In the fields of quantum communications and quantum computing, the transmission of information using single photons guarantees the security of the communication and enables calculations that would be impossible with our "classical" computers. One of the key elements in implementing such protocols is the photon source. It must emit, on demand, one and only one photon with specific quantum properties.The aim of this project is to characterise a source of single and indistinguishable photons, designed and manufactured at CEA-Grenoble, and consisting of an indium arsenide (InAs) semiconductor quantum dot integrated into a gallium arsenide (GaAs) nanocavity. With its low quality factor, the nanocavity enables photons to be extracted efficiently over a wide range of wavelengths (30nm). In addition, thanks to its small mode volume, the spontaneous emission of quantum dots within it can be accelerated by Purcell effect (Purcell factor up to 6).In this manuscript, we first detail the implementation and the optimisation of the experimental setups used to characterise the photons emitted by our sample : microphotoluminescence, radiative lifetime measurement, Hanbury-Brown and Twiss (HBT) experiment and Hong-Ou-Mandel (HOM) experiment. Then, we present the experimental results obtained for photons emitted by two distinct quantum dots : One containing neutral exciton states, and the second, containing a charged exciton state. In order to photo-create these exciton states in the quantum dots, two pulsed optical excitation schemes were implemented: the two-photon resonant excitation of the neutral biexciton, and the phonon-assisted quasi-resonant excitation of the neutral and charged excitons. We present the measurement of the three figures of merit of a single photon source for these two quantum dots : the brightness, the single-photon purity and the indistinguishability of the emitted photons. In a second phase, we will focus on the charged exciton state and measure the profile of its emission line and its autocorrelation function under continuous resonant excitation. These results, achieved within the nano-photonics group of the University of Basel, constitute the first demonstration of continuous excitation on these nanostructures, which is a good surprise given their small size.Finally, we will look at the effects of the application of mechanical stress on the energy separation of the two neutral exciton states, known as the "fine structure splitting" (FSS). During the radiative cascade of the neutral biexciton state, the two photons emitted are entangled in polarisation, and the fidelity of the entanglement depends in particular on the FSS. Here, we will consider a second sample, where the InAs quantum dots are embedded in a GaAs photonic nanowire. By bending the nanowire, mechanical stresses are applied to the quantum dots, which will modify the energy of the exciton states. We will present an experimental study aimed at observing a possible effect on the fine structure splitting.In conclusion, we have developed and optimised various experimental setups for characterising photon sources for applications in the fields of quantum communications and quantum computing. For a first version of these photonic nanostructures, the brightness, the single-photon purity and the indistinguishability of the photons emitted by our quantum dots integrated into nanocavities are very encouraging for the future. The next objectives are to improve the nanocavities and to characterise the decoherence phenomena and the mechanical coupling between the quantum dots and these nanostructures.
In the fields of quantum communications and quantum computing, the transmission of information using single photons guarantees the security of the communication and enables calculations that would be impossible with our "classical" computers. One of the key elements in implementing such protocols is the photon source. It must emit, on demand, one and only one photon with specific quantum properties.The aim of this project is to characterise a source of single and indistinguishable photons, designed and manufactured at CEA-Grenoble, and consisting of an indium arsenide (InAs) semiconductor quantum dot integrated into a gallium arsenide (GaAs) nanocavity. With its low quality factor, the nanocavity enables photons to be extracted efficiently over a wide range of wavelengths (30nm). In addition, thanks to its small mode volume, the spontaneous emission of quantum dots within it can be accelerated by Purcell effect (Purcell factor up to 6).In this manuscript, we first detail the implementation and the optimisation of the experimental setups used to characterise the photons emitted by our sample : microphotoluminescence, radiative lifetime measurement, Hanbury-Brown and Twiss (HBT) experiment and Hong-Ou-Mandel (HOM) experiment. Then, we present the experimental results obtained for photons emitted by two distinct quantum dots : One containing neutral exciton states, and the second, containing a charged exciton state. In order to photo-create these exciton states in the quantum dots, two pulsed optical excitation schemes were implemented: the two-photon resonant excitation of the neutral biexciton, and the phonon-assisted quasi-resonant excitation of the neutral and charged excitons. We present the measurement of the three figures of merit of a single photon source for these two quantum dots : the brightness, the single-photon purity and the indistinguishability of the emitted photons. In a second phase, we will focus on the charged exciton state and measure the profile of its emission line and its autocorrelation function under continuous resonant excitation. These results, achieved within the nano-photonics group of the University of Basel, constitute the first demonstration of continuous excitation on these nanostructures, which is a good surprise given their small size.Finally, we will look at the effects of the application of mechanical stress on the energy separation of the two neutral exciton states, known as the "fine structure splitting" (FSS). During the radiative cascade of the neutral biexciton state, the two photons emitted are entangled in polarisation, and the fidelity of the entanglement depends in particular on the FSS. Here, we will consider a second sample, where the InAs quantum dots are embedded in a GaAs photonic nanowire. By bending the nanowire, mechanical stresses are applied to the quantum dots, which will modify the energy of the exciton states. We will present an experimental study aimed at observing a possible effect on the fine structure splitting.In conclusion, we have developed and optimised various experimental setups for characterising photon sources for applications in the fields of quantum communications and quantum computing. For a first version of these photonic nanostructures, the brightness, the single-photon purity and the indistinguishability of the photons emitted by our quantum dots integrated into nanocavities are very encouraging for the future. The next objectives are to improve the nanocavities and to characterise the decoherence phenomena and the mechanical coupling between the quantum dots and these nanostructures.
