We study the interaction of light with matter.
This is about as fundamental a problem as you can get.
More specifically, our interest is in the applications of strongly correlated quantum light to reach new technological potentials, such as quantum spectroscopy, quantum sensing, better or even new types of light sources, and as a feed for quantum gates in a semiconductor devices (microcavities).
A recent line of research that essentially starts with our group here at Wolverhampton is the use of polaritons as qubits, a concept that we denote as ȹ (read "qp"). One polariton is basically a strongly-interacting photon, which has been a long-sought element for quantum-optical information processing. In a recent breakthrough in collaboration with the groups of D. Sanvitto, F. Sciarrino, L. Marrucci & P. Mataloni, we have demonstrated the first ȹ. We are currently trying to exploit these newcomers to make them break one of the most burning problem of contemporary science: the building of a quantum computer.
More details, at an introductory level, below! If you have read enough already to be sufficiently excited, join us! We need talented people with a taste for building .
The theory of frequency-resolved N-photon correlations
In 2012, we have developed a theory  to compute correlations between spectrally-filtered photons (computing correlations between photons is basically what "quantum optics" is about; this is a full-field of its own. Our input was to extend this quantity to photons of determined energies, or frequency, which is the same thing in quantum physics).
Roughly, there is three ways in which you can correlate photons: you can make them clutter together (this is called "bunching"), you can make them avoid each-other ("antibunching") or your can have them be uncorrelated, that is, be independent of each others. The latter case is an important one, it describes "coherent light", i.e., that of a laser. In this case, light is like a wave, rather than like a collection of particles. So if you insist in seeing the particles of this wave, what you see is that they are not correlated. This neat understanding of how photons (particles) are linked to waves won the Nobel prize in 2005. You can view a short video explaining these concepts in more details though still in simple terms in this video abstract that we did for the New Journal of Physics.
This theory is a breakthrough. It solved many problems at once: it allows us to compute such correlations...
- without approximations (previous cases were making approximations, sometimes drastic ones),
- at all times (rather than zero-delay only),
- for any number of photons (rather than only two),
- and for any quantum system (rather than the simplest possible cases: the harmonic oscillator and a two-level system).
The technique is so powerful, that it allowed us to take one of the basic problem of light-matter interactions, known as resonance fluorescence (driving a system at the same energy that it emits), and introduce a new type of observable: the full landscape of all-photon correlations, spanning over the entire set of frequencies.
The beautiful structure that we observed  is here reproduced on the bottom left, with bunching in red, antibunching in blue and white for uncorrelated photons. The structure has been measured by the solid state quantum optics group of Andreas Muller in Florida, and is shown as the bottom right part.
The juxtaposition of these two images is one of the great joy of theoretical physicists: the consecration of a prediction that is confirmed by its direct experimental observation. The red areas correspond to "photon bunching", a tendency of photons to gather together, akin to the phenomenon that sees London busses clutter:
There are various reasons why photons would tend to get closer together. One of them, the most fundamental, is their bosonic character, meaning that they are particles bearing the character of waves with, as one important consequence, a tendency to display constructive interferences and pile-up together (in constrast, fermionic particles, like electrons, bear the character of matter and avoid or repulse each other). Another reason, that we uncovered with our theory of frequency-resolved correlations, is due to "leapfrog processes"