Today, quantum photonics is one of the leading platforms for the research and development of the quantum information sciences and technologies. It is also the most promising platform for quantum networking protocols. As such, the investigation and characterization of quantum light sources is vital to the future of quantum research. In this thesis document, titled “Multimode Statistical Photonics and Quantum State Tomography for Quantum Networks,” I present my research on the multi-mode statistical properties of classical light sources, with applications to quantum state tomography methods for use in quantum communications and networking protocols. During my time here at LSU, I have worked with my fellow researchers in the quantum photonics group to investigate the quantum dynamics which govern the classical statistical behaviors of stochastically-random light sources. By extracting the correlated multiphoton, multi-mode subsystems of such sources, we have discovered nontrivial, uniquely-quantum behaviors within sources of light which were previously thought to be only classical. I have also developed a robust theoretical framework for describing such sources, utilizing techniques from quantum field theory, which we have shown to be an excellent predictor of experimental measurements. I have then applied these innovations to projects concerning plasmonic nanostructures, quantum imaging, and quantum sensing. Additionally, in collaboration with the National Institute of Standards and Technology (NIST), I have performed an experiment utilizing weak-field interference to prove the correctness of a quantum state tomography method called weak-field homodyne detection, a low-noise procedure which is ideal for quantum communications and networking protocols. As a result, I was able to completely reconstruct a coherent quantum state within the interference mode, and also to obtain some limited information about the remaining modes of that state. Throughout all of this, my research has had a particular focus on multi-mode photonics and its applications to plasmonics and quantum networking, and I believe that the state-tomography protocols and the results concerning stochastically-random quantum light sources will be of use to the quantum information and photonics communities for years to come.
Today, quantum photonics is one of the leading platforms for the research and development of the quantum information sciences and technologies. It is also the most promising platform for quantum networking protocols. As such, the investigation and characterization of quantum light sources is vital to the future of quantum research. In this thesis document, titled “Multimode Statistical Photonics and Quantum State Tomography for Quantum Networks,” I present my research on the multi-mode statistical properties of classical light sources, with applications to quantum state tomography methods for use in quantum communications and networking protocols. During my time here at LSU, I have worked with my fellow researchers in the quantum photonics group to investigate the quantum dynamics which govern the classical statistical behaviors of stochastically-random light sources. By extracting the correlated multiphoton, multi-mode subsystems of such sources, we have discovered nontrivial, uniquely-quantum behaviors within sources of light which were previously thought to be only classical. I have also developed a robust theoretical framework for describing such sources, utilizing techniques from quantum field theory, which we have shown to be an excellent predictor of experimental measurements. I have then applied these innovations to projects concerning plasmonic nanostructures, quantum imaging, and quantum sensing. Additionally, in collaboration with the National Institute of Standards and Technology (NIST), I have performed an experiment utilizing weak-field interference to prove the correctness of a quantum state tomography method called weak-field homodyne detection, a low-noise procedure which is ideal for quantum communications and networking protocols. As a result, I was able to completely reconstruct a coherent quantum state within the interference mode, and also to obtain some limited information about the remaining modes of that state. Throughout all of this, my research has had a particular focus on multi-mode photonics and its applications to plasmonics and quantum networking, and I believe that the state-tomography protocols and the results concerning stochastically-random quantum light sources will be of use to the quantum information and photonics communities for years to come.
