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).
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).
Integrated quantum photonics is a very active field of quantum information, communication, and processing. One of the main challenges to achieve massively parallel systems for complex operations is the generation, manipulation, and detection of many qubits within the same chip. Here, we present our last achievements on AlGaAs quantum photonic devices emitting nonclassical states of light at room temperature by spontaneous parametric down conversion (SPDC). The choice of this platform combines the advantages of a mature fabrication technology, a high nonlinear coefficient, a SPDC wavelength in the C-telecom band and the possibility of electrical injection.
Quantum states of photons such as entangled and single photons are indispensable components for applications in optical quantum information processing. Being able to generate and manipulate quantum states of photons on a chip is a crucial step towards realizing real-world quantum information processing systems such as an optical quantum computer. In this thesis, theoretical and experimental research was carried out for on-chip generation of quantum states of photons via the nonlinear process of spontaneous parametric down-conversion (SPDC), including polarization entangled photons, hyperentangled photons and heralded pure single photons. A majority of the study was based on III-V semiconductor AlGaAs, which could lead to monolithic integration of pump lasers and enable electrically pump sources operating at room temperature. The generation of cross-polarized entangled photons via a type-II SPDC in an AlGaAs Bragg reflection waveguide (BRW) was first investigated. The lack of material birefringence enables the direct generation of wavelength-multiplexed polarization entangled photons with a concurrence of at least 0.77 in a bandwidth of 95 nm. Second, a technique using concurrent type-0 and type-I processes on an AlGaAs chip to generate polarization entangled photon was developed. This technique allows the generation of both co-polarized and cross-polarized entangled photons on the same chip without the need of off-chip compensation, interferometer and bandpass filter. Third, a technique of generating hyperentangled photons on an AlGaAs chip was developed by utilizing spatial modes of different guiding mechanisms. High quality entanglement could be achieved with bandpass filtering of a few nanometers without off-chip phase compensation. Four, a generic technique using nonlinear photonic nanowires to engineer two-photon quantum states was proposed. Via dispersion engineering which controls the group velocity of each mode, maximally entangled polarization entangled photons and pure heralded single photons can be generated on-chip. Design examples were applied to III-V semiconductor and ferroelectric materials. Last, a strategy to enhance the bandwidths of wavelength conversion and SPDC spectrum was developed by tapering the waveguides. Combined with further integration of pump lasers, photonic circuits and detectors, the techniques developed in this thesis provides a viable route to achieve fully integrated generation and manipulations of photonic quantum qubits.
At the heart of quantum physics lies the principle of superposition, and at the heart of information theory lies the bit. Perhaps the most useful property of quantum systems is that they can be loaded with information bits, so-called qubits, that are indefinitely both 0 and 1 until a measurement is made. Another consequence is that several qubits can become entangled, which is manifested by the non-classical correlations between such quantum systems when measured in all possible bases. Within the rapidly progressing fields of quantum information and quantum communication these quantum effects are utilized to perform tasks such as quantum computing and quantum cryptography. In this thesis we present experimental and theoretical work using single photon sources to prepare ``flying'' photonic qubits. We describe work using mainly quasi-phase-matched nonlinear crystals to generate beams of entangled photon pairs, that are either encoded in polarization at near-visible wavelengths, or in time at optical fiber telecommunication wavelengths (1550 nm). The optical fiber is the medium used for transporting the qubits over a long distance, and it is therefore essential to couple the photons well into the fibers. By focusing the beams optimally, we have investigated how this problem can meet the requirement of creating photons of a narrow frequency bandwidth and a high photon flux. Furthermore, we have generated truly single photons that are heralded by an electrical signal. As a result of modifying the statistics of such sources we have been able to show the effect of photon antibunching. In two separate works, we have implemented a quantum key distribution system based on faint laser pulses at the telecom wavelength of 1550 nm, as well as protocols based on entanglement for performing authentication of key distribution in quantum cryptography.
