The AION project harnesses a new generation of quantum sensors to conduct experiments in fundamental physics, such as the discovery of dark matter, and detect hitherto unknown sources of gravitational waves, such as violent collisions far away in the universe and events that occurred when the universe was a fraction of a second old. One of the foremost candidates for dark matter is some type of very light particle that is spread throughout space with a varying density that changes in time. AION is capable of detecting the effects of these variations on cold atoms using techniques based on quantum interference effects, with much greater sensitivity than current experiments. The same quantum techniques probe small fluctuations in the fabric of space-time caused by the passage of gravitational waves, and AION will measure such effects in a different range of wavelength and frequency from the existing experiments LIGO and Virgo. In this way it will be able to observe the mergers of black holes that are much more massive, possibly casting light on the formation of the supermassive black holes at the centres of galaxies. AION may also be sensitive to gravitational waves generated in the very early universe, for example by phase transitions or by cosmic strings. AION will be operated in a network with detectors in the US and Europe that are based on similar quantum physics, and its measurements will complement those by LIGO, Virgo and the future space experiment LISA, providing many possible synergies through joint observations. We will build an instrument in the UK that brings together the advantages of state-of-the-art optical clocks based on Sr atoms, with atom interferometry. This instrument has two atom interferometers, one above the other, in a vacuum system over 10m tall, with a laser beam running vertically through both that splits and recombines atomic wave packets. Two clouds of atoms will be prepared at different heights along a long vertical vacuum pipe, and both clouds will be launched so that they travel upwards for several metres before coming to rest and falling back down under gravity. Such 'atomic fountains' allow a long measurement time and large separation between the two arms of the interferometers. The atoms must be cooled to very low temperatures, less than 1 nanokelvin in our final design, otherwise they spread out and become too dilute before falling through the detection region. A vertical laser beam that runs through both clouds of atoms, at different heights, such that common-mode rejection of noise in differential measurements can determine the gradient of gravity with an uncertainty of 1x10^-10 per shot, comparable with the state of the art. The atoms are cooled in side-arms, transported into the vertical tube, launched, subjected to multiple laser pulses that form the interferometer and then finally detected using laser light. This requires a very sophisticated set of lasers. This will be the first large-scale atom interferometer in the UK; there are currently 10m devices in the USA, Germany and China. The AION programme exploits synergies between STFC and EPSRC science and the strategic areas of quantum technology, computing and metrology. It brings together a consortium of experimental and theoretical particle physicists, as well as astrophysicists and instrumentation experts, quantum information scientists, experts in Sr based atomic clock research, and atomic physicists drawn from the STFC and EPSRC communities. AION will collaborate with leading international laboratories such as Fermilab in the US, creating new scientific partnerships also with members of the space science community. The quantum technologies of AION have potential applications in such varied areas as navigation and oil drilling. We will work closely with the UK Quantum Technologies Hub in sensors and metrology to develop these technologies and bring them to market.

Modern physics explains a stunning variety of phenomena from the smallest of scales to the largest and has already revolutionized the world! Lasers, semi-conductors, and transistors are at the core of our laptops, cellphones, and medical equipment. And every year, new novel quantum technologies are being developed within the National Quantum Technology Programme in the UK and throughout the world that impact our everyday life and the fundamental physics research that leads to new discoveries. Quantum states of light have recently improved the sensitivity of gravitational-wave detectors, whose detections to date have enthralled the public, and superconducting transition-edge-sensors are now used in astronomy experiments that make high-resolution images of the universe. Despite the successes of modern physics, several profound and challenging problems remain. Our consortium will use recent advances in quantum technologies to address two of the most pressing questions: (i) what is the nature of dark matter and (ii) how can quantum mechanics be united with Einstein's theory of relativity? The first research direction is motivated by numerous observations which suggest that a significant fraction of the matter in galaxies is not directly observed by optical telescopes. This mysterious matter interacts gravitationally but does not seem to emit any light. Understanding the nature of dark matter will shed light on the history of the universe and the formation of galaxies and will trigger new areas of research in fundamental and possibly applied physics. Despite its remarkable importance, the nature of dark matter is still a mystery. A number of state-of-the-art experiments world-wide are looking for dark matter candidates with no luck to date. The candidate we propose to search for are axions and axion-like-particles (ALPs). These particles are motivated by outstanding questions in particle physics and may account for a significant part, if not all, of dark matter. First, we propose an experiment which will rely on quantum states of light and will detect a dark matter signal or improve the existing limits on the axion-photon coupling by a few orders of magnitude for a large range of axion masses. Second, we will build a quantum sensor which will improve the sensitivity of the international 100-m long ALPS detector of axion-like-particles by a factor of 3 - 10. Our second line of research is devoted to the nature of space and time. Recent announcements of Google's Sycamore quantum computer and the detection of gravitational waves have provided additional evidence to the long list of successful experimental tests of quantum mechanics and Einstein's theory of relativity. But how can gravity be united with quantum mechanics? To seek answers that inform this question, we propose to study two quantum aspects of space-time. First, we will experimentally investigate the holographic principle, which states that the information content of a volume can be encoded on its boundary. We will exploit quantum states of light and build two ultra-sensitive laser interferometers that will investigate possible correlations between different regions of space with unprecedented sensitivity. Second, we will search for signatures of semiclassical gravity models that approximately solve the quantum gravity problems. We will build two optical interferometers and search for the first time for signatures of semiclassical gravity in the motion of the cryogenic silicon mirrors. Answering these challenging questions of fundamental physics with the aid of modern quantum technologies has the potential to open new horizons for physics research and to reach a new level of understanding of the world we live in. The proposed research directions share the common technological platform of quantum-enhanced interferometry and benefit from the diverse skills of the researchers involved in the programme.

