Cooperative quantum optics in dense thermal vapours

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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.