Collective strong coupling of light and matter with cold atoms in a ring resonator

Optical cavities have played a central role in atomic, molecular, and optical physics since the development of the laser. Within such cavities, increased field intensity gives access to a variety of nonlinear optical phenomena, and increased interaction length allows detection of even trace amounts of gases. For large numbers of atoms in small enough cavities, the system enters a regime of collective strong coupling. In this case the atoms and cavity field behave cooperatively, giving rise to a rich nonlinear dynamics.

The simplest resonator design comprises a pair of parallel mirrors, but ring geometries involving three or more mirrors are used in a number of important applications. Ring laser gyroscopes form the basis of rotation sensing in navigation systems, and passive ring resonators have been used to measure optical activity in chiral liquids. Here we propose experiments with ultracold gases in an optical ring cavity, operating in the regime of collective strong coupling of atoms and light. This system has applications to laser cooling of molecules, self-organisation, quantum simulation, and precision metrology.

The first phase of the experiment aims to understand cavity-enhanced cooling as a way to generalise laser cooling for use with molecules. Preliminary work in this field suggests that spatial self-organisation of atoms plays a crucial role in enhancing the coherent scattering into the cavity mode. Although the majority of theoretical work on this topic has been carried out within the ring geometry, the few existing cavity cooling experiments have utilised standing wave resonators. We will therefore be perfectly placed to investigate proposed advantages of the ring geometry, related to translational invariance and the coherent exchange of momentum between degenerate cavity eigenmodes.

In the second phase of the experiment we will perform direct quantum simulation of condensed matter systems. There has been phenomenal success using quantum gases in optical lattice potentials to mimic a variety of solid-state crystalline systems. However one ingredient missing in most of this work is any atomic backaction onto the lattice potential. In typical experiments, the underlying optical lattice is unaffected by the position or motion of the atoms, in contrast with solids and with optical lattices in resonators. Recent experiments with Bose-Einstein condensates in high-finesse cavities have begun to study cavity optomechanics, and a quantum phase transition to a supersolid has been demonstrated. We will exploit the ring geometry to study systems with moving lattices in a regime where atom density modifies the optical potential. This will provide a key testing ground for condensed matter systems where lattice excitations play an important role.

Our programme includes three essential Pathways to Impact. These may be characterised as outreach and public communication, collaboration, and application and exploitation. Each is summarised in turn below.

I. Communication and Outreach
We would like to engage the public with our work. However the complex nature of the topic makes this difficult, and often leads to oversimplification and misinformation. Our impact plan therefore begins with public communication training for the Principle Investigator. This training is aimed at providing the skills necessary to engage the public as well as the wider scientific community. This training will then be exploited in the production of short videos describing our work for non-specialist audiences. These short clips will be made freely available on the MUARC web site, as well as on popular internet sites such as YouTube. More technical documents, such as machine drawings, circuit schematics, and users' guides will also be made available on the MUARC site. These are intended for scientists within and outside our field as well as advanced hobbyists, who may wish to replicate and even improve on various home-built devices used in our work. Finally the web site will be regularly updated with experimental results. Significant news events will be coordinated with the University of Birmingham press department. We will also disseminate our results through the usual peer-reviewed publications and conference presentations. For example, we are preparing a pair of technical manuscripts, and our postgraduate students have presented posters at two international conferences.

II. Collaboration
Our plan for academic collaboration is centred on researchers in our immediate professional circle, as described in the Academic Benificiaries section of this proposal. However, we believe there are many ways our research will have a broader scientific impact. For example, our work on cavity-enhanced cooling may lead to cooling and trapping of objects which are larger and more complex than simple atoms or molecules, such as nano-mechanical resonators, biological samples, and dielectric microspheres. Furthermore, phenomena related to self-assembly may shed light on the nascent field of optical binding. As another example, our work with driven optical lattices is expected to elucidate systems of electrons in semiconductor superlattices, which can be formally similar. These systems have recently been demonstrated to be useful for amplification in terahertz resonators, with applications to high-bandwidth communication technologies.

II. Application and Exploitation
Our work addresses emerging fields, and therefore direct application is expected on a longer timescale than is relevant to the First Grant scheme. However, we will evaluate a number of technological applications of our work. As an example in addition to those mentioned above, we will assess the utility of our system for inertial sensing. The ability to measure gravity or gravity gradients with high sensitivity has applications from archaeology to oil and mineral exploration. The use of atom interferometers for such purposes is already the subject of existing work in the Birmingham Cold Atoms group, as evidenced by collaborations such as iSense and GGTop. There is therefore considerable local expertise both in the science of sensing as well as the technology. Our current collaboration with theorists at the University of Nottingham who are already working with Chelmsford-based e2V will help strengthen this foundation.