Quantum information science (QIS) is an interdisciplinary field that explores the principles and applications of quantum mechanics in information processing, communication and computation. QIS leverages the unique properties of quantum systems such as entanglement and superposition to develop new ways of encoding, transmitting and manipulating information. Eventhough quantum technologies hold enormous promises for quantum processing or quantum communication, their potential is still limited by the ability to transmit quantum information and control quantum systems with high precision. Distributing entanglement over large distances holds vast potential for communication applications. However, the inherent fragility of entangled states presents a significant challenge in their distribution. In Part I of this thesis, we investigate the distribution of entanglement over noisy channels. Concerning bipartite entanglement, we introduce novel hardware-agnostic protocols and tools for various tasks that are relevant for reliable quantum communication in the context of quantum computing, including entanglement purification, entanglement verification and fidelity witnessing. Our protocols rely on auxiliary high-dimensional entangled systems to gather non-local information and on new approaches that use but do not consume all available resources. These new tools enhance the performance of our protocols compared to previous strategies, working particularly well for the verification of entangled states where without using auxiliary resources an exponential improvement even with ``optimal'' strategies is obtained. We also investigate the distribution of tripartite entanglement. On the one hand, we extend our protocols to operate with GHZ states, and on the other hand, we develop a new entanglement swapping protocol and enhance the only known entanglement purification protocol for W states. These two strategies collectively enable us to introduce the first quantum repeater for W states. In Part II of the thesis, we explore quantum processing architectures based on logically encoded systems. We investigate many-body systems subject to non-tunable distant-dependent interactions. We demonstrate how using logical encodings we can execute multiqubit non-local gates in a controlled manner solely using local control. This method presents different functionalities which can be exploited for various applications. (1) Efficient way to control the interaction topology of the system, which we use for quantum control where we show how a logical system can be used to control another system by mediating non-local interactions. Based on this functionality, we also develop a quantum simulator with enhanced performance compared to prior methodologies. (2) Interactions resilient against position fluctuations of the system. We demonstrate a significant enhancement in entangling gate fidelities in different systems subjected to thermal noise. (3) Amplification of interactions. We exploit this capability to design a modular quantum computing architecture, which uses multipartite entanglement to implement different intermodular gates.
Quantum information science (QIS) is an interdisciplinary field that explores the principles and applications of quantum mechanics in information processing, communication and computation. QIS leverages the unique properties of quantum systems such as entanglement and superposition to develop new ways of encoding, transmitting and manipulating information. Eventhough quantum technologies hold enormous promises for quantum processing or quantum communication, their potential is still limited by the ability to transmit quantum information and control quantum systems with high precision. Distributing entanglement over large distances holds vast potential for communication applications. However, the inherent fragility of entangled states presents a significant challenge in their distribution. In Part I of this thesis, we investigate the distribution of entanglement over noisy channels. Concerning bipartite entanglement, we introduce novel hardware-agnostic protocols and tools for various tasks that are relevant for reliable quantum communication in the context of quantum computing, including entanglement purification, entanglement verification and fidelity witnessing. Our protocols rely on auxiliary high-dimensional entangled systems to gather non-local information and on new approaches that use but do not consume all available resources. These new tools enhance the performance of our protocols compared to previous strategies, working particularly well for the verification of entangled states where without using auxiliary resources an exponential improvement even with ``optimal'' strategies is obtained. We also investigate the distribution of tripartite entanglement. On the one hand, we extend our protocols to operate with GHZ states, and on the other hand, we develop a new entanglement swapping protocol and enhance the only known entanglement purification protocol for W states. These two strategies collectively enable us to introduce the first quantum repeater for W states. In Part II of the thesis, we explore quantum processing architectures based on logically encoded systems. We investigate many-body systems subject to non-tunable distant-dependent interactions. We demonstrate how using logical encodings we can execute multiqubit non-local gates in a controlled manner solely using local control. This method presents different functionalities which can be exploited for various applications. (1) Efficient way to control the interaction topology of the system, which we use for quantum control where we show how a logical system can be used to control another system by mediating non-local interactions. Based on this functionality, we also develop a quantum simulator with enhanced performance compared to prior methodologies. (2) Interactions resilient against position fluctuations of the system. We demonstrate a significant enhancement in entangling gate fidelities in different systems subjected to thermal noise. (3) Amplification of interactions. We exploit this capability to design a modular quantum computing architecture, which uses multipartite entanglement to implement different intermodular gates.