The advent of quantum computation promises exciting advances, not only in fields like medicine and metrology, but many industries that rely on parameter-heavy calculations or simulation of molecular interaction. At the same time Shor's algorithm for quantum computers presents a threat to current asymmetric encryption protocols used in everyday communication. Flying qubits, i.e. single-photons, can help mitigate this problem via quantum key distribution, which is insusceptible to an increase in computational power. In addition, they can link quantum computers, forming a quantum network, so that quantum states can be transmitted between them. Sources of flying qubits need good performance in key metrics like single--photon purity, repetition rate, indistinguishability and brightness to become useful in these applications. They should ideally emit strongly entangled pairs of photons and be matched to other quantum technologies in bandwidth and emission energy. In this thesis the emission characteristics of single epitaxial quantum dots, the single-photon source of our choice, are investigated. Strongly entangled photon-pair emission is demonstrated for three different quantum-dot systems: InAsP quantum dots embedded in nanowire waveguides, suitable for integration into photonic circuits, show emission of single photons and entangled photon pairs under non-resonant and quasi-resonant excitation. Violation of Bell's inequality is demonstrated using the traditional set of polarization angles. GaAs quantum dots grown in droplet--etched nanoholes are tested with two resonant excitation methods: Using resonance fluorescence, near-unity indistinguishability and re-excitation limited single-photon purity, albeit not simultaneously with laser-inherited bandwidth, are measured. Using two-photon resonant excitation we set a new standard for single-photon purity, can generate pairs of entangled photons but suffer from reduced indistinguishability. In addition, nanofabrication of paraboloid shaped reflectors for enhanced extraction efficiency of photons and strain-tuning of the emission energy into resonance with the 87Rb D1-line are demonstrated. Strain-tunable InAs quantum dots emitting in the telecom C-band are investigated under above-band excitation and two different resonant two-photon excitation techniques, all of which cause pure single-photon emission. Using the robust phonon-assisted two-photon excitation technique, close-to ideal entangled photon-pair emission is demonstrated. For many of these findings photon arrival times were recorded over many hours with temporal precision on the order of 10 ps. We have developed a user-friendly, yet versatile piece of software in order to extract as much information as possible from this vast amount of data. These results will facilitate integration of quantum dot based single- and entangled-photon sources into future quantum networks and quantum key distribution systems.
In the pursuit of developing a quantum information network, photons appear to be the most convenient carriers to interconnect distant ports. The need to get on-demand single photons that is one and only one photon, with a high reliability, is the main driving force for the development of bright solid-state sources. One important parameter is the brightness defined as the probability that one collected pulse contains a single photon. For some applications like quantum information processing or long distance quantum communications, the emitted single photons must be also indistinguishable, so that can make use of their quantum interference to implement effective photon-photon interactions. To reach the ideal source of single and indistinguishable photons, different systems are explored : defects in diamond (NV centers), trapped atoms or ions, single molecules or semiconductor quantum dots.By coupling a semiconductor quantum dot to an optical cavity, the spontaneous emission of the emitter can be modified to obtain bright single-photon sources. An innovative technique was developed by Pr. Pascale Senellart and her team at the Laboratory of Photonics and Nanostructures (LPN) from CNRS that allows making such sources in a very reproducible way.This work explores the performance of single quantum dot coupled to micropillars. Various techniques are used in order to increase the single photon purity and indistinguishability while keeping a high source brightness. First, the cavity was modified using an adiabatic architecture such that a strong acceleration of the spontaneous emission was implemented. Then, a technique to apply an electric bias on the micropillars has been developed. The combination of the electric bias with a resonant optical excitation of the quantum dot allows to generation purely indistinguishable photons with a high brightness.The results developed in this thesis open a vast field of novel applications in quantum technologies, from quantum cryptography, metrology to quantum imaging.
