Molecular Microcavity Photon Source

Photons - quantum particles of light - have an important role to play in quantum science and technology. