While theoretical ideas from quantum information science have long provided a foundation for understanding communication protocols and thermalization in many-body systems, the advent of near-term quantum devices offers the prospect of application and experimental study. In this thesis, we explore their interplay. The postulates of quantum mechanics generalize classical probability distributions and thus transmission of information, enabling fundamentally novel protocols for communication and cryptography. These algorithms motivate the deployment of quantum networks, a distributed model of computation where universality and fault-tolerance are often not required. We revisit the application of astronomical interferometry and provide a practical scheme based on distributed source coding, enabling powerful telescopes. Based on constructions from communication complexity, we design a voting scheme with efficient scaling of quantum communication and computation, and prove its security. The spread of quantum correlations, besides underlying the performance of many communication protocols, characterizes the process of objects away from equilibrium attaining a uniform state. Evolution is natural, requiring little control and easing study by quantum simulators. We analyze traversable wormholes, initially motivated by gauge-gravity duality, from this perspective. The effect of a gravitational scrambling mechanism is distinguished from generic mixing. Nonetheless, transfer of information is still found in common many-body systems, such as interacting spins. The two themes above are accompanied by proposals for physical realization in atomic and optical platforms. Communication protocols are carried out by photons transmitting quantum information and cavities mediating processing by atomic qubits. Thermalization is studied in strongly interacting systems of Rydberg atoms controlled by electromagnetic fields. These platforms represent the current generation of quantum computers, and so we probe its capabilities.
While theoretical ideas from quantum information science have long provided a foundation for understanding communication protocols and thermalization in many-body systems, the advent of near-term quantum devices offers the prospect of application and experimental study. In this thesis, we explore their interplay. The postulates of quantum mechanics generalize classical probability distributions and thus transmission of information, enabling fundamentally novel protocols for communication and cryptography. These algorithms motivate the deployment of quantum networks, a distributed model of computation where universality and fault-tolerance are often not required. We revisit the application of astronomical interferometry and provide a practical scheme based on distributed source coding, enabling powerful telescopes. Based on constructions from communication complexity, we design a voting scheme with efficient scaling of quantum communication and computation, and prove its security. The spread of quantum correlations, besides underlying the performance of many communication protocols, characterizes the process of objects away from equilibrium attaining a uniform state. Evolution is natural, requiring little control and easing study by quantum simulators. We analyze traversable wormholes, initially motivated by gauge-gravity duality, from this perspective. The effect of a gravitational scrambling mechanism is distinguished from generic mixing. Nonetheless, transfer of information is still found in common many-body systems, such as interacting spins. The two themes above are accompanied by proposals for physical realization in atomic and optical platforms. Communication protocols are carried out by photons transmitting quantum information and cavities mediating processing by atomic qubits. Thermalization is studied in strongly interacting systems of Rydberg atoms controlled by electromagnetic fields. These platforms represent the current generation of quantum computers, and so we probe its capabilities.
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.
Quantum information science has completely changed the way we think about and process information. From the simple realization that information is physical, we have been able to use the peculiar features of quantum mechanical phenomena to our advantage. Designing algorithms whose performance exceeds those running on classical computers, and performing secret communication whose security can actually be proven from sound assumptions are two main catalysts for the establishment of the field. This thesis discusses some progress in quantum key distribution and quantum randomness generation. Quantum key distribution, which is motivated by the increasing need to communicate securely, is on the verge of becoming an established technology. With the goal of extending the reach of quantum key distribution to more realistic scenarios, we present a study on reference-frame-independent protocols whose knowledge can help design more efficient protocols, and a framework to the security analysis of distributed-phase-reference protocols, which have been missing for many years. This allows these protocols to be used in practice against the most general adversary, although the key rate is rather pessimistic. In quantum randomness generation, the amount of extractable randomness from a quantum system depends on the level of trust or characterization of the devices; we present a study into such interaction. In the extreme situation of distrust, i.e. device-independent scenarios, we study the effect of the input randomness on the certifying power of such scenarios, and realize that one cannot amplify an arbitrary min-entropy source device-independently. Finally we discuss the amount of randomness in post-selected data, which has consequences on practical randomness generation protocols. The in depth study of randomness generation from quantum processes is well justified by the important role of randomness in modern computer science and other fields.
Quantum communication represents a revolutionary advancement over classical information theory, which leverages unique quantum mechanics properties like entanglement to achieve unprecedented capabilities in secure and efficient information transmission. Unlike bits in classical communication, quantum communication utilizes qubits in superposition states, allowing for novel information storage and processing. Entanglement, a key quantum phenomenon, enables advanced protocols with enhanced security and processing power. This paper provides a comprehensive overview of quantum communication, emphasizing the role of entanglement in theoretical foundations, practical protocols, experimental progress, and security implications. It contrasts quantum communications potential applications with classical networks, identifying areas where entanglement offers significant advantages. The paper explores the fundamentals of quantum mechanics in communication, the physical realization of quantum information, and the formation of secure quantum networks through entanglement-based strategies like Quantum Key Distribution (QKD) and teleportation. It addresses the challenges of long-distance quantum communication, the role of quantum repeaters in scaling networks, and the conceptualization of interconnected quantum networks. Additionally, it discusses strides towards the Quantum Internet, Quantum Error-Correcting codes, and quantum cryptographys role in ensuring secure communication. By highlighting the role of entanglement, this paper aims to inspire further research and innovation in secure and efficient information exchange within quantum networks.
Current communication networks are based on classical physics and classical information-processing. However, for nearly a century, we have known that at its most fundamental level, the universe is governed by the laws of quantum mechanics. With quantum communication, new possibilities arise in our capabilities to transmit and process information which, in many cases, lead to advantages compared to what is classically possible. The entire scope of tasks for which quantum communication can offer improvements has not yet been fully explored, but several quantum protocols are known that can either perform tasks which are impossible with classical resources or can outperform classical protocols. These quantum protocols are well understood from a theoretical point of view, but many of them have never been demonstrated in practice. Thus, in the context of quantum communication, there is a significant gap between theory and experiment that must be removed in order to harness the advantages provided by quantum mechanics in a practical setting. \n \nIn this thesis, we develop a series of tools for developing and testing practical quantum communication protocols. Our main technique is a theoretical reformulation of existing quantum communication protocols that converts them into a form in which they can be demonstrated with existing experimental techniques. More precisely, they can be implemented using only coherent states of light and linear optics circuits while still retaining the crucial properties of the original abstract protocols. We use this result to construct practical protocols for the Hidden Matching problem and quantum fingerprinting. \n \n \nIn the case of quantum fingerprinting, we make a thorough analysis of the role played by experimental errors and show that our practical protocol can still be implemented in the presence of these imperfections. In fact, we report a proof of concept experimental demonstration of a quantum fingerprinting system that is capable of transmitting less information than the best known classical protocol for this problem. Our implementation is based on a modified version of a commercial quantum key distribution system using off-the-shelf optical components over telecom wavelengths, and is practical for messages as large as 100 Mbits, even in the presence of experimental imperfections. \n \nSimilarly, in the context of cryptography, we propose a multiparty quantum signature protocol that can be implemented from any point-to-point quantum key distribution network, proving its security against forging, repudiation and non-transferability. Crucially, since quantum key distribution is already a practical technology, so is this protocol. However, unlike other tasks in quantum communication, there has not been significant theoretical work on establishing a security model for quantum signature schemes. Consequently, we also constructed a security framework for these schemes and proved several properties that these protocols must satisfy in order to achieve their security goals. \n \nFinally, in addition to proposing new practical protocols, we provide a reliable data analysis technique to verify an important property of many quantum communication protocols: the presence of entanglement. Our technique is based on entanglement witnesses and it does not require the specification of a prior distribution nor the assumption of independent measurements. The technique is suitable to be used with nonlinear entanglement witnesses, which we show can be constructed from any linear witness and evaluated from the same experimental data. We also develop numerical tools necessary to employ this approach in practice, rendering the procedure ready to be applied to current experiments. We demonstrate this by analyzing the data of a photonic experiment generating two-photon states whose entanglement is verified with the use of an accessible nonlinear witness.
