The past two decades have witnessed tremendous advances and breakthroughs in quantum information science and technology, due mainly to the use of quantum physical resources such as coherence and entanglement. The formalization of the concept of universal quantum computing by D. Deutsch in 1985 has matured into commercial initiatives that aim to accelerate the physical implementation of a practical quantum computer. So far, such quantum technology has been pushed forward as information processors that are mainly on superconducting quantum bits (qubits) based. Other developments make use of quantum states of photons in conjunctions with other quantum registers based on electrons, atoms, molecules, artificial systems, between others. Although in any case, multiple qubit technologies are still under intense research and developments and the temperature and size of the quantum register are essential issues. However, all possible physical implementation of quantum information processing devices have in common the fundamental properties of quantum systems: interference, coherence, and entanglement. In this thesis, we concerned the study of quantum coherence and entanglement in qubits (encoded on the polarisation basis) and quantum materials (operating at room temperature) to analyze the role of quantum correlations and decoherence for information processing purposes. The current research is divided into two main parts, the first one begins with the analysis of the influence of a birefringent medium over the entanglement of a photonic qubit state. We employ a polarisation maintaining fiber (PMF) as a decohering environment to test the theoretical model in which the symmetry of the coupling between the qubit and environment defines the death and revival of the information correlations. This finding establishes a tool to keep the entanglement independent of the fiber length employing the symmetry properties of the physical system. Additionally, to demonstrate that the entanglement is not the only crucial factor in information schemes, we employ the prisoner’s dilemma game (in a two parameters strategy space and extended up to three) to evidence that quantum advantages in this protocol are by the quantum superposition instead of the entanglement of the physical system. Here we also pose an experimental setup with photons to verify our findings with photonic qubits. The second part of the thesis examines a novel nanomaterial which could serve up a bridge to the interaction of photons and electrons toward a physical representation of photonic qubits conditioned by external registers (as electrons or ions). The first stage in this direction considers the implementation of single photon emitters. However, previous to it is necessary to recognize the photophysical capabilities of the Perovskites as structure selected. As a complement, the novelty of this nanostructure allow us to give answers to some open questions in this characterization direction in the frame of this research. The MA-halide (methylammonium-halide) perovskites are structurally composed by small domains which ranging from nano to micrometer sizes presenting strong photoluminescence intermittency (blinking). We attribute this response to the Auger- nonradiative recombination of additional electrons photogenerated in a trap-filling process due to defects or ion movement. We verified this through the application of an absorbing layer (quencher) of charges of PCBM (Phenyl-C61-butyric acid methyl ester) resulting in considerably decreases in blinking. We also provide a novel technique for real-time observation of the ion movement effect over this material. Additionally, this behavior indicates appropriate conditions for the existence of a few emitters which are useful in quantum protocols and become a motivation to analyze and explain this phenomenon. Then, we prepare two chemical compositions of inorganic Perovskites with a high structural order to explore the necessary conditions to produce single emitter with this material. We visualized the samples through electron microscopy, and both systems were spectrally characterized decreasing dilutions up to observe emission intermittency. We verified the nonclassical behavior of the emission employing a Hanbury Brown-Twiss interferometer by measuring the degree of the second-order coherence correlation function. Finally, but not less important, the additional information presents the laboratories implemented during this doctoral research and the prototype of signals counting device to analyze temporal coincidences in correlated photonic events.
