Rydberg crystals and supersolids

High-temperature superconductors are a technologically important example of a strongly correlated quantum system. They owe their exotic electronic properties to interactions between the electrons, which give rise to correlated behaviour.

Strongly correlated materials are very difficult to model they lie in between - the electrons interact, and so can't be described as individual particles, but the strong interactions also mean that we can't use a ``bulk'' description based on average properties either.

To bridge the gap between simple models and real materials, there is growing interest in simulating strongly correlated behaviour using laser cooled atoms, where the external and internal state of each atom can be completely controlled. Recently it has become possible to extend this control to the interactions, by exciting the outermost electron to a highly excited (or Rydberg state) using a laser pulse. This switches on a dipole-dipole interaction which is 12 orders of magnitude stronger than that between the ground state atoms. The interactions completely dominate the kinetic energy of the cold (5 microKelvin) atoms, and the system becomes strongly correlated.

In this proposal, we will exploit the unique properties of strontium Rydberg atoms to explore how these strong interactions lead to spatial correlations. In particular we will examine the interplay between these correlations and superfluidity.

The first challenge is to develop a technique for imaging the Rydberg atoms. To do this, we will develop a new kind of scanning microscopy that exploits the fact that strontium atoms have two valence electrons.
Next, we will use this new technique to observe how the interactions lead to the dynamical formation of ``Rydberg crystals'' where the Rydberg excitations form an ordered lattice.
Then, to combine these strong interactions with superfluidity, we will use a weak coupling to the Rydberg state to ``dress'' atoms in their ground state with a small amount of Rydberg character. By combining this ``Rydberg dressing'' with a Bose-Einstein condensate, we will be able to controllably introduce spatial correlations into a superfluid, providing a new laboratory for studying the physics of strongly correlated systems. Recent theoretical proposals have suggested that this could lead to the observation of a ``Rydberg supersolid'', where a crystalline spatial distribution can coexist with superfluid flow - similar to a phase predicted to exist in solid helium over 40 years ago.

In the immediate term, the UK economy will benefit from the supply of highly trained personnel. Two PG students and one PDRA will receive high quality scientific training and training in transferable skills. The skills they will acquire could be applied in the academic, defence and R&D sectors and beyond. This proposal will also drive the development of high-tech equipment, with the potential to benefit UK SMEs in e.g. photonics.

Local schools will benefit from targeted outreach activities undertaken during the proposal. We will develop our web pages and links to social networks to maximise the wider societal and cultural impact.

In the long term, significant economic benefit would arise from an improved understanding of strongly correlated materials, some of which (such as high-Tc superconductors) already have important technological applications. Researchers in this area, in both the condensed matter and atomic physics communities, benefit strongly from this proposal, which provides a new approach to studying strong correlations in a clean environment.

Economic benefits could also arise by applying the techniques developed here to the creation of large-scale entangled sates, with applications in precision measurement.