Quantum key distribution (QKD) is a rapidly maturing quantum photonic technology that allows for the distribution of secure cryptographic keys under no assumptions of the attacker's computing power. A key hurdle for its large-scale adoption is the reduction of its hardware size and cost, for which photonic integration is a promising solution. However, there exists no perfect material platform for this task, and for QKD this leads to either design compromises or core functionalities left off-chip. This thesis makes the assertion that hybrid integrated photonics, where one combines two chip materials into one structure, can resolve some of these issues. Firstly, we present an edge-coupled hybrid transceiver that combines a silicon nitride waveguide chip with an indium phosphide modulator chip. Secondly, we pursue the hybrid integration of the single photon detectors. However, the appropriate solution of fast-gated telecom-wavelength avalanche diodes did not exist, and so in the second experimental chapter of this thesis we develop and characterise a linear array of GHz-gated InGaAs/InP SPADs. Finally, we demonstrate a hybrid integrated QKD receiver, consisting of a silica waveguide chip with the aforementioned SPAD array. This represents the first integrated QKD receiver without requiring either external single photon detectors or cryogenic cooling. The results support the benefits of hybrid integration for quantum key distribution, but also more widely for quantum photonic technology as it continues to emerge.
Quantum key distribution (QKD) is a rapidly maturing quantum photonic technology that allows for the distribution of secure cryptographic keys under no assumptions of the attacker's computing power. A key hurdle for its large-scale adoption is the reduction of its hardware size and cost, for which photonic integration is a promising solution. However, there exists no perfect material platform for this task, and for QKD this leads to either design compromises or core functionalities left off-chip. This thesis makes the assertion that hybrid integrated photonics, where one combines two chip materials into one structure, can resolve some of these issues. Firstly, we present an edge-coupled hybrid transceiver that combines a silicon nitride waveguide chip with an indium phosphide modulator chip. Secondly, we pursue the hybrid integration of the single photon detectors. However, the appropriate solution of fast-gated telecom-wavelength avalanche diodes did not exist, and so in the second experimental chapter of this thesis we develop and characterise a linear array of GHz-gated InGaAs/InP SPADs. Finally, we demonstrate a hybrid integrated QKD receiver, consisting of a silica waveguide chip with the aforementioned SPAD array. This represents the first integrated QKD receiver without requiring either external single photon detectors or cryogenic cooling. The results support the benefits of hybrid integration for quantum key distribution, but also more widely for quantum photonic technology as it continues to emerge.
Photonic integration is a promising route to miniaturise the hardware of quantum key distribution (QKD), yet the monolithic integration of single-photon detectors remains a significant challenge. QKD receiver chips integrating superconducting detectors have been demonstrated, but their requirement for cryogenic cooling restricts their practical applications. High-frequency gated single-photon avalanche diodes (SPADs) provide a mature non-cryogenic alternative, and their fabrication into compact arrays would enable scalable hybrid integration. However, this faces several challenges related to efficient GHz array gating, inter-pixel crosstalk, and scalable waveguide coupling, which to date remain unaddressed. Here, we overcome the key challenges and develop GHz-gated InGaAs/InP SPAD arrays with performance viable for QKD and negligible inter-pixel crosstalk. We combine the arrays with low-loss silica waveguide chips to produce compact hybrid QKD receivers and perform BB84 protocol experiments, achieving secure key rates over 2 Mbps at short distances and 15 kbps over 100 km of fibre. Our work provides a method for flexible and scalable integration of waveguide-coupled SPADs for quantum information processing applications.
Abstract Quantum photonic technologies, such as quantum key distribution, are already benefiting greatly from the rise of integrated photonics. However, the flexibility in design of these systems is often restricted by the properties of the integration material platforms. Here, we overcome this choice by using hybrid integration of ultra-low-loss silicon nitride waveguides with indium phosphide electro-optic modulators to produce high-performance quantum key distribution transceiver chips. Access to the best properties of both materials allows us to achieve active encoding and decoding of photonic qubits on-chip at GHz speeds and with sub-1% quantum bit error rates over long fibre distances. We demonstrate bidirectional secure bit rates of 1.82 Mbps over 10 dB channel attenuation and positive secure key rates out to 250 km of fibre. The results support the imminent utility of hybrid integration for quantum photonic circuits and the wider field of photonics.