The past two decades have witnessed tremendous advances and breakthroughs in quantum information science and technology, due mainly to the use of quantum physical resources such as coherence and entanglement. The formalization of the concept of universal quantum computing by D. Deutsch in 1985 has matured into commercial initiatives that aim to accelerate the physical implementation of a practical quantum computer. So far, such quantum technology has been pushed forward as information processors that are mainly on superconducting quantum bits (qubits) based. Other developments make use of quantum states of photons in conjunctions with other quantum registers based on electrons, atoms, molecules, artificial systems, between others. Although in any case, multiple qubit technologies are still under intense research and developments and the temperature and size of the quantum register are essential issues. However, all possible physical implementation of quantum information processing devices have in common the fundamental properties of quantum systems: interference, coherence, and entanglement. In this thesis, we concerned the study of quantum coherence and entanglement in qubits (encoded on the polarisation basis) and quantum materials (operating at room temperature) to analyze the role of quantum correlations and decoherence for information processing purposes. The current research is divided into two main parts, the first one begins with the analysis of the influence of a birefringent medium over the entanglement of a photonic qubit state. We employ a polarisation maintaining fiber (PMF) as a decohering environment to test the theoretical model in which the symmetry of the coupling between the qubit and environment defines the death and revival of the information correlations. This finding establishes a tool to keep the entanglement independent of the fiber length employing the symmetry properties of the physical system. Additionally, to demonstrate that the entanglement is not the only crucial factor in information schemes, we employ the prisoner’s dilemma game (in a two parameters strategy space and extended up to three) to evidence that quantum advantages in this protocol are by the quantum superposition instead of the entanglement of the physical system. Here we also pose an experimental setup with photons to verify our findings with photonic qubits. The second part of the thesis examines a novel nanomaterial which could serve up a bridge to the interaction of photons and electrons toward a physical representation of photonic qubits conditioned by external registers (as electrons or ions). The first stage in this direction considers the implementation of single photon emitters. However, previous to it is necessary to recognize the photophysical capabilities of the Perovskites as structure selected. As a complement, the novelty of this nanostructure allow us to give answers to some open questions in this characterization direction in the frame of this research. The MA-halide (methylammonium-halide) perovskites are structurally composed by small domains which ranging from nano to micrometer sizes presenting strong photoluminescence intermittency (blinking). We attribute this response to the Auger- nonradiative recombination of additional electrons photogenerated in a trap-filling process due to defects or ion movement. We verified this through the application of an absorbing layer (quencher) of charges of PCBM (Phenyl-C61-butyric acid methyl ester) resulting in considerably decreases in blinking. We also provide a novel technique for real-time observation of the ion movement effect over this material. Additionally, this behavior indicates appropriate conditions for the existence of a few emitters which are useful in quantum protocols and become a motivation to analyze and explain this phenomenon. Then, we prepare two chemical compositions of inorganic Perovskites with a high structural order to explore the necessary conditions to produce single emitter with this material. We visualized the samples through electron microscopy, and both systems were spectrally characterized decreasing dilutions up to observe emission intermittency. We verified the nonclassical behavior of the emission employing a Hanbury Brown-Twiss interferometer by measuring the degree of the second-order coherence correlation function. Finally, but not less important, the additional information presents the laboratories implemented during this doctoral research and the prototype of signals counting device to analyze temporal coincidences in correlated photonic events.
For future quantum technologies and in particular for quantum architecture systems, one needs to consider scalability and reproducibility in order to be able to handle and add easily as many qubits of information as possible on a given platform. Quantum coherence and quantum information are fragile and thus requires the need for many qubits to be created, to interact and quantum information to be preserved. Currently there are either very good qubits in terms of fidelity but poor scalability or very good scalability but poor fidelity thus requiring more qubits to compensate. Most of the current platforms are in solid-state physics such as superconducting, dopants in silicon, quantum dots in III-V semiconductors or defects in diamond. The main interest for condensed matter systems is the fact that it is potentially scalable as integration is possible in the future, already demonstrated in the ‘classical’ semiconductor industry. Nevertheless, there are some true challenges in controlling and understanding the mechanisms of decoherence, losses and unwanted effects in these ‘dirty’ systems. As quantum technologies require ultimate control of quantum effects such as maintaining quantum coherence, quantum superposition and entanglement, it pushes towards deeper knowledge of underlying material and condensed matter physics. Photons are of particular interest as they are good carriers of quantum information and one aim is to explore a fully integrated photonic quantum circuit which would be a hybrid system made of stationary solid-state qubits (we will call them quantum emitters) coupled together via single photons travelling within a common optical bus that is photonics-ready for quantum communications within a network of quantum nodes. In this presentation, we will present our latest developments and results towards the integration of quantum emitters with photonic structures with nanosize features. In the first part, our platform is described which consists of glass and thus directly compatible with optical fibres. It is based on the technique of exchanged ions within the glass in order to create locally a particular confinement of the light. The next part will concern our latest results on two different quantum emitters. The first one is based on perovskite nanocristals that can be synthetised chemically and giving rise to quantum optics-type of emitters. The second one is based on the so-called silicon-vacancy defect centre in nanodiamonds. These emitters are produced by the high-pressure/high-temperature method and lead to promising results for quantum technologies. Finally, the last part will treat the nanophotonics approaches in order to couple efficiently these emitters with our platform.
