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.
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.
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.
A quantum emitter (QE) is a physical system that can be used to encode a quantum state via some internal degree of freedom (e.g. exciton, electron spin, valley) and is coupled to light via a dipolar transition that enables the conversion of that quantum state into the state of a photon and vice versa. Such QEs can be applied to sources of single and entangled photons for applications in quantum cryptography or quantum imaging and a collection of coupled QEs can be used to realize a small quantum simulator or quantum memory node in a distributed quantum network. Among solid state QE systems, semiconductor quantum dots (QD) are particularly attractive due to their high radiative quantum efficiencies, their strong optical coupling, enabling fast and arbitrary qubit rotations, and their tunable emission in the range of standard telecommunication wavelengths. Adiabatic rapid passage (ARP) is a state preparation scheme that enables the excitation and de-excitation of a QE with a high fidelity even in the presence of fluctuations in experimental parameters such as intensity and frequency of the control laser pulse. This excitation scheme may be used for quantum state initialization in quantum information platforms and for triggering quantum light sources. This thesis work concerns an exploration of the efficacy of ARP for the quantum control of excitons in semiconductor QDs. The experiments carried out in this work show that the impact of phonons may be suppressed for ARP, pointing to the possibility of high-fidelity inversion at elevated temperatures. We also investigate the dependence of the threshold for decoherence suppression on the size and shape of the QD. The experiments performed in this thesis work also quantify the detuning tolerance of ARP. ARP was also used to demonstrate parallel quantum state initialization in two different QDs with different transition energies and dipole moments.
Excitation of a semiconductor quantum dot with a chirped laser pulse allows excitons to be created by rapid adiabatic passage. In quantum dots this process can be greatly hindered by the coupling to phonons. Here we add a high chirp rate to ultrashort laser pulses and use these pulses to excite a single quantum dot. We demonstrate that we enter a regime where the exciton-phonon coupling is effective for small pulse areas, while for higher pulse areas a decoupling of the exciton from the phonons occurs. We thus discover a reappearance of rapid adiabatic passage, in analogy to the predicted reappearance of Rabi rotations at high pulse areas. The measured results are in good agreement with theoretical calculations.
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.
Single semiconductor quantum dots, which are considered as two-level systems in the artificial atom model, are promising structures for the realization of integrated devices such as single photon sources for quantum information applications. However, contrary to genuine atoms, they are condensed matter systems which suffer from the coupling to their solid environment, leading to a degradation of the coherence of the emitted photons. One possibility to avoid this coupling, thereby improving the coherence of the photons until the one imposed by the radiative limit T2 = 2T1, is to perform strictly resonant excitation of a quantum dot at low temperature. Using an experimental setup that spatially decouples the excitation and detection paths, we showed that a fine control of the electrostatic environment related to the nanostructures residual doping remains a major challenge to answer the widely encountered problem of the inhibition of the resonance fluorescence of a quantum dot. Meanwhile, thanks to quantum optics experiments and high resolution optical spectroscopy, we demonstrated that the resonant Rayleigh scattering regime where quantum dots emit single photons with the laser coherence time, paves the way to the development of "ultra-coherent" sources of single photons with high degrees of indistinguishability. In this regime where the laser tailors the coherence of the photons and where the radiative limit becomes a secondary requirement, two photons emitted successively by the same quantum dot are indistinguishable on time scales never obtained before for a solid nano-emitter (up to ten nanoseconds, comparing to the carriers lifetime and decoherence time of about a hundred of picoseconds).
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.