The technologies utilizing quantum states of light have been in the spotlight for the last two decades. In this regard, quantum metrology, quantum imaging, quantum-optical communication are some of the important applications that exploit fascinating quantum properties like quantum superposition, quantum correlations, and nonclassical photon statistics. However, the state-of-art technologies operating at the single-photon level are not robust enough to truly realize a reliable quantum-photonic technology. In Chapter 1, I present a historical account of photon-based technologies. Furthermore, I discuss recent efforts and encouraging developments in the field of quantum-photonic technologies, and major challenges for the experimental realization of reliable quantum technologies utilizing photons, setting up a stage for unveiling our smart methodologies to cope with them. Similarly, in Chapter 2, I review the fundamental concepts such as states of light, spatial modes of light, and machine learning. In Chapter 3 of this dissertation, I present a theoretical work on a nonlinear quantum metrology scheme, showing a sub-shot-noise limited phase estimation using the displaced-squeezed light and on/off detection. Furthermore, I discuss a camera-based squeezed-light detection technique that can be a smart and time-efficient alternative to conventional balanced-homodyne detection. In Chapter 4, I discuss our efforts to incorporate artificial intelligence in a quest to improve the efficiency of discriminating thermal light sources from coherent light sources. The conventional identification technique requires a large number of measurements. We utilize artificial neural networks to dramatically reduce the number of measurements required to distinguish thermal light and coherent light. In Chapter 5, I present a communication protocol that utilizes the spatial modes of light. Despite being valuable resources for a wide variety of quantum technologies, spatial modes of light are extremely vulnerable to random phase fluctuations. The conventional techniques to cope with these challenges are relatively inefficient. We utilize convolutional neural networks to perform the spatial mode correction of single photons, resulting in a near-unity fidelity of correction. I wrap up my dissertation in Chapter 6 by summarizing the historical context of quantum technologies, challenging problems facing state-of-art quantum technologies, and the importance of our efforts to introduce artificial intelligence in photonic technologies.
Quantum plasmonics explores how light interacts with collective charge oscillations at metal-dielectric interfaces, enabling strong confinement and enhanced quantum effects at the nanoscale. While traditional quantum optics focuses on single photons, this thesis explores an intermediate regime - multiparticle quantum optics - where classical light, analyzed using photon-number-resolving detection and projective measurements, reveals nontrivial quantum correlations. We begin by establishing the theoretical foundation of multiparticle quantum plasmonics, introducing key concepts like photon-plasmon interactions, coherence, and statistical fluctuations. The first study shows that multiparticle scattering can alter quantum statistics in plasmonic systems, offering new control over fluctuations. The second reveals nonclassical near-field plasmon dynamics, showing how quantum coherence emerges from bosonic and fermionic contributions in subsystems. The third study presents a quantum plasmonic sensing method, using conditional detection to boost signal-to-noise ratio and improve phase estimation. The final study extends this approach to quantum imaging with natural light: by isolating multiphoton correlations from thermal light via photon-number-resolving detection and a single-pixel protocol, we enhance image contrast under noisy conditions. These results show that multiparticle interactions can control quantum statistical properties, even in classical fields. By bridging theory and application, this thesis advances quantum plasmonics and highlights the potential of multiphoton methods in imaging, sensing, and information processing.
Quantum information science and technology is experiencing rapid development. The process raises an emerging demand for experiments to implement and demonstrate novel ideas to overcome the challenges towards the further application of quantum information. Linear optical systems have long been the most available and efficient platform for the demonstration of quantum information methods and concepts due to its robustness against the influence of environment. However, the large-scale multi-qubit linear optical information systems have long been suffering the low generated efficiency of the single photon sources, leading to a limited availability and scalability. In recent years, novel techniques such as beam-like SPDC open up new possibilities to break this bottleneck and further develop linear optical quantum information systems. Driven by the increasing demand and inspired by the technical progress, in my degree candidature, I carried out a project to build an advanced linear optical multi-qubit experimental platform to implement quantum information experiment and to discover novel methodologies of quantum optics research. In this thesis, I will present our progress and future perspectives. In Chapters 1-2, I will introduce the background and the motivation of this project first, and then present the basic concepts and principles of linear optical quantum information that are involved in my research. Chapter 3 describes my work on the generation and measurement of linear optical multi-qubit entanglement based on beam-like SPDC sources. By analyzing and containing the entanglement destruction from time-space correlation and group delay dispersion, the specifications of linear optical multi-qubit system can reach a higher level that allows for more applications. In Chapter 4, we discuss the methodology we developed to statistically estimate quantum states based on measurement results. In this project, we developed and applied a new algorithm named Bayesian mean estimation based on Gibbs sampling to estimate quantum states. I will describe the background, principle, and advantages compared to other widely accepted methods of this algorithm. In Chapter 5, I report our progress in experimentally demonstrating quantum overlapping tomography, a scheme focusing on characterizing critical information of a many-body quantum system in logarithmic time complexity. We compare the estimate results of full-state tomography and overlapping tomography to confirm that overlapping tomography gives accurate information of the system with much fewer state measurements than full-state tomography. Furthermore, under the condition that the system is too large to perform a tomography, we confirm that the state is still able to be measured with overlapping tomography. Chapter 6 begins with the conclusion of the thesis. Furthermore, I present outlooks on some potential directions to further increase the linear optical quantum information system.
