Increasing the science reach for Quantum Enhanced Interferometry.

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.