Quantum dots (QDs) are nanoscale regions of semiconductor, embedded within a much larger host of a second semiconductor. The differing properties of the two semiconductors mean that single particles of charge (electrons) can be trapped within a QD, allowing for study of light-matter interactions on a single particle level. In particular, QDs form an excellent source of the quantum states of light (photons) that are required for many exciting new quantum technologies such as secure communication and enhanced sensing. A consequence of the solid-state host is that the QD interacts with its local environment, a particularly important example being quantised vibrations of the lattice, termed phonons. These interactions have typically been considered an unwelcome but unavoidable consequence of working with QDs and other similar solid-state systems. This proposal aims to demonstrate that through appropriate nano-fabrication and control of the QD geometry, the interaction of the QD with both its optical (photonic) and vibrational (phononic) environments can be controlled. By realising such control over environmental interactions, the impact of phonon interactions on the photons emitted can be almost eliminated, increasing the efficiency and quality of the QD photon source to support new applications. Furthermore, the need for extreme cryogenic cooling can be greatly reduced, removing a significant barrier to quantum technologies applications. Harnessing these developments, several novel quantum technologies will be developed based on the QD platform. Quantum 2-photon microscopy offers the potential to perform imaging of delicate samples that would be damaged by the intense light fields required for current methods. Meanwhile, high sensitivity optical sensing can be realised by using phonon interactions to "squeeze" the uncertainty in photons emitted by the QD. Finally, quantum data locking offers the potential for quantum-secured communication with a significantly higher efficiency than existing methods.

Light emitting semiconductor materials and devices dominate many aspects of everyday life. Their influence is all pervasive providing the sources which enable the internet, large area displays, room and street lighting to give just a few examples. Their existence relies on the high quality semiconductor structures which may be prepared by advanced crystal growth and sophisticated nanofabrication. Our proposal aims to capitalise on the advanced growth and fabrication to achieve similar advances in the quantum world where often counter-intuitive behaviour is governed solely by the laws of quantum mechanics. Our overall aim is to explore the behaviour of nano-devices operating in regimes where fundamentally new types of quantum-photonic phenomena occur, with potential to underpin the next generation of quantum technologies. We focus on two complementary systems: III-V semiconductors with their highly perfect crystal lattices, proven ability to emit photons one by one and long coherence quantum states, and atomically-thin graphene-like two dimensional (2D) semiconductors enabling new band structures, stable electron-hole bound states (excitons) and easy integration with patterned structures. The combination of the two material systems is powerful enabling phenomena ranging from the single photon level up to dense many-particle states where interactions dominate. A significant part of our programme focusses on on-chip geometries, enabling scale-up as likely required for applications. The semiconductor systems we employ interact strongly with photons; we will achieve interactions between photons which normally do not interact. We will gain entry into the regime of highly non-linear cavity quantum electrodynamics. Excitons (coupled electron-hole pairs) and photons interact strongly, enabling ladders of energy levels leading to on-chip production of few photon states. By coupling cavities together, we will aim for highly correlated states of photons. These advances are likely to be important components of photonic quantum processors and quantum communication systems. In similar structures, we access regimes of high density where electrons and holes condense into highly populated states (condensates). We aim to answer long-standing fundamental questions about the types of phase transitions that can occur in equilibrium systems and in out-of-equilibrium ones which have loss balanced by gain. We will also study condensate systems up to high temperatures, potentially in excess of 100K, and of the mechanisms underlying phase transitions to condensed states. The condensed state systems, besides their fundamental interest, also have potential as new forms of miniature coherent light sources. Nanofabrication will play a vital role enabling confinement of light on sub-wavelength length scales and fabrication of cavities for photons such that they have very long lifetimes before escaping. The ability to place high quality emitters within III-V nanophotonic structures will receive enhancement and potential world lead from a crystal growth machine we have recently commissioned, specially designed for this purpose, funded by the UK Quantum Technologies programme. Similar impact is expected from our ability to prepare 2D heterostructures (atomically thin layers of two separate materials placed one on top of the other) under conditions of ultrahigh vacuum free from contamination, enabling realisation of bound electron-hole pair states of very long lifetime, the route to condensation to high density states. The easy integration of 2D heterostructures with patterned photonic structures furthermore enables nonlinear and quantum phenomena to be studied, including in topological structures where light flow is immune to scattering by defects. Taken all together we have the ingredients in place to achieve ground-breaking advances in fundamental quantum photonics with considerable potential to underpin next generations of quantum technologies.