Harnessing quantum optical phenomena in cold atomic ensembles

Light can mediate strong interactions between cold and dense atomic ensembles, such that the atoms respond to light cooperatively. Some of the recent findings in experimental laboratories and numerical simulations for these systems have been incompatible with the textbook wisdom of standard optics in a dielectric medium. In this programme we want to explain some of these failings and also to develop a novel platform for utilizing these strong interactions in the manipulation of light. Individual quantum particles have now been accurately controlled in experiments already for quite some time. However, technologically important many-particle systems have so far been beyond such control. The goal of the programme is to improve the control and manipulation of atomic ensembles and to guide experimentalists to develop better next generation optical devices and technologies based on quantum physics.

Modern society relies on light-based technologies from data storage to telecommunications and healthcare. The control, manipulation and detection of atomic systems, ions, molecules, solid-state resonators, nano-emitters, quantum dots, metamaterials, superconducting microwave systems, surface plasmons and photonic crystals by electromagnetic fields are rapidly progressing. These systems are composed of resonant emitters and the goal is to make them smaller, denser and more coherent, resulting in collective and cooperative phenomena. Cooperative response can be achieved in media composed of "artificial" atoms (quantum dots, metamaterials, etc) via high-quality fabrication when broad distributions of resonances no longer mask light-mediated interactions, such that the systems behave more similarly to those composed of cold atoms. Although our project is focussed on cold atom ensembles, it therefore also has broad importance to a wide range of other media and to the development of optical devices in these systems.

Resonant emitters appear in entanglement generation, quantum memories, networks and communication in quantum information processing, in quantum sensing and metrology and, e.g., in laser technology, where cooperative effects in superradiant lasers may allow the development of very high frequency stability. Cooperative phenomena could also be useful in understanding and exploiting resonant energy transfer that is important in chemical and biological processes.

The UK government has initiated the UK National Quantum Technologies (QTs) Programme with a network of QT Hubs that transfers quantum physics from fundamental research to commercial products and services. The Hubs bring together physicists, engineers, industry and end-users. From sensing to metrology and quantum information processing, the interaction of light with atoms is a key element. QTs are already outperforming conventional sensors in the measurements of gravity, rotation and magnetic fields. The potential applications the QT Hub Sensors and Metrology include medical imaging, brain research, oil exploration, civil engineering (detecting groundwater, pipes, holes, leaks, etc.) and archaeology. Atomic clocks are essential, e.g., in high precision measurements and satellite navigation, while accelerometers are needed for navigation without satellites and could in the future revolutionize vehicle transport. The goal of the Hub is to engage with industry to develop small, light and cheap quantum devices. Although our proposal addresses fundamental research topics that are separate from the present QT Hubs, it aims to develop concepts and understanding that will have a long-term future technological impact. QT plays an important role also at DSTL where the development includes compact atomic clocks, gravity gradiometers and a precise navigation system from clocks, accelerometers and gyroscopes.

The proposed cooperative dispersion band engineering can have far-reaching future impacts. Enhancing the optical properties and functionalities of atomic medium to entirely new regimes, such as negative refraction and strong magnetic excitation of atoms by light, can open new gateways for utilizing atomic ensembles as optical devices. Previously, such regimes have only been attained in artificial media. Importantly, atoms do not absorb light that is turned into heat, and their purity and long coherence times can provide unprecedented opportunities in the development of applications, such as diffraction-free lensing that could revolutionize imaging. The pristine single-atom control and strongly-coupled 2D lattices could provide accurate wavefront shaping of light and effectively flat ultra-thin quantum optical components or a quantum photonic crystal. This could then act also as a hologram. Here the characteristic length scale is determined by the cooperative interactions between the atoms, allowing subwavelength manipulation of photons and near-field control.