Photons are very promising resources for the whole field of quantum information processing. Bulk optics are currently not suitable for the realization of complex quantum optical schemes due to several drawbacks. The ongoing miniaturization of photonic structures due to the availability of sophisticated nanofabrication has provided huge opportunities for physical research of novel phenomena in nanophotonic systems and quantum technological applications. These include quantum information processing such as quantum computing, quantum communication, quantum metrology, and quantum sensing. Quantum communication is already quite advanced with several quantum key distribution (QKD) systems commercially available offering absolute security. Photonic quantum technologies are enabling breakthroughs in experimental tests of quantum physics and are close to practical applications of quantum technologies in imaging, secure communications, ultrasensitive metrology and quantum computing. There are several advantages to encoding a quantum bit onto a photon: the preparation and manipulation of single photons is easy, and the photonic qubit is a natural candidate for quantum imaging and communications. Furthermore, the qubit can be encoded in various degrees of freedom such as on the position, phase, time-bin, energy, angular momentum, polarization of the photon—or a combination of these. This flexibility leads to enormous opportunities for scientific exploration and in practical applications. This special issue on “Photonic Quantum Technologies” presents selected papers on photonic quantum systems from leading groups in the world. The articles represent a flavor of the cutting-edge research that is going on in this area. The studied material systems range from molecules to semiconductor quantum emitters. A major challenge is the design of telecom photon emitters with high efficiency, good yield, spectral purity, stability and the possibility of mass manufacture. We believe that this Special Issue will provide an overview of recent exciting developments in the field of photonic quantum technologies, serving to keep researchers up-to-date with the latest progress and spurring on new concepts in this field.
Even though the quantized interaction between light and matter in the photoelectric effect is considered one of the cornerstones in the development of quantum mechanics, it was long thought impossible to directly observe the quantized nature of light itself. The advent of light-intensity autocorrelation techniques in the 1950s, first developed to study the size of stars, changed little in this perception, until true single-photon correlation spectroscopy from trapped ions was proven to be possible in the late 1970s: Photons from a single quantum emitter tend to arrive in bunches, but each light quantum is spaced apart in time—it is antibunched. Initially considered only as the workhorse of atomic quantum optics, over the past two decades, photon correlation techniques have become a standard in fields as diverse as quantum-information processing, biological imaging, polymer physics, and materials science, in general. The technique is particularly useful for probing materials that are specifically designed to emit or absorb light in optoelectronic devices, such as light-emitting diodes, solar cells, or lasers. The central question in such studies is how large a mesoscopic piece of material can become and still behave as an atom-like source of single photons. What interactions may arise between multiple excitations within a material, and what sort of dark states may give rise to intermittency in the stream of photons? We review the many different classes of optoelectronic materials for which photon-correlation spectroscopy has proven to offer useful insight into excited-state dynamics, ranging from molecular, over semiconductor to metallic nanostructures. The technique is particularly suited to probing mesoscopic aggregates of organic semiconductors since each single molecule acts as a quantum emitter itself.