Advances in quantum computing pose significant threats to the security of conventional encryption standards. Although quantum key distribution systems offer inherent security, they face challenges in achieving practical secret-key rates over long transmission distances. Photonic Integrated Circuit technologies provide scalability and system efficiency for high key rates using multiplexed time-bin protocols. We present the architecture of a 16-channel multiplexed QKD system at a GHz clock rate realized on integrated photonic chips, promising further scalability. The sender module revolves around an indium-phosphide chip featuring tunable lasers, modulators, and combiners. On the receiver side, low fiber-to-chip coupling loss and waveguide-integrated superconducting nanowire single-photon detectors on silicon nitride provide the necessary detection efficiency.
The security of current public key cryptosystems, such as RSA, depends on the difficulty of computing certain functions known as trapdoor functions. However, as computational resources become more abundant with the fast development of super- and quantum computers, relying on such methods for communication security becomes risky. Quantum key distribution (QKD), is a potential solution that can allow theoretically secure key exchange for future communications. Chip-scale integration of this solution for securing communication of embedded systems and hand held devices demands miniaturizing the optical components that are used in typical QKD boxes, hence reducing its size and cost. The aim of the work in this thesis is firstly investigating novel approaches to realising integrable single photon sources and detectors for applications such as QKD, and secondly proposing a chip-scale integrated QKD system with efficient and optimised optical components. In the first part of the thesis, a model for coupling 2D material emitters to rod-type photonic cavities is studied for room temperature single photon sources. Our investigated approach allows better coupling between the emitter and the cavity modes than conventional methods, while increasing light collection ratio. In the second part, site-controlled growth of semiconductor III-V nanowires on Si for photodetection applications is achieved by fabricating the sites using electron-beam lithography and wet etching. Studies were also carried out to investigate the effect of the wafer’s growth temperature on the nanowire formation. Finally, a model was proposed for realising a chip-scale QKD system using photonic crystals as a photonic circuit platform. The work involves increasing the Q-factor of the cavity single photon source, increasing cavity waveguide coupling, reducing losses in beam splitters and out-couplers. A final model of a chip-scale QKD system which involves the optimised components is proposed at the end of the thesis.
Quantum communication promises unconditional security by leveraging the fundamental laws of quantum physics, which make eavesdropping detectable through the disturbance it introduces in quantum states. A cornerstone of this technology is quantum key distribution (QKD), enabling the distribution of a shared secret key between distant users with information- theoretic security, rather than relying on assumptions about an eavesdropper’s computational power, as in classical cryptography. However, current QKD systems rely largely on bulk-optical solutions, which limits scalability, stability, and integration into existing telecommunication infrastructures. Photonic integrated circuits (PICs) offer a promising platform to overcome these challenges by providing compact, stable, and CMOS-compatible architectures. This PhD work investigates integrated photonic platforms for practical quantum communication at telecommunication wavelengths, with a focus on demonstrating QKD protocols across three different encoding schemes: time-bin, frequency-bin, and path. First, a time-bin QKD receiver using a borosilicate-glass integrated imbalanced Mach–Zehnder interferometer (iMZI) is developed and compared with a fiber-based receiver, showing enhanced phase stability and improved long-distance secure key rates. Next, frequency-bin entanglement-based QKD is implemented using silicon microring resonators, demonstrating high-visibility quantum interference and active phase-stabilization techniques. Finally, a chip-to-chip path-entangled QKD link is realized using silicon photonic spiral sources, representing a crucial step toward fully integrated quantum transceivers. Across these experiments, this PhD thesis establishes a unified experimental framework for integrated QKD and evaluates performance in terms of quantum bit error rate, secure key rate, and system stability under realistic operating conditions. Together, the results provide a proof-of-concept demonstration of scalable, high-performance quantum communication using PIC technology, advancing toward practical deployment in future quantum networks and laying foundations for multidimensional and fully integrated quantum communication nodes.
Improvement in secure transmission of information is an urgent need for governments, corporations and individuals. Quantum key distribution (QKD) promises security based on the laws of physics and has rapidly grown from proof-of-concept to robust demonstrations and deployment of commercial systems. Despite these advances, QKD has not been widely adopted, and large-scale deployment will likely require chip-based devices for improved performance, miniaturization and enhanced functionality. Here we report low error rate, GHz clocked QKD operation of an indium phosphide transmitter chip and a silicon oxynitride receiver chip-monolithically integrated devices using components and manufacturing processes from the telecommunications industry. We use the reconfigurability of these devices to demonstrate three prominent QKD protocols-BB84, Coherent One Way and Differential Phase Shift-with performance comparable to state-of-the-art. These devices, when combined with integrated single photon detectors, pave the way for successfully integrating QKD into future telecommunications networks.