Technological advances have led to the availability of electronic devices like laptops, mobile devices and global positioning systems. In order to increase performance, modern technology has followed the path of miniaturising the components to reduce the overall size of commercial devices. Following this trend, we have now reached the point where matter can be controlled at the smallest scale: the single atom. It is in this new realm of physics that unconventional effects take place: when we deal with structures composed of just a few atoms or when we manipulate single electronic charges, the physics follows rules described by quantum mechanics. A completely new range of effects take place and devices with novel functionalities can be created: the quantum information revolution seems to be within reach. A very exciting research field focuses on the study of nanostructures, entities whose dimensions are of the order of 0.000000001m. Such small structures can be used for controlling single particles of light: single photons. Conventional light sources emit a large number of photons in a wide angular range and are mainly used for lighting and imaging. The ability to control light at the single-photon level is technologically challenging but tremendously interesting. If we can store information encoded on single photons, we can transfer it at the speed of light with a guaranteed secure communication. Single-photon emitters also find applications in imaging and medical sensing. Unfortunately, many single-photon sources operate at very low temperatures, which require the use of liquid helium, which is expensive and inconvenient for real-world applications. A material called Gallium Nitride (GaN) offers opportunities to overcome these limitations. GaN is a semiconductor crystal, and defects in that crystal can act as single-photon emitters, as can indium gallium nitride (InGaN) nanostructures embedded in a GaN matrix. Such nanostructures can emit single photons at room temperature, across a very wide range of wavelengths. However, incorporating these emitters into practical devices is very challenging. They tend to form at random locations in the crystal, which makes it hard to ensure that a device contains an optimally-positioned single emitter and that the light is emitted in the desired direction with high efficiency, as required for applications. In this project, we will develop technologies which allow us to control where an emitter forms, and integrate those site-controlled emitters with structures which extract the light from the device efficiently and channel it in a desired direction. We will create devices where the light extraction structures are integrated with the electrical injection of charge carriers into the emitter. That means that we will be able to use an applied voltage to either drive the single-photon emission or to alter the wavelength (or colour) of the emitted photon. The approach we will take to improving light extraction uses technologies that are easily incorporated into a standard manufacturing routine. We will put mirror-like structures underneath the single-photon emitters; above them, on the crystal surface, we will place tiny rings of metal, which can act like a lens, directing the light into the application system. In addition to being relatively easy to manufacture, relative to other possible technologies, this approach has additional advantages: it avoids etching the GaN crystal, which can damage device performance, and it also places less stringent requirements on achieving a very specific wavelength from the single-photon emitter. The metallic ring also doubles up as a contact for electrical injection. Overall, this provides a scalable, robust route to creating a new quantum technology - which addresses UK government priorities for advanced materials and manufacturing, and represents a crucial step forward in the implementation of quantum emitters in real-life devices.

The internet data rate of Mb/s is currently available to UK homes thanks to installation of fibre network. Recently Fujitsu, a major telecom company, outlined their plan to lay Gb/s fibre network in UK, which can increase the data rate to 10 Gb/s and beyond. Therefore optical fibre will play an ever increasing importance in our life and hence there is a clear need to carry out research in ultrafast optical components such as photodiodes, used to convert optical signal to electrical signal. In photodiodes the energy from light is used to release an electron from an atom and a detectable current is generated when the electron is swept by an electric field. In a specially designed avalanche photodiode (APD) the electric field is increased such that a single electron generated by the photoelectric effect can produce an avalanche of electrons and holes. Consequently a much larger signal is produced, leading to a better signal to noise ratio. Unfortunately current commercial APD can only work up to 10 Gb/s and is therefore not future proof. In this proposal, we will develop extremely thin 10-50 nm semiconductor layer to achieve the avalanche effect at ps time scale such that our APDs can operate at bit rates of Tb/s. The new semiconductor materials that will be developed in this project are AlAsSb and AlGaPSb since they have great potential to withstand extremely high electric field while maintaining low dark current (essential to minimise errors in digital signal). Crucially since our materials are only nm thick, we can engineer the electric field in APD to impose some degree of coherence in the electron and hole behaviours so that the avalanche effect occurs with minimal noise. We believe our APDs can be designed to approach the performance of an ideal noiseless APD with high bandwidth for optical communications. We recently demonstrated that the avalanche effect in thin AlAsSb is relatively immune to temperature change. Therefore in addition