In this thesis, I present experimental and theoretical work which assesses and develops a range of tools that are required for performing quantum information processing, particularly in photonic systems. I investigate the three degrees of freedom of a single photon—its polarisation, spatialmomentum and time-frequency distributions. For polarisation, I show that using wave plates to implement arbitrary, single-qubit rotations is more complicated than is commonly appreciated. In the spatial-momentum and time-frequency domains, I develop ways to perform tomographic analysis of quantum states, and I report the first demonstrations of these techniques. In the time-frequency domain, the tomography technique utilises entanglement in the photon polarisation as a resource to store and provide access to the time-frequency information. In my first two experiments, I use spontaneous parametric down-conversion to produce entanglement between pairs of single photons in all three degrees of freedom. I demonstrate the first characterisation of entanglement in spatial modes and the time-frequency domain, the first quantitative measurement of entangled qutrit states, and the highest quality entangled states yet measured in both polarisation and spatial modes. I also report the first realisation of complete hyperentanglement, and a full, black-box tomography of a 36-dimensional two-photon state—the largest system to be characterised in this way to date. In my final experiment, I model and implement a new architecture for a controlled-Z gate which is much simpler to align than previous implementations. This gate requires only one non-classical interference condition, the visibility of which is the main limitation to its performance. I show that the gate operates effectively as a means of both creating entanglement and discriminating between the four elements in a basis of maximally entangled, Bell-type states. Indeed, its observed performance as a Bell analyser would be sufficient to build a quantum state teleporter which would guarantee that the recipient would be left with a better copy of an unknown input state than any eavesdropper. In a series of numerical simulations, I investigate some of the practicalities that arise when using tomographic reconstruction techniques, including how to estimate errors, which measurements to make and what is actually the optimal reconstruction. In particular, I show that tomographies perform better when based on the results from over-complete sets of measurements. Finally, I discuss the important issue of how to compare two processes, particularly when trying to assess the quality of a measured process by comparing it to some expected ideal. Judging possible candidate measures against a set of experimentally and theoretically motivated
One of the main issues in the domain of quantum information and communication is the generation, manipulation and detection of several qubits on a single chip. Several approaches are currently investigated for the implementation of qubits on different types of physical supports and a variety of quantum information technologies are under development: for quantum memories, spectacular advances have been done on trapped atoms and ions, while to transmit information, photons are the ideal support thanks to their high speed of propagation and their almost immunity against decoherence. My thesis work has been focused on the conception, fabrication and characterization of a miniaturized semiconductor source of entangled photons, working at room temperature and telecom wavelengths. First the theoretical concepts relevant to understand the work are described (chapter 1). Then the conception and fabrication procedures are given (chapter 2). Chapter 3 presents the optoelectronics characterization of the device under electrical pumping, and chapter 4 the results on the optical losses measurements and the nonlinear optical characterization (second harmonic generation, spontaneous parametric down conversion and joint spectral intensity reconstruction). Chapters 5 and 6 focus on the characterization of the quantum state generated by a passive sample (demonstration of indistinguishability and energy-time entanglement) and its utilization in a multi- user quantum key distribution protocol (polarization entanglement). Finally the work on the first electrically driven photon pairs source emitting in the telecom range and working at room temperature is presented (chapter 7).
