Quantum mechanics - our best fundamental theory of atoms and molecules - presents several remarkable properties which if harnessed would generate major scientific and technological breakthroughs. For example: quantum particles, such as electrons, have an intrinsic property called spin which has no direct analogy to our usual notions of rotation; these spins can exist in multiple orientations at the same time (a superposition state); and they can be entangled such that physically separated particles must be described as a composite entity. My research seeks to understand and deploy these spin states in chemically synthesised molecules, with applications in two broad themes: 1. Molecular spins for quantum sensing. The sensitivity of spin states to their environment makes them promising sensors for a range of properties including magnetic and electric fields, strain, and temperature. This spin-based sensing approach offers exciting applications ranging from thermometry inside biological cells to nanoscale imaging of new phases of matter. Molecular systems could potentially revolutionise quantum sensing through their unique combination of properties: they can be chemically tuned to match a specific sensing target, self-assembled into multi-spin structures for applications ranging from entanglement-enhanced sensing to wide-field imaging, and readily brought close to a sensing target due to their nanoscale, self-contained nature. However, a foundational understanding of how to harness such molecular spin systems is needed. My research seeks to address this challenge by studying the interface of molecular spins with external stimuli such as light, strain, temperature, and electric fields, and controlling molecular spin superpositions and entanglement. Key aims include: demonstrating a spin-based sensor in an organic molecule, achieving room temperature sensor readout with light, efficiently coupling molecular spins to strain and thermal fields, and generating entanglement among single spins. From these fundamental insights, a broad class of tailor-made molecular quantum sensors could be realised, with implications for understanding both physics and biology. 2. Spins in molecular materials and devices. In addition to being a powerful resource for quantum sensing, spin also offers a native nanoscale window into the function of molecular devices, such as next-generation light-emitting diodes and solar cells. These systems naturally generate long-lived spin states, offering a sensitive intrinsic means to map structure and dynamics down to nanometre length scales that would otherwise be extremely challenging to access. This spin-based window provides a means to unravel phenomena ranging from light harvesting and photocatalysis to light emission and charge recombination, understand the role of spin-dependent processes on device performance, and ultimately aid deterministic design of future molecular devices. As a second complementary research theme, I will use spin as a native probe to understand the microscopic processes behind next-generation light-emitting and light-harvesting materials. Using spin-sensitive methods down to the ultimate limit of single molecules, I aim to provide unprecedented insight into these photophysical phenomena, and the foundations from which novel optoelectronic devices can be constructed. By focusing on two conventionally distinct but symbiotic themes of quantum sensing and optoelectronic materials, I aim to cross-pollinate these fields: the quantum sensing theme will provide new approaches to understand next-generation optoelectronic devices, while the molecular devices theme will provide new materials and architectures that could be utilised for quantum sensing. Overall, these efforts will lead to new possibilities for quantum-engineered molecular materials and devices.

Atoms and molecules are very well described by quantum mechanics, but what about much larger things? Erwin Schrödinger pointed out that cats have never been shown to exist in a quantum superposition, but recent experiments are pushing back the boundaries of which objects have been shown to do this. The strongest tests require a large mass in a superposition for a long time with a large superposition distance. The superposition distance is the distance between the two components of the superposition. Pioneering experiments have been done with superconductors, superfluids and vibrating cantilevers but the most macroscopic superposition state created so far is a variant of the famous two-slit experiment for molecules made of 2000 atoms. Our project has the ambitious goal of testing whether levitated nanodiamonds made up of more than a million times more atoms can display this quantum behaviour. The most exciting thing about this experimental frontier is that it could, in 10-15 years, lead to a test of quantum gravity. Einstein's general relativity explains gravity and is needed to make GPS work, but we don't know how to combine it with quantum mechanics to explain the gravitational effects produced by a quantum object. Successfully combining these two most fundamental theories of physics would produce a theory of quantum gravity, which has been sought for 100 years. Theories of quantum gravity such as string theory and loop quantum gravity have been proposed, but suffer from a lack of empirical evidence. Physicists such as Stephen Hawking and Roger Penrose worked on black holes and showed they are fertile playgrounds to constrain theories of quantum gravity, but black holes are not practical to experiment on. A new proposal from us and others shows a way to test one key aspect of quantum gravity with a lab experiment on a table-top. The idea is to create two of the nanodiamond Schrödinger cats and see how they interact gravitationally. This project is only possible thanks to the advances already demonstrated by the quantum technology community, and indeed this research will, in time, lead to a new class of more sensitive sensors that would be used to detect acceleration, rotation, tilt, gravity and magnetic fields. Having already published our descriptions for how to test macroscopic quantum mechanics and quantum gravity, we will now transform our preliminary experiments to begin the delivery of these proposals. To reach large superposition distances and long durations we will use diamond nanoparticles (around 800 nm across) containing a single nitrogen vacancy centre (NVC). This follows our proposals which provide a clear route to achieve a superposition distance of over 1000 nm, although our initial experiments will only reach 0.1 pm. Nanodiamonds have been levitated in vacuum using optical traps by us and others, as well as in Paul traps and magnetic traps. We showed that the heating of the diamond by the trapping beam in an optical trap in vacuum is a serious obstacle. To get around this we developed (with collaborator Oliver Williams) large quantities of high-purity nanodiamonds, and have now switched to using a magnetic trap as this further minimises the heating of the levitated diamond. A magnetic trap also provides the inhomogeneous magnetic field which is required to couple the spin to the motion. The core idea is to put the NVC electron spin into a spin superposition because the inhomogeneous magnetic field then provides a superposition of forces on the diamond leading to a spatial superposition. To evidence this, we will then flip the spin to recombine the superposition components for matter-wave interferometry and repeat the interferometry as a function of experimental tilt with respect to gravity to search for interference fringes.