Quantum information processing (QIP) studies the representation, manipulation, and transmission of information using quantum systems. By leveraging counterintuitive quantum phenomena such as superposition and entanglement, QIP enables new paradigms for computation, communication, and learning that can surpass classical limits in certain regimes. Foundational tasks in QIP include quantum data compression, quantum communication, and quantum learning and optimization, which together form the basis for scalable quantum technologies. This thesis develops new frameworks and algorithms at the intersection of quantum information theory, quantum learning, and quantum-inspired modeling. The thesis is organized into three parts. The first part studies quantum compression problems; the second addresses quantum communication; and the third develops quantum learning and optimization frameworks with applications to complex systems. In the first part, a new formulation for lossy quantum-classical source coding, termed rate-channel theory, is introduced, replacing the conventional distortion-based formulation with a posterior-channel-based framework under a block-error criterion. The framework is further generalized to incorporate quantum side information at the decoder. A connection between rate-channel theory and classical rate-distortion theory is also established, showing that optimal rate-distortion performance can be recovered under suitable conditions. These results provide a unified and flexible framework for data compression, which is essential for scalable quantum computing infrastructure, enabling efficient storage of quantum data, such as outputs from quantum sensors (e.g., MRI) and information encoded in photonic systems for satellite and optical communication. In the second part, the problem of computing functions of distributed sources over classical-quantum multiple access channels is studied. A new coding framework is developed that combines algebraic structured codes with unstructured random coding techniques, enabling efficient communication of functions without reconstructing individual sources. The resulting achievable conditions, expressed in terms of single-letter quantum information quantities, strictly enlarge the class of sources for which reliable computation is possible. These results contribute to the theoretical foundations of quantum network communication and are directly relevant to deep-space satellite-based quantum communication, distributed quantum computing, and the design of scalable quantum network infrastructures. In the final part, new approaches to quantum learning and optimization are developed. A geometry-aware quantum optimization framework is proposed based on a novel quantum information metric and an unbiased estimator constructed from single-shot measurements. This leads to an optimization method with constant sample complexity per iteration and provable exponential convergence under mild conditions, while avoiding saddle points and local minima. Such advances are particularly relevant for challenging optimization tasks in quantum sensing and quantum computational modeling of many-body systems. Building on this optimization framework, a Hamiltonian learning approach is introduced to infer interaction structures from time-resolved measurement data, with finite-sample guarantees and polynomial scaling with system size. This framework is further extended to develop a quantum-inspired generative model for gene regulatory network inference, where interactions are encoded as a parameterized Hamiltonian evolving over pseudotime. The proposed methods demonstrate strong performance on synthetic benchmarks and reveal biologically meaningful structures in real-world data, with applications to multi-omics integration, cancer genomics, and systems biology. Overall, this thesis advances the theoretical and algorithmic foundations of quantum information processing, with contributions spanning compression, communication, and learning. These developments support the design of resource-efficient and scalable quantum technologies, enabling applications in quantum sensing, quantum networks, distributed quantum computation, and the modeling of complex physical and biological systems.
Precise control over quantum systems can enable the realization of fascinating applications such as powerful computers, secure communication devices, and simulators that can elucidate the physics of complex condensed matter systems. However, the fragility of quantum effects makes it very difficult to harness the power of quantum mechanics. In this thesis, we present novel systems and tools for gaining fundamental insights into the complex quantum world and for bringing practical applications of quantum mechanics closer to reality. We first optimize and show equivalence between a wide range of techniques for storage of photons in atomic ensembles. We describe experiments demonstrating the potential of our optimization algorithms for quantum communication and computation applications. Next, we combine the technique of photon storage with strong atom-atom interactions to propose a robust protocol for implementing the two-qubit photonic phase gate, which is an important ingredient in many quantum computation and communication tasks.