to ultra high speed optical communication, our proposed nm scaled AlAsSb and AlGaPSb avalanche layers are envisaged to work as an ultra fast photon counter with high immunity to ambient temperature fluctuation. Since a photon is the basic unit of light, the "ultimate" light sensor is achieved by increasing the avalanche gain to approximately a million so that the APD works as a photon counter. Our thin avalanche layer has the potential to register a photon count in a few ps, which is at least an order of magnitude faster than current APD photon counters. If successful one of the major impacts of our photon counter will be to improve the data encryption technique called quantum key distribution in which the data is encrypted using a single photon. This is believed to be the most secure encryption technology. Any unauthorised detection of the photon will cause a significant error rate, and hence alerting the sender of the attempted hacking. Therefore the high thermal stability and fast response time of our APDs will enhance the robustness of future quantum cryptography systems. We also believe our new technology will bring significant improvement to medical X-ray imaging as the APD can improve the signal to noise ratio of X-ray detection system. Typically the avalanche effect increases the electrical signal, induced by the X-ray absorption, to above the electronic circuit noise and hence enhancing the image quality. Our recent work showed that having a thin avalanche layer is essential for high performance X-ray APD. Hence our work will enable a new generation of X-ray APDs for imaging applications. To achieve the goals discussed above we will carry out very systematic development of AlAsSb and AlGaPSb APDs via advanced growth of the semiconductor crystals and optimised chemical etching process as well as meticulous measurements to extract key material properties for design of high performance APDs utilising nm avalanche regions.

Quantum technologies (QT) are new, disruptive information technologies that can outperform their conventional counterparts, in communications, sensing, imaging and computing. The UK has already invested significantly in a national programme for QT, to develop and exploit these technologies, and is now investing further to stimulate new UK industry and generate a supply of appropriately skilled technologists across the range of QT sectors. All QT exploit the various quantum properties of light or matter in some way. Our work is in the communications sector, and is based on the fundamental effect that measuring or detecting quantum light signals irreversibly disturbs them. This effect is built into Nature, and will not go away even when technologies (quantum or conventional) are improved in the future. The fundamental disturbance of transmitted quantum light signals enables secure communications, as folk intercepting signals when they are not supposed to (so-called eavesdroppers) will always get caught. This means Alice and Bob can use quantum light signals to set up secure shared data, or keys, which they can then use for a range of secure communications and transactions - this is quantum key distribution (QKD). The irreversible disturbance of light can also be used to generate random numbers - another very important ingredient for secure communications, cryptology, simulation and modelling. In the modern world where communications are so ubiquitous and important, there is increasing demand for new secure methods. Technologies and methods widely used today will be vulnerable to emergent quantum computing technologies, so encrypted information being sent around today which has a long security shelf-life will be at risk in the future. New "quantum safe" methods that are not vulnerable to any future QT have to be developed. So QKD and new mathematical encryption must be made practical and cost effective, and soon. The grand vision of the Quantum Communications Hub is therefore to pursue quantum communications at all distance scales, to offer a range of applications and services and the potential for integration with existing infrastructure. Very short distance communications require free space connections for flexibility. Examples include between a phone or other handheld device and a terminal, or between numerous devices and a fixed receiver in a room. The Hub will be engineering these "many-to-one" technologies to enhance practicality and real-world operation. Longer distance conventional communications - at city, metropolitan and inter-city scales - already use optical fibres, and quantum communications have to leverage and complement this. The Hub has already established the UK's first quantum network, the UKQN. We will be extending and enhancing the UKQN, adding function and capability, and introducing new QKD technologies - using quantum light analogous to that used in conventional communications, or using entanglement working towards even longer distance fibre communications. The very longest distance communications - intercontinental and across oceans - require satellites. The Hub will therefore work on a new programme developing ground to satellite QKD links. Commercial QKD technologies for all distance scales will require miniaturisation, for size, weight and power savings, and to enable mass manufacture. The Hub will therefore address key engineering for on-chip operation and integration. Although widely applicable, key-sharing does not provide a solution for all secure communication scenarios. The Hub will therefore research other new quantum protocols, and the incorporation of QKD into wider security solutions. Given the changing landscape worldwide, it is becoming increasingly important for the UK to establish national capability, both in quantum communication technologies and their key components such as light sources and detectors. The Hub has assembled an excellent team to deliver this capability.

