Atomically thin materials offer a new paradigm for control of electronic excitations in the extreme two-dimensional (2D) limit in condensed matter. Recently this concept has been developed further when artificial potentials for electrons were created in heterostructures consisting of stacked 2D layers held together by van der Waals forces, and light was used to access and manipulate electronic spin and valley degrees of freedom in atomically-thin semiconducting transition metal dichalcogenides (TMDCs). A significant world-wide effort in the last 5 years has resulted in intense studies of optical properties of TMDC atomic layers in the linear regime. Here, we propose to use this new class of (2D) semiconducting crystals to demonstrate unexplored approaches to exploiting nonlinear optical phenomena on the nano-scale in regimes unattainable by other ultra-fast photonic materials. To achieve this, we will exploit robust excitonic complexes observable up to room T, which will be generated and controlled in artificially created vertical stacks of 2D atomic layers. Giant nonlinearities enabling ultra-fast control of light with light of low intensity will be realised and explored in such van der Waals heterostructures placed in optical microcavities, operating in the strong light-matter coupling regime that we demonstrated recently. In this regime part-light-part-matter polaritons are formed, with the exciton part responsible for the strong nonlinearity and the photon part providing efficient coupling to light. This work will open a new route to development of highly nonlinear nano-photonic devices such as miniature ultra-fast modulators and switches, with high potential to impact on a new generation of signal processing and quantum technology hardware.

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.

Quantum technologies exploit the intriguing properties of matter and light that emerge when the randomizing processes of everyday situations are subdued. Particles then behave like waves and, like the photons in a laser beam, can be split and recombined to show interference, providing sensing mechanisms of exquisite sensitivity and clocks of exceptional accuracy. Quantum measurements affect the systems they measure, and guarantee communication security by destroying cryptographic keys as they are used. The entanglement of different atoms, photons or circuits allows massively powerful computation that promises complex optimizations, ultrafast database searches and elusive mathematical solutions. These quantum technologies, which EPSRC has declared one of its four Mission-Inspired priorities, promise in the near future to stand alongside electronics and laser optics as a major technological resource. In this 'second quantum revolution', a burgeoning quantum technology industry is translating academic research and laboratory prototypes into practical devices. Our commercial partners - global corporations, government agencies, SMEs, start-ups, a recruitment agency and VC fund - have identified a consistent need for hundreds of doctoral graduates who combine deep understanding of quantum science with engineering competence, systems insight and a commercial head. With our partners' guidance, we have designed an exciting programme of taught modules to develop knowledge, skills and awareness beyond the provision of traditional science-focused PhD programmes. While pursuing leading-edge research in quantum science and engineering, graduate students in the EPSRC CDT for Quantum Technology Engineering will follow a mix of lectures, practical assignments and team work, peer learning, workshops, and talks by our commercial partners. They will strengthen their scientific and engineering capabilities, develop their computing and practical workshop skills, study systems engineering and nanofabrication, project and risk management and a range of commercial topics, and receive professional coaching in communication and presentation. An industrial placement and extended study visit will give them experience of the commercial environment and global links in their chosen area, and they will have support and opportunities to break their studies to explore the commercialization of research inventions. A QT Enterprise Club will provide fresh, practical entrepreneurship advice, as well as a forum for local businesses to exchange experience and expertise. The CDT will foster an atmosphere of team working and collaboration, with a variety of group exercises and projects and constant encouragement to learn from and about each other. Students will act as mentors to junior colleagues, and be encouraged to take an active interest in each other's research. They will benefit from the diversity of their peers' backgrounds, across not just academic disciplines but also career stages, with industry secondees and part-time students bringing rich experience and complementary expertise. Students will draw upon the wealth of experience, across all corners of quantum technologies and their underpinning science and techniques, provided by Southampton's departments of Physics & Astronomy, Engineering, Electronics & Computer Science, Chemistry and its Optoelectronics Research Centre. They will be given training and opening credit for the Zepler Institute's nanofabrication facilities, and access to the inertial testing facilities of the Institute of Sound & Vibration research and the trials facilities of the National Oceanography Centre. Our aim is that graduates of the CDT will possess not only a doctorate in the exciting field of quantum technology, but a wealth of knowledge, skills and awareness of the scientific, technical and commercial topics they will need in their future careers to propel quantum technologies to commercial success.