Novel superfluid phenomena in semiconductor microcavities

Superfluidity is one of the most remarkable consequences of macroscopic quantum coherence in interacting condensed matter systems, and manifests itself in a number of fascinating effects, such as for example dissipationless flow of a superfluid via obstacles, quantised circulation, and metastable persistent currents. First observed in liquid Helium in 1937, it is closely related to the phenomena of Bose-Einstein condensation (BEC), and to the Bardeen-Cooper-Schrieffer (BCS) collective state of fermions, which is responsible for superconductive behaviour of some materials.The phenomenon of macroscopic coherence and superfluidity is not restricted to systems close to thermodynamic equilibrium, such as liquid Helium, superconductors and ultra-cold atomic gases. It has also been recently observed in systems far from equilibrium, where a steady-state is obtained by a dynamical balance of driving and losses. Semiconductor microcavities currently play a leading role in the study of non-equilibrium superfluidity. Strong interactions between confined light and bosonic excitations in the semiconductor (called excitons) lead to new quasi-particles - microcavity polaritons, in which interactions can be manipulated externally by changing the driving power and the energy detuning between photons and excitons, and where the matter component can be accurately probed by measuring the properties of the emitted light.An additional advantage of semiconductor microcavities is that the temperatures for BEC and superfluidity in current experiments are of the order of 10K, and are only limited by relatively small dipole interactions between excitons and photons in GaAs. Other materials, such as GaN, have already hosted polariton lasing at room temperature, and it is now only a question of technological progress in manufacturing samples of a better quality (less of the inhomogeneous disorder) for polariton BEC and superfluidity to be realised at room temperature. Quantum collective effects at such high temperatures are likely to lead to device applications, for example in quantum storage and computation, and for transporting light-matter pulses without loss over macroscopic distances.However, properties which characterise non-equilibrium superfluids in dissipative and driven quantum systems are fundamentally different compared to superfluids in thermal equilibrium, and thus we are faced with an exciting opportunity to discover and explore brand-new physical phenomena. This project is aimed at exploring novel superfluid properties of non-equilibrium condensates, using polaritons in semiconductor microcavities. It is a very broad subject since essentially all phenomena discovered and discussed in the context of equilibrium superfluidity are likely to be affected by non-equilibrium. Our aim is to provide a comprehensive theoretical description, and experimental realisation by our project partners, of a broad range of phenomena connected with superfluid behaviour.

Who will benefit from this research? There are three classes of beneficiaries: 1) general public as recipients of fundamental knowledge about properties of matter (for example science minded pupils, students, people interested in basic knowledge); 2) technological industry (optical/electronic device designers etc...); 3) other academics in mine and related research areas. How will they benefit from this research? 1 class) The proposed project is concerned with a very fundamental problem of a novel type of superfluidity which can exist in highly dissipative and driven light-matter systems. The scientific advances which are likely to take place as a result of this research are substantial. Thus, if successful, this research will make its way to text-books, will broaden our general understanding of fundamental properties of matter, and be part of this exciting knowledge about the quantum states, which inspire imagination of new generation of students and encourage them to take physics degrees. The idea of light-matter particles flowing without resistance in a solid is likely to capture the imagination of young people and attract them to science. Additionally, the project will have a direct educational impact on Warwick Physics undergraduates (final year project students) by exposing them to challenging theoretical problems, relevant to the state-of-the-art experiments. It will motivate them to continue onto research degrees and teach them skills relevant also in other type of employment (analysing real data, comparing theoretical models and their predictions to experimental results). 2 class) It is well known that the conventional electronics will not be keeping up with the Moore's law, and that new technologies need to be considered for progress to continue. If electrons are replaced by photons new regimes of miniaturisation and speed can be reached. Semiconductor microcavities will be particularly suitable for such applications because polaritons in the superfluid regime can travel large distances without dissipation, and decoherence processes are largely reduced. At the same time, due to the very light effective mass of polaritons, such devices could potentially operate at room temperature. 3 class) PDRA and PhD positions, funded by this project, will provide an excellent career development and training opportunities by combining analytical and numerical research with the direct interaction with experiment. Outcomes of the proposed research will also benefit other researchers working in the area of polariton BEC and superfluidity, solid-state condensation and coherence in general, and related phenomena in ultra-cold atomic gases. What will be done to ensure they have the opportunity to benefit from this research? To ensure that these potential beneficiaries can benefit from the project, we will disseminate the results of our investigations widely at international and national conferences, workshops, summer schools, meetings and research visits. Also I will keep my web-page at the University of Warwick updated with the outcomes of our research in the form of uploaded talks, preprints, publications and summaries of current investigations. We expect that the numerical codes, developed within this project, can benefit other researchers: we will be open to either performing requested calculations or providing other scientists with a copy of the codes. We will also undertake some school outreach activities.