Magnetism is one of the most long-appreciated physical properties of matter. An early example of the exploitation of magnetic materials is the ancient lodestone compass, made from a magnetic mineral of iron oxide and used as a navigational device some 2000 years ago. However, it was not until the advent of quantum mechanics at the beginning of the 20th century that we developed an understanding of the atomic origin of magnetism in materials. We now know that magnetism is a phenomenon that arises from the behaviour of the electrons that make up matter. What makes magnetic materials so remarkable is that their chemical structure and bonding can allow their electrons to strongly interact in a variety of ways, giving rise to a rich diversity of magnetic properties that we can tune and harness for our benefit. Indeed, today we make use of magnetic materials in a range of technological devices that have revolutionised modern life. The basis of the read-head in a computer hard drive, for example, is two layers of magnetic materials, where the relative orientation of the magnetism within each layer controls the flow of current to read digital information. In other magnetic materials known as rare-earth magnets, the magnetic effect is so strong that it can be used to levitate trains and forms the basis of powerful, compact motors that are used to propel cars and to generate electricity from wind turbines. At the forefront of the interdisciplinary research field of advanced materials is the need to discover and understand the properties of novel magnetic materials to drive breakthroughs in the development of new technologies for the 21st century. This will involve discovering alternative sources of magnetic materials to overcome our over-reliance on their critical global supply chains and hazardous mining practices, exploiting the phenomenon of magnetic refrigeration to develop environmentally-friendly cooling technology, and uncovering never-before-seen magnetic properties in materials that may underpin the next generation of paradigm-shifting quantum technologies. To achieve these ambitious goals, access to - and development of - state-of-the-art equipment for the magnetic characterisation of materials are essential. With the Midlands Mag-Lab, we will establish a unique user facility at the University of Birmingham based on a cutting-edge Superconducting Quantum Interference Device (SQUID) magnetometer - the premier tool for the magnetic characterisation of materials. A versatile suite of measurement options will provide access to a broad range of temperatures, magnetic fields and pressures at which to probe the properties of a diverse range of advanced materials. This includes reaching temperatures ten times colder than outer space, magnetic fields one hundred thousand times stronger than the earth's magnetic field and applied pressures ten thousand times greater than atmospheric pressure. The equipment will be essential to enabling a wide-ranging portfolio of advanced materials research, with over 40 academic groups from across the Midlands region requiring the capacity and capability afforded by Mag-Lab, as well as international and industrial organisations demonstrating the wider requirement for the facility. With core establishing principles of fair and transparent equipment access and a significant proportion of early-career researchers within the initial user group, Mag-Lab will play a key role in ensuring the future success and strength of UK advanced materials research.

