Cooperative quantum optics in dense thermal vapours

During this century there has been an explosion of research activity into manipulating individual quantum entities (single atoms, single ions, single photons...). This was the topic of the 2012 Nobel Prize in Physics, see http://www.nobelprize.org/nobel_prizes/physics/laureates/2012/. One of the great breakthroughs was that the mysterious quantum property of entanglement was identified not only as being at the heart of Einstein's "spooky action at a distance" but also as a resource for the emerging topic of quantum information processing. It is noteworthy that the fundamental theory and the technological breakthroughs advance in a symbiotic fashion. Specifically, there is a drive towards harnessing the powers of single quantum entities such as qubits in a quantum computer, which could yield computing devices with unprecedented power exploiting the exponential scale up of complexity in a quantum system.
In the experiments described in this proposal we will produce pairs of photons which are entangled, by using our understanding of the interactions among hot atoms in a thermal vapour. We will also exploit a phenomenon known as "heralding" where for entangled photon pairs the presence of one photon can be triggered, or "heralded", by the presence of the other photon at a detector. Entangled photon pairs and heralded single photons will find great utility in the burgeoning fields of quantum information processing and quantum cryptography, in addition to fundamental tests of quantum mechanics. There are many possible sources of such useful photons which are currently being studied; the advantages offered by our scheme include simplicity (such that scalability - building many identical copies of key components of the apparatus - is not an issue), and the characteristics of the photons (their frequency (colour) and bandwidth (purity of colour)) are ideally suited to interface with quantum memories and quantum repeaters (also based on atomic ensembles).

In addition to performing the research experiments we plan to disseminate our work further by incorporating a similar experiment into the teaching laboratories at our department, such that undergraduates can experiment with entangled photon pairs, and measure a violation of Bell's inequality. John Bell was a Physicist from Belfast who formulated a mathematical expression to encapsulate how "weird" quantum mechanics can be. A violation of Bell's inequality can not be explained by classical physics, but can be interpreted as spooky action at a distance. The results from our experiments will also be incorporated into talks for the public and schoolchildren given by the investigators.

We believe that the research outlined in this proposal will have impact through many routes, some short term and others long term:

1. Supplying highly-trained personnel
Impact will be achieved through the training of undergraduates, postgraduates and post doctoral research assistants. The researchers associated with the project will gain expertise in state-of-the-art laser techniques, photonics, computer modelling, computer interfacing, and use of analysis software, in addition to generic transferable-skills training.

2. Outreach activities
We have a strong record in our group of presenting our research to a wider community, which we will continue with this project. This will take the form of activities at science festivals, school visits, hosting school visits to the laboratory, and public lectures. This project hinges on exploiting some of the "weird" properties of quantum mechanics, specifically entangled photon pairs. These counter-intuitive results are ideal to include in activities designed to enthuse the up and coming generation of scientists.

3. Longer-term impact of knowledge generation
In the field of "Quantum Physics for New Quantum Technologies" a major issue is scalability; thus there is a drive for miniaturization of devices. In the context of this work this means using compact high-density vapour cells. The reduction in cell length is associated with a concomitant increase in atomic number density in order to maintain a workable optical depth, therefore our work on the impact of atom-atom interactions will be of broad interest. There already exist many devices which exploit miniaturized vapour cell technology, including frequency references, magnetometers, gyroscopes, frequency stabilization and atomic sensors for accelerometers and gravimeters. We will be mindful of opportunities to exploit any potential industrial applications of our research.
The financial sector is already exploiting quantum-information based protocols for secure communication using light transmitted through fiber optics. As we plan to deliver a single-photon source in this programme this will aid with development of improved sources. Future quantum technologies are also likely to use atomic ensembles in quantum memories and quantum repeaters. The fact that the photons produced by our single-photon source are well matched to resonance lines of atomic vapours means that our work has potential impact in this field.