Une source efficace de photons uniques et indiscernables Dans les domaines des communications quantiques et du calcul quantique, la transmission d'information à l'aide de photons uniques garantis la sécurité de la communication et permet la mise en œuvre de calculs irréalisables avec nos ordinateurs « classiques ». L'un des éléments primordiaux à la réalisation de tels protocoles est la source de photons. Elle doit émettre, à la demande, un et un seul photon aux propriétés quantiques déterminées.L'objectif de ce projet de thèse est de caractériser une source de photons uniques et indiscernables, conçue et fabriquée au CEA-Grenoble, et qui consiste en une boite quantique semiconductrice d'arséniure d'indium (InAs) intégrée à une nanocavité d'arséniure de gallium (GaAs). Présentant un faible facteur de qualité, la nanocavité permet l'extraction efficace de photons sur une grande plage de longueurs d'ondes (30nm). De plus, grâce à son petit volume de mode, l'émission spontanée des boites quantiques en son sein peut être accélérée par un effet Purcell pouvant aller jusqu'à 6.Dans ce manuscrit de thèse, nous détaillons tout d'abord la mise en place et l'optimisation des dispositifs expérimentaux utilisés pour caractériser les photons émis par notre échantillon : Micro-photoluminescence, mesure du temps de vie radiatif, expérience Hanbury-Brown et Twiss (HBT) et expérience Hong-Ou-Mandel (HOM). Ensuite, nous présentons les résultats expérimentaux obtenus pour les photons émis par deux boites quantiques distinctes : L'une contenant des états d'excitons neutres, et la seconde, un état d'exciton chargé. Afin de photo-créer ces états d'excitons dans les boites quantiques, deux dispositifs d'excitation optique impulsionnelle ont été mis en place : L'excitation résonante à deux photons du biexciton neutre, et l'excitation quasi-résonante assistée par phonons longitudinaux acoustiques de l'exciton neutre et de l'exciton chargé. Nous présentons la mesure des trois figures de mérite d'une source de photon unique pour ces deux boites quantiques : La brillance, l'unicité des photons émis et leur indiscernabilité. Dans un second temps, nous nous intéresserons particulièrement à l'état d'exciton chargé et mesurerons le profil de sa raie d'émission et sa fonction d'autocorrélation sous excitation résonante continue. Ces résultats, réalisés au sein du groupe de nano-photonique de l'université de Bâle, constituent la première démonstration d'excitation continue sur ces nanostructures, ce qui est une agréable surprise étant donné leur petite taille.Pour terminer, nous nous pencherons sur l’effet de l’application de contraintes mécaniques sur la séparation en énergie des deux états de l’exciton neutre, appelée la « séparation de structure fine ». Lors de la cascade radiative de l’état du biexciton neutre, les deux photons émis sont intriqués en polarisation, et la fidélité de l’intrication dépend notamment de la séparation de structure fine. Nous étudierons ici un second échantillon, où les boites quantiques InAs sont intégrées à un microfil photonique de GaAs. En faisant fléchir le microfil, des contraintes mécaniques sont appliquées sur les boites quantiques, ce qui a pour effet de modifier l’énergie des états de l'excitons. Nous présenterons une étude expérimentale visant à observer un possible effet sur la séparation de structure fine.En conclusion, nous avons développé et optimisé divers dispositifs expérimentaux permettant la caractérisation de sources de photons pour des applications dans le domaine des communications quantiques et du calcul quantique. Pour une première version de ces nanostructure photonique, la brillance, l'unicité de l'émission et l'indiscernabilité des photons émis par nos boites quantiques intégrées dans des nanocavités sont très encourageantes pour la suite. Les prochains objectifs portent sur l'amélioration des nanocavités et sur la caractérisation plus approfondie des phénomènes de décohérence et du couplage mécanique entre les boites quantiques et ces structures.
