In the first half of this Thesis, we introduce a framework of “quantum cryptographic agility,” which allows for a resource-efficient swap of an underlying cryptographic protocol. Specifically, we introduce several schemes which perform the tasks of Digital Signatures and Secret Sharing. Our first achievement is an investigation of Quantum Digital Signatures (QDS) over a continuous-variables platform, consisting of phase-encoded coherent states and heterodyne phase detection. QDS allows for secure authentication of a classical message, while guaranteeing message transferability. For the first time, we prove security of CV QDS in the presence of an eavesdropper on the quantum channels. We then introduce a continuous variable (CV) Quantum Secret Sharing (QSS) protocol. Our security proof allows for classical information to be split and shared between multiple potentially dishonest recipients, while retaining security against collective beamsplitter and entangling-cloner attacks. In the last chapter of this half, we introduce another QDS scheme which runs over identical hardware setup to our QSS protocol. We analyse experimental data in which quantum coherent states were distributed at a rate of 1 GHz, which for QDS allows us to securely sign a message in less than 0.05 ms. In the second half of this Thesis we suggest and discuss a deterministic source of nonclassical light, which we call “PhoG”. Our source is based on the coherent diffusive photonics, relying on both coherent and dissipative evolution of the quantum state, and may be realised in an array of dissipatively-coupled laser-inscribed waveguides in a χ⁽³⁾ glass. We analyse the PhoG device with several analytical and numerical models and demonstrate that a coherent state input leads to a bright output state with strong photon-number squeezing. With minor reconfiguration our system can generate entanglement between spatially separated modes via a process analogous to four-wave mixing.
In the first half of this Thesis, we introduce a framework of “quantum cryptographic agility,” which allows for a resource-efficient swap of an underlying cryptographic protocol. Specifically, we introduce several schemes which perform the tasks of Digital Signatures and Secret Sharing. Our first achievement is an investigation of Quantum Digital Signatures (QDS) over a continuous-variables platform, consisting of phase-encoded coherent states and heterodyne phase detection. QDS allows for secure authentication of a classical message, while guaranteeing message transferability. For the first time, we prove security of CV QDS in the presence of an eavesdropper on the quantum channels. We then introduce a continuous variable (CV) Quantum Secret Sharing (QSS) protocol. Our security proof allows for classical information to be split and shared between multiple potentially dishonest recipients, while retaining security against collective beamsplitter and entangling-cloner attacks. In the last chapter of this half, we introduce another QDS scheme which runs over identical hardware setup to our QSS protocol. We analyse experimental data in which quantum coherent states were distributed at a rate of 1 GHz, which for QDS allows us to securely sign a message in less than 0.05 ms. In the second half of this Thesis we suggest and discuss a deterministic source of nonclassical light, which we call “PhoG”. Our source is based on the coherent diffusive photonics, relying on both coherent and dissipative evolution of the quantum state, and may be realised in an array of dissipatively-coupled laser-inscribed waveguides in a χ⁽³⁾ glass. We analyse the PhoG device with several analytical and numerical models and demonstrate that a coherent state input leads to a bright output state with strong photon-number squeezing. With minor reconfiguration our system can generate entanglement between spatially separated modes via a process analogous to four-wave mixing.
Agile cryptography allows for a resource-efficient swap of a cryptographic core in case the security of an underlying classical cryptographic algorithm becomes compromised.Conversely, versatile cryptography allows the user to switch the cryptographic task without requiring any knowledge of its inner workings.In this paper, we suggest how these related principles can be applied to the field of quantum cryptography by explicitly demonstrating two quantum cryptographic protocols, quantum digital signatures (QDS) and quantum secret sharing (QSS), on the same hardware sender and receiver platform.Crucially, the protocols differ only in their classical postprocessing.The system is also suitable for quantum key distribution (QKD) and is highly compatible with deployed telecommunication infrastructures, since it uses standard quadrature phase-shift keying encoding and heterodyne detection.For the first time, QDS protocols are modified to allow for postselection at the receiver, enhancing protocol performance.The cryptographic primitives QDS and QSS are inherently multipartite, and we prove that they are secure not only when a player internal to the task is dishonest, but also when (external) eavesdropping on the quantum channel is allowed.In our first proof-of-principle demonstration of an agile and versatile quantum communication system, the quantum states are distributed at GHz rates.A 1-bit message may be securely signed using our QDS protocols in less than 0.05 ms over a 2-km fiber link and in less than 0.2 s over a 20-km fiber link.To our knowledge, this also marks the first demonstration of a continuous-variable direct QSS protocol.