Las últimas dos décadas han sido testigos de tremendos avances y desarrollos en la ciencia y tecnología de la información cuántica, debido principalmente al uso de recursos físico-cuánticos, tales como la coherencia y el entrelazamiento. La formalización del concepto de computación cuántica universal por D. Deutsch en 1985 ha madurado hacia iniciativas comerciales que apuntan a acelerar la implementación física de una computadora cuántica práctica. Hasta ahora, dicha tecnología cuántica se ha impulsado como procesadores de información que se basan principalmente en bits cuánticos (qubits) superconductores. Otros desarrollos hacen uso de los estados cuánticos de los fotones en conjunción con otros registradores cuánticos basados en electrones, átomos, moléculas, sistemas artificiales, entre otros. Aunque, para cualquier caso, las tecnologías de qubit múltiples aún están bajo intensa investigación y desarrollos en los que la temperatura y el tamaño del registrador cuántico son cuestiones esenciales. Sin embargo, toda posible implementación física de dispositivos de procesamiento de información cuántica tiene en común las propiedades fundamentales de los sistemas cuánticos: interferencia, coherencia y entrelazamiento. En esta tesis, tratamos el estudio de la coherencia cuántica y el entrelazamiento en qubits (codificados en base a la polarización) y los materiales cuánticos (que operan a temperatura ambiente) para analizar el papel de las correlaciones cuánticas y la decoherencia para fines de procesamiento de información. La investigación actual se divide en dos partes principales: la primera comienza con el análisis de la influencia de un medio birrefringente sobre el entrelazamiento de un estado de qubit fotónico. Empleamos una fibra que mantiene la polarización (PMF) como un entorno de decoherencia para probar el modelo teórico en el que la simetría del acoplamiento entre el entorno y el qubit define la muerte y reactivación de las correlaciones de información. Este hallazgo establece una herramienta para mantener el entrelazamiento independiente de la longitud de la fibra empleando las propiedades de simetría del sistema físico. Como complemento, para demostrar que el entrelazamiento no es el único factor crucial en los esquemas de información, empleamos el juego del dilema del prisionero (en un espacio de estrategia de dos parámetros y extendido hasta tres) para demostrar que las ventajas cuánticas en este protocolo son debido a la superposición cuántica en lugar del entrelazamiento del sistema físico. Aquí también presentamos una configuración experimental con fotones para verificar nuestros hallazgos con qubits fotónicos. La segunda parte de la tesis examina un nuevo nanomaterial que podría servir de puente para la interacción de fotones y electrones hacia una representación física de qubits fotónicos condicionados mediante registros externos (como electrones o iones). La primera etapa en esta dirección considera la implementación de emisores de fotones individuales. Sin embargo, antes de esto es necesario reconocer las capacidades fotofísicas de las Perovskites como estructura seleccionada. Como complemento, la novedad de esta nanoestructura nos permite dar respuestas a algunas preguntas abiertas en esta dirección de caracterización en el marco de esta investigación. Las perovskitas de MA-haluro (metilamonio-haluro) están estructuralmente formadas por pequeños dominios que van desde tamaños de nano a micrómetros y que presentan una fuerte intermitencia de fotoluminiscencia (Blinking). Atribuimos esta respuesta a la recombinación noradiativa de Auger de electrones adicionales fotogenerados en un proceso de relleno de trampas ocasionadas por defectos o móvimiento de iones. Esto lo verificamos mediante la aplicación de una capa absorbente (quencher) de cargas de PCBM (fenil-éster metílico del ácido butírico C61), lo que resultó en una disminución considerable del parpadeo. También proporcionamos una técnica novedosa para la observación en tiempo real del efecto del movimiento de iones sobre este material. Adicionalmente, este comportamiento indica condiciones adecuadas para la existencia de pocos emisores que son útiles en protocolos cuánticos y se convierten en una motivación para analizar y explicar este fenómeno. Luego, preparamos dos composiciones químicas de perovskitas inorgánicas con un alto orden estructural para explorar las condiciones necesarias para producir un emisor cuántico con este material. Visualizamos las muestras a través de microscopía electrónica, y ambos sistemas se caracterizaron espectralmente para disminuir las diluciones hasta observar la intermitencia de la emisión. Se verificó el comportamiento no clásico de la emisión empleando un interferómetro Hanbury-Brown y Twiss mediante la medición del grado de función de correlación de coherencia de segundo orden. Finalmente, pero no menos importante, la información adicional presenta los laboratorios implementados durante esta investigación doctoral y el prototipo de un dispositivo de conteo de señales para analizar las coincidencias temporales en eventos fotónicos correlacionados.