Quantum information science is an interdisciplinary field that investigates the principles and applications of quantum mechanics in information processing. Upcoming quantum technologies promise a range of valuable applications, including powerful simulation of complex systems, enhanced computational capabilities, high-precision sensing and metrology, and secure communication via quantum key distribution. A central concept is entanglement, a unique type of correlation between quantum systems that has no classical counterpart. Entanglement plays a pivotal role in many quantum protocols, particularly as a key resource in quantum communication, where protocols such as distributed quantum computation and secret sharing are executed in a distributed setting. Hence, distributing entanglement over long distances and manipulating it locally is essential for enabling quantum communication applications. However, the effectiveness of such applications is limited by the challenges of transmitting quantum information and maintaining precise control over quantum systems. For example, quantum information carriers, such as photons traveling through optical fibers, may be lost, or the operations performed may not be perfect. Consequently, it is crucial to develop robust tools for distributing and manipulating entanglement in distributed settings in order to fully unlock the capabilities of quantum technologies. In this thesis, we explore the influence of noise on the manipulation of entanglement in entanglement-based quantum networks. First, we examine the noise resilience of various resource states, i.e., pre-established entangled states, used to achieve bipartite or tripartite communication. This is followed by an analysis of a fully asymmetric network scenario, incorporating the noise introduced by the delays from classical messaging within the quantum network. We then propose classical communication protocols to achieve bipartite communication and evaluate their noise impact under the optimization of the arrangement of the pre-established entanglement. In addition, we propose an alternative approach to quantum repeaters, a key device for entanglement distribution, based on growing multipartite states, allowing entanglement to be recycled and patched. All investigations have been carried out employing a novel formalism, also developed in this thesis, for efficiently and accurately simulating noisy quantum processes on classical hardware, with broader applicability beyond quantum communication.
Quantum entanglement, a peculiar phenomenon in quantum mechanics, presents a significant challenge to the development of quantum computation. The inherent fragility of entangled states necessitates sophisticated techniques for encoding, transmitting, and analyzing quantum information. This paper proposes a novel method leveraging the unique properties of quantum entanglement to enhance the efficiency and security of quantum computation, focusing on the encoding and analysis of entangled states. The core mechanism centers around utilizing quantum entanglement to achieve enhanced data fidelity and facilitate more robust quantum algorithms. We outline the key steps involved, including the establishment of a secure entanglement distribution protocol, the implementation of a quantum entanglement analysis framework, and the demonstration of its potential impact on quantum algorithm performance. The research explores the practical implications of this approach, emphasizing its capacity to overcome existing limitations and pave the way for significant advancements in quantum information processing.
Quantum communications enable secure key transmission and entanglement distribution, but their rates and maximum transmission distances are limited due to the losses and noise introduced by quantum channels. Various techniques have been developed to address these limitations, tailored to the platform and the nature of the decoherence. In this thesis, we explore ways to enhance the performance of quantum communications, with a particular focus on increasing the key rates and transmission distances in quantum key distribution (QKD), a method that leverages quantum mechanics to securely share keys between trusted parties. Additionally, we aim to improve the purity of entangled states shared between two parties, contributing to broader advancements in quantum communications. We begin by introducing a software-enhanced continuous-variable QKD (CV-QKD) protocol, where keys are encoded onto the phase and amplitude degrees of the optical field. This protocol enhances both transmission distance and key rates through a post-selection method employing arbitrary filters, experimentally demonstrated on two different platforms. Its adaptability makes it highly promising, as it can be implemented on existing CV-QKD platforms and dynamically optimises key rates for rapidly changing channels. We then present a protocol for the pure-loss channel leveraging quantum repeaters to mitigate the channel losses by segmenting the total communication distance into smaller, more manageable intervals. This repeater-like protocol introduces a more efficient entanglement-swapping measurement at the repeater station than the state-of-the-art twin-field protocols. Instead of polarisation encoding, our protocol relies on encoding keys in the photon-number basis, optimising the states to achieve the highest key rates for each distance. Notably, this protocol shows strong potential for improving long-distance quantum communication by surpassing the capacity of the direct communication channel at a shorter distance than the existing repeater protocols such as twin-field QKD while achieving an overall higher transmission distance. Finally, in this thesis, we demonstrate that for the Pauli-dephasing channel, placing a node in the middle of the quantum channel to perform a Bell-state measurement as an entanglement-swapping operation does not result in improved key rates, even with reduced decoherence. As a result, we propose an entanglement purification protocol that refines a larger set of noisy entangled states into a smaller set of high-fidelity ones using local operations and classical communication. This purification protocol builds upon earlier purification methods, extending their application from two Bell pairs to any number of Bell pairs and thereby achieving the capacity of the dephasing channel. Through numerous recursive iterations, it produces near-perfect Bell pairs, making it well-suited for quantum repeaters and correcting dephasing errors in quantum computers.