Quantum Technologies (QT) are at a pivotal moment with major global efforts underway to translate quantum information science into new products that promise disruptive impact across a wide variety of sectors from communications, imaging, sensing, metrology, simulation, to computation and security. Our world-leading Centre for Doctoral Training in Quantum Engineering will evolve to be a vital component of a thriving quantum UK ecosystem, training not just highly-skilled employees, but the CEOs and CTOs of the future QT companies that will define the field. Due to the excellence of its basic science, and through investment by the national QT programme, the UK has positioned itself at the forefront of global developments. There have been very recent major [billion-dollar] investments world-wide, notably in the US, China and Europe, both from government and leading technology companies. There has also been an explosion in the number of start-up companies in the area, both in the UK and internationally. Thus, competition in this field has increased dramatically. PhD trained experts are being recruited aggressively, by both large and small firms, signalling a rapidly growing need. The supply of globally competitive talent is perhaps the biggest challenge for the UK in maintaining its leading position in QT. The new CDT will address this challenge by providing a vital source of highly-trained scientists, engineers and innovators, thus making it possible to anchor an outstanding QT sector here, and therefore ensure that UK QT delivers long-term economic and societal benefits. Recognizing the nature of the skills need is vital: QT opportunities will be at the doctoral or postdoctoral level, largely in start-ups or small interdisciplinary teams in larger organizations. With our partners we have identified the key skills our graduates need, in addition to core technical skills: interdisciplinary teamwork, leadership in large and small groups, collaborative research, an entrepreneurial mind-set, agility of thought across diverse disciplines, and management of complex projects, including systems engineering. These factors show that a new type of graduate training is needed, far from the standard PhD model. A cohort-based approach is essential. In addition to lectures, there will be seminars, labs, research and peer-to-peer learning. There will be interdisciplinary and grand challenge team projects, co-created and co-delivered with industry partners, developing a variety of important team skills. Innovation, leadership and entrepreneurship activities will be embedded from day one. At all times, our programme will maximize the benefits of a cohort-based approach. In the past two years particularly, the QT landscape has transformed, and our proposed programme, with inputs from our partners, has been designed to reflect this. Our training and research programme has evolved and broadened from our highly successful current CDT to include the challenging interplay of noisy quantum hardware and new quantum software, applied to all three QT priorities: communications; computing & simulation; and sensing, imaging & metrology. Our programme will be founded on Bristol's outstanding activity in quantum information, computation and photonics, together with world-class expertise in science and engineering in areas surrounding this core. In addition, our programme will benefit from close links to Bristol's unique local innovation environment including the visionary Quantum Technology Enterprise Centre, a fellowship programme and Skills Hub run in partnership with Cranfield University's Bettany Centre in the School of Management, as well as internationally recognised incubators/accelerators SetSquared, EngineShed, UnitDX and the recently announced £43m Quantum Technology Innovation Centre. This will all be linked within Bristol's planned £300m Temple Quarter Enterprise Campus, placing the CDT at the centre of a thriving quantum ecosystem.

The last 20 years have witnessed a remarkable growth in the field of THz frequency science and engineering, which has matured into a vibrant international research area. The modern THz field arguably began with the development of a pulsed (single-cycle) THz emitter - the semiconductor photoconductive switch - and the subsequent development of THz time-domain spectroscopy (TDS). Since then, considerable success has been achieved in the further development of this and other THz sources, including the uni-travelling carrier (UTC) photodiode and the quantum cascade laser (QCL). However, notwithstanding this, it is only the THz-TDS technology that has been developed sufficiently for commercialization as a complete system, leaving other THz devices, components and techniques still restricted to the academic laboratory. This is unfortunate, since despite the success of THz-TDS, the technique has a number of shortcomings including its high fs-laser dominated cost, low power, and limited frequency and spatial resolution, which could be addressed by QCL and UTC technologies if they were to be engineered into appropriate instruments. In fact, a cursory comparison with the neighbouring microwave and optical regions of the spectrum reveals that THz frequency science and technology is still in its infancy, and not just in the context of commercial uptake. For example, the THz region significantly lags in the availability of precision spectroscopy instrumentation required to address sharp spectral features inherent to gases, for example, in atmospheric analysis, or in materials with long excited state lifetimes. THz technology also significantly lags in the fields of non-linear spectroscopy and coherent control, where powerful and controlled pulses of electromagnetic radiation interact with matter and manipulate its properties. In the optical and microwave regions, fascinating phenomena including electron-spin resonance and nuclear magnetic resonance were major breakthroughs, revealing a wealth of new science and engineering applications. These techniques, now standard across many disciplines, support much contemporary research and technology activity. A further example of how THz technology compares unfavourably with other spectral ranges is in the context of THz microscopy and analysis below the diffraction limit, which intrinsically restricts such measurements to ensemble sampling of physical properties averaged over the size, structure, orientation and density of, for example, nanoparticles, nanocrystals or nanodomains. Although near-field imaging approaches have been adapted from the visible/infrared regions enabling THz measurements on the micro/nano-scale, no THz instrument currently provides the required spatial resolution and sensitivity, nor can address the enormous range of length-scales (spanning five orders of magnitude from electron confinement lengths (<10 nm) to the THz wavelength (~300 um)), nor can operate at cryogenic temperatures. In fact, on this point, the THz field is deficient even in the provision of basic technologies such as waveguides and coupling optics required to deliver THz signals with low loss into cryostats or industrial apparatus. In this programme we will create the first comprehensive instrumentation for precise THz frequency spectroscopy, microscopy, and coherent control. This will be based upon our unique and proprietary capabilities to generate, and manipulate photonically, THz signals of unprecedentedly narrow (Hz) linewidth and with sub-wavelength spatial resolution. The instrumentation will then be exploited to create new challenge-led applications in non-destructive testing and spectroscopic analysis for electronics and atmospheric sensing, inter alia, as well as discovery-led opportunities within physics, quantum technologies, materials science, atmospheric chemistry and astronomy.