Quantum Coherence Phenomena in Solid State and Atomic Condensates

One of the most exciting aspects of conducting research in physics, and very often the basis of unexpected discoveries, is the possibility of bringing together two apparently very different fields of research. First principles and physical mechanisms in some cases, and methodologies and techniques in others, surprisingly can operate and be applied to a different domain, enriching otherwise separate communities. One can refer to this as the `cross-fertilisation process'. The solid state and atomic physics communities have recently developed strong connections and common interests. In the last few years, both fields have received vigorous new stimuli, mainly due to the extensive experimental effort invested and the exceptional progress achieved in these areas. United by the common themes of `macroscopic quantum coherence', `condensation' and `superfluidity', my research project seeks to consolidate and develop the cross-fertilisation between these two fields.In the microscopic world, such as one finds at the atomic or molecular level, the familiar laws of Newtonian classical physics give way to those of quantum mechanics. Here, particles exhibit wave-like properties, leading for instance to interference phenomena. Although quantum effects hardly affect the macroscopic world of our everyday experience, under particular circumstances a large number of quantum particles can become locked together and `condense' into a single quantum state, giving rise to macroscopically detectable effects. This can happen when the temperature is so low that wavefunctions of individual particles begin to overlap, interfere and eventually behave identically as a whole, giving rise to a `macroscopic quantum coherent' phase.The mechanism of condensation depends strongly on the type of particles involved. `Bosons' are gregarious and, at low temperatures, have the tendency to occupy the same low energy state -- the Bose-Einstein condensation. In contrast, `fermions' have solitary properties and cannot occupy the same quantum state -- the Pauli exclusion principle. This lonely quantum behaviour of fermions underlies many of the characteristic properties of matter, ranging from the existence of the periodic table of the elements to the stability of neutron stars. However, an attractive interaction between fermionic particles can cause them to pair, generating composite bosonic molecules. As such, their characteristics change and, now showing gregarious properties, they may condense. Both bosonic or paired fermionic condensed phases share the characteristic of complete absence of viscosity, where particles flow without friction, a property which takes the name of `superfluidity'.Two distinct research areas are covered by my project, where the phenomena described above develop. From one side, in semiconductor structures such as quantum wells and microcavities, it is possible to generate composite bosonic particles known as excitons and to manipulate their properties and interactions with the help of the light emitted by lasers. From the other side, clouds of fermionic atoms have been trapped in magnetic and optical potentials, and cooled down to extremely low temperatures. Notably, it has been possible to manipulate and control the interaction properties between the atoms by means of an external magnetic field. The possibility of generating new phases of matter has inaugurated a new era of atomic and solid state physics. After the solid-liquid-gas phases, superfluidity in liquids and superconductivity in solids, the realisation of a superfluid phase of fermionic atoms or excitonic particles represents one of the new frontiers in physics. The main goal of my research project involves the study of the properties of both systems connected with the emergence of condensation, macroscopic quantum coherence and in particular with the identification of signatures which may allow experiments to distinguish between the normal and superfluid phase.