Quantum mechanics led us to reconsider the scope of physics and its building principles, such as the notions of realism and locality. More recently, quantum theory has changed in an equally dramatic manner our understanding of information processing and computation. On one hand, the fundamental properties of quantum systems can be harnessed to transmit, store, and manipulate information in a more efficient and secure way than possible in the realm of classical physics. On the other hand, the development of systematic procedures to manipulate systems of a large number of particles in the quantum regime, crucial to the implementation of quantum-based information processing, has triggered new possibilities in the exploration of quantum many-body physics and related areas. In this thesis, we present new results relevant to two important problems in quantum information science: the development of a theory of entanglement, intrinsically quantum correlations, and the exploration of the use of controlled quantum systems to the computation and simulation of quantum many-body phenomena. In the first part we introduce a new approach to the study of entanglement
We consider quantum computer architectures where interactions are mediated between hot qubits that are not in their mechanical ground state. Such situations occur, e.g., when not cooling ideally, or when moving ions or atoms around. We introduce quantum gates between logically encoded systems that consist of multiple physical ones and show how the encoding can be used to make these gates resilient against such imperfections. We demonstrate that, in this way, one can improve gate fidelities by enlarging the logical system, and counteract the effect of unknown positions or position fluctuations of involved particles. We consider both a classical treatment of positions, in terms of probability distributions, and a quantum treatment using mechanical eigenmodes. We analyze different settings including a cool logical system mediating interactions between two hot systems, as well as two logical systems consisting of hot physical systems whose positions fluctuate collectively or individually. In all cases, assuming ideal local control to logical systems, we demonstrate a significant improvement in gate fidelities, which provides a platform-independent tool to mitigate thermal noise in the context of trapped-particle-based architectures. Published by the American Physical Society 2024
Quantum computing and quantum information science are two recently discovered and rapidly growing fields of physics that show substantial promise in providing new and valuable technologies in the foreseeable future. Large-scale quantum computers, if ever realized experimentally, are likely to outperform their classical counterparts in a number of important computational tasks, the most important of which may be the accurate simulation of many-body quantum systems such as the ones encountered in physics, chemistry and life sciences. This thesis investigates the problem of controlling quantum systems for the purpose of performing quantum information processing tasks. The problem is approached from a theoretical and simulational viewpoint. The work contained here encompasses a range of levels of abstraction. Firstly, we discuss the decomposition of abstract multiqubit logic gates into sequences of simple elementary gates. Secondly, we study the local commutational properties of two-qubit gates using local gate invariants. Thirdly, we develop methods for the physical implementation of the elementary gates through the control of specific quantum systems, possibly in the presence of noise and decoherence. We present a new, almost optimal n-qubit gate decomposition based on the cosine-sine decomposition, which utilizes a likewise new intermediate quantum circuit structure we call a uniformly controlled gate. We then show how they can be used in constructing a general state transformation circuit. Both of the resulting circuits can be efficiently implemented using nearest-neighbor gates which makes their physical realization simpler. A local gate invariant is introduced which can be used to assess the suitability of two-qubit gates for serving as the entangling gate in elementary gate libraries. Finally, we develop numerical optimization methods for finding near-optimal control sequences for generating oneand two-qubit gates, both in closed quantum systems and in the presence of Markovian noise.