Agile cryptography allows for a resource-efficient swap of a cryptographic core in case the security of an underlying classical cryptographic algorithm becomes compromised. Conversely, versatile cryptography allows the user to switch the cryptographic task without requiring any knowledge of its inner workings. In this paper, we suggest how these related principles can be applied to the field of quantum cryptography by explicitly demonstrating two quantum cryptographic protocols, quantum digital signatures (QDS) and quantum secret sharing (QSS), on the same hardware sender and receiver platform. Crucially, the protocols differ only in their classical postprocessing. The system is also suitable for quantum key distribution (QKD) and is highly compatible with deployed telecommunication infrastructures, since it uses standard quadrature phase-shift keying encoding and heterodyne detection. For the first time, QDS protocols are modified to allow for postselection at the receiver, enhancing protocol performance. The cryptographic primitives QDS and QSS are inherently multipartite, and we prove that they are secure not only when a player internal to the task is dishonest, but also when (external) eavesdropping on the quantum channel is allowed. In our first proof-of-principle demonstration of an agile and versatile quantum communication system, the quantum states are distributed at GHz rates. A 1-bit message may be securely signed using our QDS protocols in less than 0.05 ms over a 2-km fiber link and in less than 0.2 s over a 20-km fiber link. To our knowledge, this also marks the first demonstration of a continuous-variable direct QSS protocol.
Ensuring the security of digital communication and data transfer has become essential in the modern era. Classical cryptographic techniques are increasingly vulnerable due to advances in quantum technologies and algorithms. Consequently, quantum computers and quantum communication offer promising solutions for secure data transfer and encryption. This study explores the generation of Quantum Digital Signatures (QDS) using Quantum Continuous Variables (QCV), providing a novel approach to secure digital signature technologies. The paper outlines the core principles of QDS generation with QCV, detailing the signature creation and verification processes. It highlights the advantages of this technology in secure communication and data transfer and discusses potential security vulnerabilities and future development prospects. Quantum Continuous Variables (QCVs), typically used in quantum optical systems, represent physical quantities with continuous spectra, such as the wavelength or phase of light. These variables enable efficient and secure quantum information processing and communication. Despite significant progress in quantum cryptography protocols using QCVs, practical application and optimization of these technologies face numerous challenges. These include complexities in preparing and measuring quantum states, managing quantum errors, and achieving higher efficiency and security standards. Moreover, the practical applications of QCV in industrial contexts remain limited, highlighting the need for further experimental and applied research. The methodology for generating QDS using QCVs involves employing specific quantum states, such as coherent and squeezed states. The process includes key distribution, signature creation and verification, and addressing potential quantum attacks. The system model comprises a sender (Alice), a receiver (Bob), and an arbiter (Charlie), facilitating secure and authenticated message transmission.
Digital signatures ensure the integrity of a classical message and the authenticity of its sender. Despite their far-reaching use in modern communication, currently used signature schemes rely on computational assumptions and will be rendered insecure by a quantum computer. We present a quantum digital signatures (QDS) scheme whose security is instead based on the impossibility of perfectly and deterministically distinguishing between quantum states. Our continuous-variable (CV) scheme relies on phase measurement of a distributed alphabet of coherent states and allows for secure message authentication against a quantum adversary performing collective beamsplitter and entangling-cloner attacks. Crucially, in the CV setting we allow for an eavesdropper on the quantum channels and yet retain shorter signature lengths than previous protocols with no eavesdropper. This opens up the possibility to implement CV QDS alongside existing CV quantum key distribution platforms with minimal modification.
Digital signatures are frequently used in data transfer to prevent impersonation, repudiation and message tampering. Currently used classical digital signature schemes rely on public key encryption techniques, where the complexity of so-called ‘one-way’ mathematical functions is used to provide security over sufficiently long timescales. No mathematical proofs are known for the long-term security of such techniques. Quantum digital signatures offer a means of sending a message, which cannot be forged or repudiated, with security verified by information-theoretical limits and quantum mechanics. Here we demonstrate an experimental system, which distributes quantum signatures from one sender to two receivers and enables message sending ensured against forging and repudiation. Additionally, we analyse the security of the system in some typical scenarios. Our system is based on the interference of phase-encoded coherent states of light and our implementation utilizes polarization-maintaining optical fibre and photons with a wavelength of 850 nm. Quantum digital signatures exploit quantum mechanics to provide verification of messages at the limits of information theory. Clarkeet al.demonstrate a photonic system that provides quantum digital signatures for messages sent to two receivers and is secure against forgery and repudiation.
Quantum Key Distribution (QKD) provides a method for securely sharing a secret key between two distant parties, Alice and Bob, by leveraging the principles of quantum mechanics. This thesis focuses on Continuous-Variable (CV) QKD, a variant of QKD where information is encoded in the quadrature components of coherent light states, allowing for efficient and cost-effective detection using homodyne or heterodyne receivers. The thesis is divided into three main parts: an introduction to quantum cryptography concepts, the presentation of a CV-QKD protocol, and a detailed security proof of the protocol against different types of attacks. The security proof considers both finite and infinite symbol transmission and tackles various eavesdropping attacks, including individual, collective, and coherent attacks. The use of Gaussian states and operations provides a promising approach to enhance QKD security, particularly in the context of practical quantum cryptography. The final sections provide a mathematical framework for the optimization of secure key rates and strategies for addressing the photon-number cutoff assumption in practical implementations.