Quantum technologies harness the properties of controlled quantum mechanical systems for applications in computation, communication and metrology, and enable us to further our understanding of fundamental physics. Quantum hardware is designed to manipulate complex and fragile many-particle quantum states, which requires exquisite control machinery, advanced software and near-complete isolation from the surrounding environment. With sufficient capabilities, quantum computers with large registers of quantum bits will perform classically intractable calculations such as quantum chemistry simulations. Quantum key distribution will provide completely secure communications across the world and quantum metrology will enable measurement precision beyond today's capabilities. While quantum technologies are far from surpassing classical hardware today, the vision and potential impact has sparked a world-wide research effort, both theoretical and experimental, to develop a quantum computer. The most sophisticated current quantum technologies have control over small numbers of quantum bits and are limited by environmental decoherence processes. While progress in developing quantum hardware is ongoing, designing and demonstrating new algorithms and protocols for quantum information is a thriving research field. Photonics provides an ideal platform for small-scale proof-of-concept quantum experiments, which are the focus of this thesis. In the coming decades, we will see the realisation of quantum hardware capable of applications that outperform any classical computer. In this thesis, I present several protocols and algorithms for quantum information science and technology, which are implemented in quantum photonic experiments. The applications of these works include robust quantum tomography, quantum state relocation, quantum enhanced data recovery and probing fundamental causality in quantum mechanics. The works presented here are based on two photon experiments from a prototypical spontaneous parametric down-conversion source, which provides an excellent test-bed for quantum information experiments. Photons can be used to encode quantum information in a range of different degrees of freedom. This versatility, along with compatibility with existing technology gives photonics a great advantage in the development of quantum hardware.
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).
The field of high-dimensional quantum photonics involves the use of multimode photonic degrees-of-freedom such as the spatial, temporal, or spectral structure of light to encode multi-level quantum states. Recent years have seen rapid progress in the development of methods to generate, manipulate, and distribute such quantum states of light and their use in a range of quantum technology applications that offer practical advantages over conventional qubit-based approaches. High-dimensional quantum states of light encoded in photonic time-bins, frequency-bins, transverse-spatial modes, waveguide paths, and temporal modes have enabled noise-robust fundamental tests of quantum mechanics, error-resilient and high-capacity quantum communication protocols, andas well as efficient approaches for quantum information processing, to name just a few examples. However, research in this field has progressed fairly independently, with little exchange across different photonic degrees-of-freedom or between experiment and theory and no comprehensive comparison between degrees-of-freedom. This roadmap aims to bridge this gap by surveying progress in each area and identifying shared challenges and opportunities that cut across two or more photonic degrees-of-freedoms. We review early work and state-of-the-art experimental techniques under development for high-dimensional quantum states encoded in single and entangled photons, as well as theoretical tools for their measurement and certification. We outline the main outstanding challenges for theory and each experimental degree-of-freedom, identifying promising future directions of research that may enable these to be overcome. We end by discussing interconnections and shared challenges centered around their distribution, measurement, and manipulation, with a view towards their integration into next-generation quantum technology platforms and applications.