Dispositifs photoniques en AlGaAs : de la génération d'etats quantiques aux communications quantiques Un des plus grands challenges dans le domaine de l’information quantique est la génération, manipulation et détection de plusieurs qubits sur des micro-puces. On assiste ainsi à un véritable essor des technologies pour l’information quantique et pour transmettre l’information, les photons ont un grand avantage sur les autres systèmes, grâce à leur grande vitesse et leur immunité contre la décohérence.Mon travail de thèse porte sur la conception, fabrication et caractérisation d’une source de photons intriqués en matériaux semiconducteurs d’une très grande compacité. Ce dispositif fonctionne à température ambiante, et émet dans la bande de longueurs d’onde télécom. Après une présentation des concepts fondamentaux (chap. 1), le chap. 2 explique la conception et la fabrication des dispositifs.Le chap. 3 présente les caractérisations opto-électroniques des échantillons pompés électriquement, et le chap. 4 les résultats des mesures de pertes et des caractérisations non-linéaires optiques (génération de seconde harmonique, conversion paramétrique spontanée et reconstruction de l’intensité spectrale jointe). Les chap. 5 et 6 se concentrent sur la caractérisation des états quantiques générés par un dispositif passif (démonstration de l’indiscernabilité et de l’intrication en énergie-temps) et leur utilisation dans un protocole de distribution de clés quantiques multi-utilisateurs (intrication en polarisation). Finalement le travail sur le premier dispositif produisant des pairs de photons dansles longueurs d’onde télécoms, injecté électriquement et fonctionnant à température ambiante est présenté (chap. 7).
Entangled photons are at the heart of experimental quantum physics. They were used for the first fundamental tests of quantum theory, and became a basic building block for many novel quantum protocols, such as quantum cryptography, dense coding or teleportation. Therefore, the efficient generation of entangled photons, as well as their distribution and accurate analysis are of paramount importance, particularly with regard to the practicability of many applications of quantum communication. This thesis deals largely with the problem of efficient generation of photonic entanglement with the principal aim of developing a bright source of polarization-entangled photon pairs, which meets the requirements for reliable and economic operation of quantum communication prototypes and demonstrators. Our approach uses a correlated photon-pair emission in nonlinear process of spontaneous parametric down-conversion pumped by light coming from a compact and cheap blue laser diode. Two alternative source configurations are examined within the thesis. The first makes use of a well established concept of degenerate non-collinear emission from a single type-II nonlinear crystal and the second relies on a novel method where the emissions from two adjacent type-I phase-matched nonlinear crystals operated in collinear non-degenerate regime are coherently overlapped. The latter approach showed to be more effective, yielding a total detected rate of almost 10^6 pairs/s at >98 % quantum interference visibility of polarization correlations. This performance, together with the almost free of alignment operation of the system, suggest that it is an especially promising candidate for many future practical applications, including quantum cryptography, detector calibration or use in undergraduate lab courses. The second issue addressed within the thesis is the simplification and practical implementation of quantum-assisted solutions to multiparty communication tasks. While the recent rapid progress in the development of bright entangled photon-pair sources has been followed with ample experimental reports on two-party quantum communication tasks, the practical implementations of tasks for more than two parties have been held back, so far. This is mainly due to the requirement of multiparty entangled states, which are very difficult to be produced with current methods and moreover suffer from a high noise. We show that entanglement is not the only non-classical resource endowing the quantum multiparty information processing its power. Instead, only the sequential communication and transformation of a single qubit can be sufficient to accomplish certain tasks. This we prove for two distinct communication tasks, secret sharing and communication complexity. Whereas the goal of the first is to split a cryptographic key among several parties in a way that its reconstruction requires their collaboration, the latter aims at reducing the amount of communication during distributed computational tasks. Importantly, our qubit-assisted solutions to the problems are feasible with state-of-the-art technology. This we clearly demonstrate in the laboratory implementation for 6 and 5 parties, respectively, which is to the best of our knowledge the highest number of actively performing parties in a quantum protocol ever implemented. Thus, by successfully solving and implementing a cryptographic task as well as a task originating in computer science, we clearly illustrate the potential to introduce multiparty communication problems into real life.
In view of real world applications of quantum information technologies, the combination of miniature quantum resources with existing fibre networks is a crucial issue. Among such resources, on-chip entangled photon sources play a central role for applications spanning quantum communications, computing and metrology. Here, we use a semiconductor source of entangled photons operating at room temperature in conjunction with standard telecom components to demonstrate multi-user quantum key distribution, a core protocol for securing communications in quantum networks. The source consists of an AlGaAs chip emitting polarization entangled photon pairs over a large bandwidth in the main telecom band around 1550 nm without the use of any off-chip compensation or interferometric scheme; the photon pairs are directly launched into a dense wavelength division multiplexer (DWDM) and secret keys are distributed between several pairs of users communicating through different channels. We achieve a visibility measured after the DWDM of 87% and show long-distance key distribution using a 50-km standard telecom fibre link between two network users. These results illustrate a promising route to practical, resource-efficient implementations adapted to quantum network infrastructures.