled photon pairs based on individual quantum dots.

DOI
10.1103/physrevb.88.201305

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We present a detailed theoretical analysis of biexciton state generation in InAs-GaAs quantum dots by strong, chirped laser pulses. Specifically, we derive an accurate analytical expression, which not only provides a clear physical picture of the process, but also allows identifications of laser parameter regimes where efficient biexciton generation should be possible, even at temperatures up to 80 K. The results are confirmed by numerical simulations, in very good agreement with the model proposed. A clear choice of parameters is proposed, which might pave the way towards the optimal design of high-fidelity sources of entangled photon pairs based on individual quantum dots.
Motivated by recent experimental results, we present a detailed account of adiabatic population transfer in InGaAs/GaAs quantum dots by strong chirped laser pulses. Specifically, numerical simulations in very good agreement with experimental results allow for a clear physical picture of this process. An analytical expression is developed, which is used to analyze the strong-field adiabatic population transfer in the presence of the phonon environment. Optimal pulse parameters to achieve robust population transfer at temperatures as high as 80 K are identified and are confirmed by numerical simulations.
Abstract Nanoscale bright sources that produce high-purity single photons and high-fidelity entangled photon pairs are the building blocks to realize high security quantum communication devices. To achieve high communication rates, it is desirable to have an ensemble of quantum emitters that can be collectively excited, despite their spectral variability. In case of semiconductor quantum dots, Rabi rotations are the most popular method for resonant excitation. However, these cannot assure a universal, highly efficient excited state preparation, due to the sensitivity to excitation parameters. In contrast, adiabatic rapid passage (ARP), relying on chirped optical pulses, is immune to quantum dot spectral inhomogeneity. Here, we show that the robustness of ARP holds true for the simultaneous excitation of the biexciton states in multiple, spatially separated and spectrally different quantum dots. For positive chirps, we also find a regime where the influence of phonons relax the sensitivity to spectral detunings and lower the needed excitation power. Being able to generate high-purity photons from spatially multiplexed quantum dot sources using the biexciton to ground state cascade is a big step towards the implementation of high photon rate, entanglement-based quantum key distribution protocols.
A pulsed laser scenario, designed to prepare a variety of self-assembled quantum dots with nearly 100% population in a biexciton state, is proposed. We use realistic parameters for typical self-assembled quantum dots and show that two-colour sequential as well as concurrent excitation of the biexciton state can virtually eliminate unwanted exciton population and prepare a quantum dot in the biexciton state with near-deterministic accuracy. The ensuing radiative emission would be a prerequisite for a high rate, on-demand, source of entangled photons.
Hereby, we study the generation of pairs of polarization-entangled photons at telecom C-band by biexciton-exciton cascade from non-resonantly excited epitaxial InAs/InP quantum dots (QDs). It is realized without external tuning of the fine structure splitting (FSS), which does not exceed 10 μμμeV in as-grown nanostructures, due to their high in-plane symmetry. Excitonic complexes are identified by means of excitation power-dependent and polarization-resolved magneto)microphotoluminescence. Their origin from different carrier configurations confined in the same QD is confirmed by time correlations of emitted photons. Experimental results are supported by 8-band kp calculations, followed by the configuration interaction method to include excitonic effects. This comparison reveals the structure of higher energy states, allowing for the reconstruction of the QD structural parameters. To verify and quantify the entanglement, we perform quantum state tomography and reconstruct the two-photon density matrix. Its diagonalization allows a detailed analysis of the entangled state and reveals an unfavourable interplay between phase accumulation and decoherence, pointing to a clear route for boosting entanglement by using a XX resonant, pulsed excitation scheme and shortening the radiative lifetimes.
Semiconductor quantum dots have been featured in a number of proposals for quantum computing because of the advantages afforded by confinement on the nanoscale, such as well separated, discrete energy levels and ease of optical manipulation. In addition, they can leverage established semiconductor fabrication techniques and, like the quantum dots used in this work, can be designed to match the telecommunication band, improving the scalability and potential integration of the platform into existing technologies. This thesis work applied optical pulse engineering to manipulate exciton qubits in self-assembled InAs quantum dots. Optimal quantum control theory was used to design pulse shapes that implement high fidelity single-qubit and two-qubit operations, with constraints on the numerical optimization that ensure that the pulses can be accurately implemented using a commercial 4f pulse shaper. In the case of single-qubit operations, the results showed that two uncoupled qubits can be manipulated in parallel using a single phase-shaped laser pulse, provided their optoelectronic properties are sufficiently different. It was demonstrated that targeted differences in inversion and phase between the qubits can be achieved on an ultrafast timescale with high fidelity. In the case of two-qubit operations, the controlled-rotation gate was optimized using amplitude-only and phase-only pulse shaping schemes. The shaped pulses for both schemes were shown to produce higher fidelity operations. The ability to implement robust state inversion on short timescales is particularly useful for technologies such as ultrafast optical switches, single-photon sources, and entangled-photon sources. This work has demonstrated state inversion in a single InAs quantum dot via adiabatic rapid passage using linearly chirped laser pulses. The achieved gate times were an order of magnitude shorter than previous demonstrations. Theoretical predictions of the dependence of the inversion efficiency of the exciton on the sign of the pulse chirp were also verified experimentally, allowing for the identification of phonon-mediated dephasing as the dominant source of decoherence.
ABSTRACT The generation of entangled photon pairs is highly useful for many types of quantum technologies. In this work an entangled photon pair generator that utilizes the biexciton–exciton cascade in semiconductor quantum dots is described on a physical, mathematical, and software level. The system is implemented and simulated as a self‐contained component in a framework for bigger quantum optical experiments. Thus, it is a description to further the holistic understanding of the system for interdisciplinary audiences in a hopefully simple yet sufficient manner. It is described from the condensed matter physics fundamentals, over the most important quantum optical properties, to a mathematical description of the used model, and finally a software description and simulation, making it an executable description of such a system. We provide a compact description in the Kraus operator formalism to seamlessly incorporate such an entangled photon pair generator simulation component into bigger simulations consisting of multiple components at a reasonable computational cost. The simulation accommodates a wide range of parameter regimes and makes it possible to simulate many different excitation strategies. This includes resonant two‐photon excitation, adiabatic rapid passage chirped excitation, and dichromatic pulsed excitation.