Semiconductor Quantum dots are potentially an attractive source of non-classical light. They can confine bound electron-hole pairs, excitons, in all three spatial dimensions giving them discrete optical emission energies, much like the electronic states in atomic systems. The semiconductor nature of the system allows quantum dots to be integrated into complicated device structures including optical cavities [1] and electrical injection [2]. The radiative exciton emission from a semiconductor quantum dot can provide the simplest form of quantum light, a single photon source [2]. The finite re-excitation time following the radiative decay of an exciton in a quantum dot prevents the dot from emitting more than one photon if excited with a short optical or electronic pulse. In a similar fashion excitation of the biexciton state, consisting of two electrons and holes, in the quantum dot results in the emission of a pair of photons at the energy of the biexciton and exciton transitions. The biexciton state was, in fact, proposed as a possible source of polarisation-entangled photons in 2000 by Benson et al. [3] as it consists of two polarisation-correlated decay paths which could potentially be indistinguishable. The condition of indistinguishability is fundamental to the generation of entangled photons and is, in general, not true of the two decay paths from the biexciton state in a quantum dot; physical anisotropy causes the intermediate exciton states to hybridise and separate in energy providing ‘which-path’ information about the route taken through the biexciton decay. We demonstrate how the splitting between the intermediate exciton states in a single InAs quantum dot can be controlled by growth [4], rapid thermal annealing [5] and the application of an external magnetic field [6]. Each of these three techniques can be used to make the intermediate exciton states degenerate allowing the biexciton decay to produce singe pairs of entangled photons triggered by short laser pulses [7,8]. Entangled photon pair sources are essential for applications such as entanglement based quantum key distribution, and optical quantum computing. For these applications it is important that no more than one entangled photon pair is generated in a cycle, a requirement met by our quantum dot source which is not possible with traditional methods for generating entangled pairs of photons. Entanglement also enables quantum interferometry and metrology, the basis of image resolution beyond that possible with equivalent classical light. Biphoton interferometry using entangled emission from the biexciton decay reveals fringes with half the period of classical light [9], and less that that of the laser. In addition, interferometry suggests that this type of entangled light source is not limited by decoherence, we fine the biphoton is surprising robust against dephasing, in stark contrast to the devastating effect of decoherence upon single photons emitted by similar structures.
Resonant laser excitation of a two-level system with subsequent single-photon emission can be used to generate single photons with high indistinguishability or Hong-Ou-Mandel (HOM) visibility. However, spectral overlap between excitation laser and emitted photons generally poses significant challenges. Furthermore, emitter re-excitation intrinsically limits achievable single-photon purity. Established solutions mitigate these issues at significant cost to source efficiency and with increased source complexity. This motivates the use of few-level systems with spectral separation of excitation and emission pathways. One option is a three-level cascade. However, without targeted lifetime engineering of emitting states, the cascade naturally limits achievable photon indistinguishability. Here we study a semiconductor quantum dot with resonant and selective cavity-enhancement of biexciton-to-exciton transition. Following resonant two-photon excitation of the biexciton state, we collect the emitted single photon with the cavity. This approach circumvents emitter re-excitation and naturally introduces spectral separation of excitation laser and emitted single photon. Supported by first experimental results, we demonstrate theoretically that with selective Purcell enhancement, the observed quality quantifiers of single-photon emission (purity, equivalently g(2)(0), and HOM visibility V, equivalently indistinguishability) are competitive with respect to high-quality deterministic quantum-dot single-photon sources. This is already achieved without systematic optimization or targeted system engineering, which firmly places the reported approach as a viable route to the next generation of highest-quality quantum-dot based deterministic single-photon sources.
The characteristics of single photons emitted by columnar microstructures based on epitaxial microcavity heterostructures with distributed Bragg reflectors, which include self-organized InAs/GaAs quantum dots and have a comparatively low Q factor in the range of 2000–3000, have been studied. It has been shown that a state with a given spin configuration—exciton with a certain polarization or trion—can be initialized in a single quantum dot under the coherent resonant linearly polarized optical pumping by a π pulse. The measurement of two-photon interference by the Hong–Ou–Mandel scheme has demonstrated that the degree of indistinguishability of successively emitted single photons is 97 and 93% at a time delay of 2 and 250 ns, respectively. Prospects of application of such sources in optical quantum computing schemes have been discussed.