# Ultracold Gases

Collective quantum behaviours in macroscopic systems have revolutionised the modern world through the discovery of such fundamental phenomena as superconductivity and magnetism. The premise of our research program, outlined below, seeks to understand complex quantum condensed matter phases at the most fundamental level through the theoretical investigation of ultracold gases of neutral atoms.

A full list of the groups publications can be found here.

### Dynamical properties of dilute gas Bose-Einstein condensates

Since the first the first observation of the condensation of a dilute gas of bosonic atoms (1995) there has been an explosion of experimental and theoretical interest in this field. In most cases the interactions between the atoms can be modeled as a short range anisotropic interaction. Our theoretical work focuses on advancing our understanding of the dynamical properties of this fundamental state of matter. In particular, recent work has focused on the adiabatic transport properties of BECs in multi-well systems (an example of which is shown in the figure), the formation of vortex lattices, the Bragg reflection of BECs in optical lattices and the quantum reflection of BECs off silicon surfaces.

### Properties of dilute gas dipolar Bose-Einstein condensates

One of the key recent developments in ultracold atom research has been the formation of BECs with significant dipole-dipole interactions. This long-range anisotropic interaction, the strongest allowed interaction between two neutral particles, significantly alters the fundamental properties of this highly correlated state of matter. Our current work focuses on the simulation of condensed matter phases with long-range interactions using dipolar BECs.

### Fundamental properties of ultracold dipolar Fermi gases

In addition, to the study of ultracold Bose gases it is also possible to produce ultracold Fermi gases with significant dipolar interactions. As such we are currently developing new theoretical approaches to predict the properties of such systems, with particular focus on the formation of superfluid and quantum Hall phases.

## Recent Research Highlights

### Dynamical instabilities in rotating dilute gas dipolar Bose-Einstein condensates.

R.M.W. van Bijnen et al., Physical Review Letters 2007

Under rotation, instabilities in the dynamics of a BEC can lead to the formation of a vortex lattice. We showed that dipolar interactions can significantly alter the regimes for which rotation can become unstable and as such at what rotation frequencies one would expect a vortex lattice to form.

### Quantum Reflection of dilute gas Bose-Einstein condensates.

R.G. Scott et al., Physical Review Letters 2005

It is well known that a quantum particle can quantum reflect off a potential drop. As such it is possible, via the Casimir-Polder potential, to quantum reflect a BEC off a silicon surface. Initial experiments, by the group of Prof. W. Ketterle (MIT, USA), showed anomalously low reflection probabilities observed for a BEC quantum reflecting off a silicon surface. Our work showed that this anomalous result is directly related to the strength of the short range inter-particle interactions in the BEC.

### Bragg reflection of dilute gas Bose-Einstein condensates.

R.G. Scott et al., Physical Review Letters 2003

One of the key developments of our modern understanding of materials is Bragg reflection. Our work, quantitatively describes the breakdown of BECs undergoing multiple Bragg reflections in a periodic optical lattice. We showed that the formation of solitons and vortices, in the BEC, at the point of Bragg reflection damp the Bloch oscillation amplitude. Our predictions were subsequently experimentally test by the group of Prof. E. Arimondo (Pisa, Italy).

# Emergent Collective Behaviour in Biological Systems

An emergent behaviour can appear when a number of simple entities interact in an environment. Within the context of physics simple interactions can lead to such emergent behaviour as superconductivity, superfluidity and magnetism. However, in our everyday life we surrounded by many examples of emergent behaviour in biological systems, such as the flocking of birds, traffic flow and the behaviour of crowds. The primary aim of our research is to provide minimal models to describe such behaviour qualitatively. This enables us to suggest the dominant interactions underpinning such behaviour as flocking.

### Flocking phenomenon in biological systems

The concept of spontaneous flocking is familiar to us all. The world around us is teeming with examples of this emergent behaviour, from a flock of birds to a school of fish, a herd of wildebeest to a swarm of locusts. In physics, a flock can be defined as the coherent motion of a group of self-propelled particles emerging from a single set of interactions between the constituents of that group. Our research focuses on developing minimal models to describe the known properties of biological flocks. In particular, our recent work has focused on the difference between hard and soft core repulsion between the constituents of the flock in a system with open boundary conditions.