Single-photon counting - the ability to faithfully capture the single quantum of light - is a critical capability for a wide range of new low-light sensing applications and a host of emerging photonic quantum technologies. This proposed Programme Grant aims to significantly expand the operational region of single-photon detectors well beyond silicon's 1000nm wavelength limit into the short-wave infrared (SWIR) region of wavelengths between 1400nm to 3000nm, and part of the mid-wave infrared (MWIR) region between 3000nm and 5000nm. By scaling up SWIR and MWIR semiconductor and superconductor single-photon detectors to large area focal plane arrays, we will produce revolutionary new cameras with picosecond timing resolution which can be used, for example, to see though fog in automotive lidar scenarios, as well as allowing imaging and sensing in new applications in environmental monitoring, healthcare, and security and defence. The project will involve the design and fabrication of innovative new detector platforms of Ge-on-Si and III-V semiconductor detectors. The detectors are capable of single-photon sensitivity in the SWIR and MWIR regions, and will be fabricated in detector array format. We will also examine superconducting nanowires to expand their operation into the MWIR regions and fabricate arrayed detector configurations. A key part of the project is to integrate these arrayed detector technologies with read-out circuitry capable of rapid, low latency delivery of single-photon data. In addition, we will utilise micro-optic technology to optimise detection efficiency and demonstrate multiple wavelength filtering. The cameras will be designed for use in a range of applications areas, including lidar, where the time-of-flight of the return photons can be used for the measurement of distance. In arrayed detector format, we will make cameras from which we will demonstrate three-dimensional imaging at long distance, where the sensitivity and time-resolution will enhance imaging through dense fog and other obscurants. We will demonstrate our detector technologies in quantum cryptography applications, where encryption keys can be shared between two users. By sending data encoded in single-photons it is possible for the sender and receiver to share a secure, random key known only to them. The most critical component in this form of quantum communication is the single-photon detector - we will demonstrate the use of our detectors both in optical fibre and free-space quantum key distribution scenarios. Other emerging applications in spectroscopy and biophotonics will be demonstrated.

o achieve this vision, we will address major global research challenges towards the establishment of the "quantum internet" —?globally interlinked quantum networks which connect quantum nodes via quantum channels co-existing with classical telecom networks. These research challenges include: low-noise quantum memories with long storage time; connecting quantum processors at all distance scales; long-haul and high-rate quantum communication links; large-scale entanglement networks with agile routing capabilities compatible with - and embedded in - classical telecommunicatons networks; cost-effective scalability, standardisation, verification and certification. By delivering technologies and techniques to our industrial innovation partners, the IQN Hub will enable UK academia, national laboratories, industry, and end-users to be at the forefront of the quantum networking revolution. The Hub will utilise experience in the use of photonic entanglement for quantum key distribution (QKD) alongside state-of-the art quantum memory research from existing EPSRC Quantum Technology Hubs and other projects to form a formidable consortium tackling the identified challenges. We will research critical component technology, which will underpin the future national supply chain, and we will make steps towards global QKD and the intercontinental distribution of entanglement via satellites. This will utilise the Hub Network's in-orbit demonstrator due to be launched in late 2024, as well as collaboration with upcoming international missions. With the National Quantum Computing Centre (NQCC), we will explore applications towards quantum advantage demonstrations such as secure access to the quantum cloud, achievable only through entanglement networks. Hub partner National Physical Laboratory (NPL) working with our academic partners and the National Cyber Security Centre (NCSC) will ensure that our efforts are compatible with emerging quantum regulatory standards and post-quantum cybersecurity to bolster national security. We will foster synergies with competing international efforts through healthy exchange with our global partners. The Hub's strong industrial partner base will facilitate knowledge exchange and new venture creation. Achieving the IQN Hub's vision will provide a secure distributed and entanglement-enabled quantum communication infrastructure for UK end-users. Industry, government stakeholders and the public will be able to secure data in transit, in storage and in computation, exploiting unique quantum resources and functionalities. We will use a hybrid approach with existing classical cyber-security standards, including novel emerging post-quantum algorithms as well as hardware security modules. We will showcase our ambition with target use-cases that have emerged as barriers for industry, after years of investigation within the current EPSRC QT Hubs as well as other international efforts. These barriers include security and integrity of: (1) device authentication, identification, attestation, verification; (2) distributed and cloud computing; (3) detection, measurement, sensing, synchronisation. We will demonstrate novel applications as well as identify novel figures of merit (such as resilience, accuracy, sustainability, communication complexity, cost, integrity, etc.) beyond security enhancement alone to ensure the national quantum entanglement network can be fully exploited by our stakeholders and our technology can be rapidly translated into a commercial setting.