Cooperative light propagation and spectroscopy with cold atoms

Traditional analysis of light propagation in a medium is based on more than a century-old theories of electrodynamics of a continuous polarizable medium and its consequences such as standard optics. In cold and dense atomic gases such theories can qualitatively fail. This is because light mediates strong interactions between the atoms that establish correlations between the positions of individual atoms in the cloud. In this project we theoretically study light propagation in cold and dense atomic ensembles. The work will be done in a close collaboration with experimental groups who trap cold and dense atomic gases in laboratories. The goal is to utilize the strong light-mediated interactions in the design of better atomic systems for high-precision measurements. Improved detection techniques could, for instance, lead to more accurate time measurements, better satellite navigation and better lasers.

During the year of light 2015, it was highlighted in several occasions how light technology forms an essential part of modern society. In classical and quantum light technologies resonant emitters play an increasingly important role. The goal is to make the emitter systems smaller, denser and more coherent. This will dramatically increase the interactions between the emitters. Understanding these interactions is essential for the development and design of new optical devices.

Strong interactions can be achieved in cold atomic gases as a result of homogeneous resonance broadening, but the homogeneous broadening can also be obtained, e.g., with nano-emitters, quantum dots and metamaterials via high-quality fabrication when the resonance properties of individual emitters no longer vary significantly. The proposed project therefore also has broad importance to a wide range of optical systems, and we have previously found our studies of nanofabricated resonators and atoms mutually very beneficial, with atomic physics phenomena generating ideas for nanophotonics, and vice versa.

The applications of light-atom systems include quantum light-matter interfaces that are important for quantum information processing (entanglement generation, quantum memories, quantum networks, etc) and, e.g., laser technology, where cooperative effects in superradiant lasers may allow the development of very high frequency stability. Confinement of light in waveguides could potentially be utilized in the development of photonic devices with single photon switches, transistors and networks.

Quantum Technologies (QTs) in sensing are already outperforming conventional sensors in the measurements of gravity, rotation and magnetic fields. Accurate gravity sensors can be utilized, e.g., in oil exploration or in civil engineering. 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. Faraday interaction has many applications in magnetometry, telecommunications and laser technology. The best stability, accuracy and reproducibility of atomic clocks are currently achieved with neutral strontium atoms using optical transitions (at NIST). The experiments already are in a regime where the many-atom radiative dipole-dipole interactions are important. The importance of squeezed states for precision-measurements is also widely recognized. Spin squeezed states are important for quantum-enhanced interferometric applications, where the accuracy of the interferometer can exceed the standard quantum limit of classical interferometers. Optical spin squeezing is used in LIGO gravitational wave detection. These experiments are currently trying to improve the fundamental limit of the measurement precision that can be obtained with given resources by utilizing spin squeezing to achieve quantum-enhancement in the interferometric measurements.

The UK government has launched a QT Programme with a network of QT Hubs that illustrates how quantum physics is being transferred from scientific discoveries to real-world applications. 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 crucial. Although our proposal addresses fundamental research topics, it aims to develop concepts and understanding which will have a technological impact on a time span well beyond the one set for the QT Hubs. Also DSTL funds QT research with a goal to develop compact atomic clocks, gravity gradiometers and a precise navigation system from clocks, accelerometers and gyroscopes.

Understanding science forms a profound part of general cultural knowledge. The training of students and research assistants with advanced numerical and analytic problem-solving skills will also benefit many areas of society outside academia.