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Excitonic transport in cold Rydberg gases

Periodic Reporting for period 1 - ExTRyG (Excitonic transport in cold Rydberg gases)

Reporting period: 2015-11-01 to 2017-10-31

"The aim of the Extryg project was to address fundamental scientific questions concerning mechanisms of energy transfer between molecules that rely on so-cllaed dipole-dipole interactions. It has long been known that molecules can exchange lumps or quanta of energy if they are sufficiently close. Each molecule acts like a tiny antenna or dipole, exchanging information with others nearby. The open question is to what extent quantum effects like superposition (""being in two places at once"") and entanglement play a role in these processes. There is some evidence that key biological processes such as photosynthesis might exploit these quantum properties to improve their efficiency. A related question is how we might replicate this behaviour in engineered systems.

To study this we proposed to create a model system in the laboratory consisting of chains of laser-cooled rubidium and strontium atoms. By putting the atoms in high-lying energy states known as Rydberg states, their dipole-dipole interactions can be blown up many orders of magnitude in both space and time. The aim was to deterministically place a single excitation in the chain and observe how it was transferred to its neighbours.

During the project we put in place all the key aspects of the model system, and observed the long-range interactions between the atoms. Several new and highly profitable research directions emerged at an early stage. The main conclusions were:

-using rubidium atoms, we showed that optical photons can be coupled in novel ways to the microwave excitation that propagate in the chain. As well as new readout methods for quantum effects, this provides a resource for microwave detection at the quantum level, and for the microwave control of quantum light.

-for Sr atoms, a new direction emerged whereby rather than exciting atoms to the Rydberg state, a laser is instead used to ""mix in"" the properties of the Rydberg state to lower-lying states that are long-lived. We showed that in this way Rydberg properties can be combined with laser cooling to millionths of a degree above absolute zero, opening a route to new types of transport experiment.

"For the first 12 months of the fellowship, the applicant worked on experiments using both rubidium and strontium experiments. Key results in this period include:

-the observation of ""contactless"" interactions between optical photons in spatially separated channels

-the demonstration of the a microwave controlled coherent quantum memory, where the storage and retrieval of single photons could be coherently controlled using an additional microwave field.

-the development of improved calculations of Sr Rydberg properties, which were used to perform state-of-the-art measurements of electric field induced systematic errors in a state-of-the-art atomic clock in collaboration with the National Physical Laboratory

From months 12-24, work focused on the Sr experiment, with work including the first demonstration of the continuous laser-cooling and trapping of atoms with Rydberg properties."
This project advanced the state-of-the-art in the following ways:

For the first time, a nonlinear optical effect was observed between photons that never overlap spatially and with no common optical medium. The interaction is mediated via long-range dipole-dipole interactions with a length scale many times larger than the optical wavelength. This will have important impact on future quantum technologies based on optical photons, through new approaches to single-photon gates and switches.

A second breakthrough was the combination for the first time of Rydberg properties, such as enhanced sensitivity to electric fields, with active laser cooling. This work is still at an early stage but has already had significant impact on other research groups.

Lastly, a new method for eliminating errors due to stray electric fields in optical atomic clocks - the most precise measurement devices ever made - was established. This work is already having impact at the frontier of precision measurement.
Photons propagating in spatially separate channels interact by being converted to Rydberg excitation