This report is a synthesis of my master thesis internship at the National Institute of Informatics (NII) in Tokyo, Japan, that lasted during the summer of year 2012. I worked in the Quantum Information Science Theory (QIST) group under supervision of Prof. Kae Nemoto and Dr. Simon Devitt. This group works on theoretical and experimental implementations of quantum information science. The aim of my project was to study and improve quantum optical systems. I first studied different fields and systems of quantum information science. Then I focused my research on single-photon sources, entangled photon sources and interferometric photonic switches. Finally, I found some strategies to design an efficient and optimized single-photon source that could be built with today's technologies. This report describes in details the created and optimized design of a single-photon source based on time and space multiplexing of Spontaneous Parametric Downconversion (SPDC) sources.
This report is a synthesis of my master thesis internship at the National Institute of Informatics (NII) in Tokyo, Japan, that lasted during the summer of year 2012. I worked in the Quantum Information Science Theory (QIST) group under supervision of Prof. Kae Nemoto and Dr. Simon Devitt. This group works on theoretical and experimental implementations of quantum information science. The aim of my project was to study and improve quantum optical systems. I first studied different fields and systems of quantum information science. Then I focused my research on single-photon sources, entangled photon sources and interferometric photonic switches. Finally, I found some strategies to design an efficient and optimized single-photon source that could be built with today's technologies. This report describes in details the created and optimized design of a single-photon source based on time and space multiplexing of Spontaneous Parametric Downconversion (SPDC) sources.
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).
Over the last sixty years, classical information theory has revolutionized the understanding of the nature of information, and how it can be quantified and manipulated. Quantum information processing extends these lessons to quantum systems, where the properties of intrinsic uncertainty and entanglement fundamentally defy classical explanation. This growing field has many potential applications, including computing, cryptography, communication, and metrology. As inherently mobile quantum particles, photons are likely to play an important role in any mature large-scale quantum information processing system. However, the available methods for producing and detecting complex multi-photon states place practical limits on the feasibility of sophisticated optical quantum information processing experiments. In a typical quantum information protocol, a source first produces an interesting or useful quantum state (or set of states), perhaps involving superposition or entanglement. Then, some manipulations are performed on this state, perhaps involving quantum logic gates which further manipulate or entangle the intial state. Finally, the state must be detected, obtaining some desired measurement result, e.g., for secure communication or computationally efficient factoring. The work presented here concerns the first and last stages of this process as they relate to photons: sources and detectors. Our work on sources is based on the need for optimized non-classical states of light delivered at high rates, particularly of single photons in a pure quantum state. We seek to better understand the properties of spontaneous parameteric downconversion (SPDC) sources of photon pairs, and in doing so, produce such an optimized source. We report an SPDC source which produces pure heralded single photons with little or no spectral filtering, allowing a significant rate enhancement. Our work on detectors is based on the need to reliably measure single-photon states. We have focused on optimizing the detection efficiency of visible light photon counters (VLPCs), a single-photon detection technology that is also capable of resolving photon number states. We report a record-breaking quantum efficiency of 91 +/- 3% observed with our detection system. Both sources and detectors are independently interesting physical systems worthy of study, but together they promise to enable entire new classes and applications of information based on quantum mechanics.
This doctoral dissertation deals with the subject of single-photon technology applications. Particular emphasis was placed on the use of single-photon sources in quantum communication, metrology, and further development of quantum computers. At the beginning of this thesis, the development history of cryptography is described, starting from steganography and ending with quantum key distribution protocols. Later, the key elements of the quantum network, such as end nodes and transmission channels, are presented. Various types of sources and single-photon detectors are discussed. Methods for amplifying the signal and correcting errors that occur during transmission are also mentioned. In the further part of this thesis, two examples of single-photon sources are described, along with their experimental implementation. These sources are based on the nonlinear spontaneous parametric down-conversion (SPDC) process, which has also been described in detail. Parameters that specify the quality of a single-photon source (such as conversion efficiency, phase-matching conditions, singleness and photonic density matrix) are also characterized, both mathematically and experimentally. Furthermore, this doctoral dissertation consists of definitions and elaborated descriptions of quantum optical phenomena, which should be useful for those who are not yet familiar with the subject of single-photon sources and their applications or just start their work in this field.
During my thesis I investigated several technologies and protocols aiming at practical quantum-cryptography applications. Firstly, I developed a free-running single-photon detector for telecom wavelengths featured by an extremely low noise that competes with the state-of-the-art superconducting single-photon detectors. Then I developed a testbench for metrological characterization of single-photon detectors. A special source of spontaneous emission and a radiometer have been developed during this work. Both the source and the radiometer are based on an Erbium-doped fiber and have been built using standard commercial components. In the third part of my thesis I discuss the first experimental demonstration of a bit commitment protocol provably secure. Finally I investigated an innovative approach to self-test the randomness underlying a quantum process. This approach allows generating unpredictable strings of bits with few calibrations of the device.
This thesis examines the integration of two disparate technologies in order to perform experiments in single photon quantum optics with low loss. Many technologies and experiments in quantum optics, communication, or computing require a certain fraction of the photons involved to be received at the end of the experiment, and in many cases the required efficiency has not yet been reached. This includes the famous Einstein, Podolsky, and Rosen gedanken-experiment, now implemented in the laboratory, demonstrating the existence of entanglement to unconvinced observers. Also included is the nonlocality test of John Bell, as well as technological problems such as device-independent quantum key distribution. In this thesis I perform these experiments in the high-efficiency regime. This programme requires the integration of two lines of research: improving sources of single photons, and improving detectors thereof. Until recently detector research was focussed on development, with improvements being sought for their own sake, working towards the ultimate goal of perfect photon detection. Recent years have seen these devices move into quantum photonics laboratories, allowing for previously impossible experiments to be undertaken. In this thesis, I combine high-efficiency, number-resolving, detectors with a high-efficiency entangled photon pair source, based on another line of research going back decades: the use of spontaneous parametric down-conversion to create single-photon-like modes of light, and the entanglement of the output modes in a useful way. For small demonstrations, such as the experiments mentioned above, this can emulate a single photon with enough fidelity, and low enough loss, to successfully perform the procedure. Here I push both of these technologies, and the problems of combining them, as far as I can, and solve the problems inherent with any marriage of disparate devices. I also examine the relative performance of two different steering inequalities, one linear and one quadratic, in the presence of noise, analysing my experimental data with each of them. I perform a detection- loophole free demonstration of EPR steering, violating the local bound by 200s, setting a then world record for efficiency at 62.5%.
A number of optical technologies remain to be developed and optimized for various applications in quantum information processing, especially quantum communication. We will give an overview of our approach to some of these, including periodic heralded single-photon sources based on spontaneous parametric down-conversion, ultrabright sources of tunable entangled photons, near unit efficiency single- and multi-photon detectors based on an atomic vapor interaction, quantum state transducers based on high efficiency frequency up-conversion, and low-loss optical quantum memories.