By measuring subtle changes (<10-8) in the acceleration of gravity we can infer the local density of nearby objects. When the density is lower (e.g. a tunnel) the local gravity becomes slightly less. While this technique is readily adopted in the oil & gas industry to (i) search for new resources (ii) perform long term monitoring at active wells, broader uptake of gravimetry in other fields is limited due to the high upfront cost of gravimeters ($80k for a Scintrex CG6), the fragility of devices and the time it takes to undertake field surveys with a single instrument. There are disruptive opportunities for gravimetry to breakthrough into other fields including environmental monitoring and security & defence. For environmental monitoring, a smaller-lighter-cheaper gravimeter will open-up opportunities for (i) deployment of sensors arrays of volcanos as a technique to image the magma plumbing system and provide resilience against eruptions, (ii) performing rapid field surveys to identify collapsed culverts, sinkholes or underground tunnels. Within the field of Security & Defence there are further opportunities of monitoring ports of entry with underwater sensors, detecting underground tunnels and monitoring compounds. Beyond this, we see opportunities in monitoring dam infrastructure, carbon capture, geothermal engineering detection/monitoring of underground aquifers. Wee-g is a precision MicroElectroMechanicalSensor (MEMS) that has been developed within the Institute for Gravitational Research (University of Glasgow). It is a spin-off from the Gravitational-Wave research activities led by Prof . Hammond. Wee-g is the world's first gravimeter, capable of monitoring the Earth tides; elastic deformations of the Earth caused by the tidal potential of the Moon and Sun. As typical gravity signals are 10-50% of the Earth ides, this is an essential measurement to show devices have sufficient stability and sensitivity. The Wee-g sensor has the potential to be made much cheaper than existing gravimeters, and thus can open-up these new opportunities in Environmental monitoring and Security & Defence. Field trials are underway with partners in both the Environmental and Security & Defence fields. Wee-g Mk I systems are being deployed on Mt Etna as part of a H2020 project (Newton-g) to monitor magma intrusion in volcanoes, and we estimate the TRL is 5. We also have a system being trialled by DSTL for underwater monitoring at a port of entry. We will use this proposal to further develop the Wee-g gravimeter (Mk II system) to put us in a prime position to spinout. We will address some of the challenges found in the Mk I system including temperature sensitivity of the MEMS chip, miniaturisation and temperature stability of the front-end electronics, and removing reliance on evaluation FPGA boards which are liable to be discontinued.

Modern physics explains a stunning variety of phenomena from the smallest of scales to the largest and has already revolutionized the world! Lasers, semiconductors, and transistors are at the core of our laptops, mobile phones, and medical equipment. These technologies in turn have enabled us to explore the natural world with ever greater detail, precision, and rigour. Over the last few years, novel quantum technologies are being developed within the National Quantum Technology Programme in the UK and throughout the world that could impact our everyday lives and enable 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 telescopes that capture high-resolution images of the universe. Despite these successes of modern physics, several profound and challenging questions remain open. Our consortium QI-extension will build on recent advances in quantum technologies, both within our existing consortium QI and beyond, 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. Understanding the nature of this mysterious so-called dark matter will shed light on the history of the universe and will trigger new areas of research in fundamental and possibly applied physics. A number of state-of-the-art experiments world-wide are looking for dark matter candidates with no luck so far. The candidates 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, or all of dark matter. First, we will enhance the sensitivity of our current experiment that 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 and characterise a large (8''/200 nm diameter) superconducting nanowire single photon detector to extend dark matter searches. Our second line of research is devoted to the nature of space and time. We have a long list of successful experimental tests of quantum mechanics and Einstein's theory of relativity. But should gravity be united with quantum mechanics? If so, how? As with any open question in physics, experiments can direct us towards the answers. To that end, we propose to study two quantum aspects of space-time. Firstly, 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. We will also use the data to search for scalar dark matter in the galactic halo. Secondly, we will search for signatures of semiclassical gravity models that approximately solve the quantum gravity problems. Building on our existing work on experimentally testing semiclassical models of gravity, we will seek to design table-top experiments that may provide direct signatures of the quantum nature of gravity. 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.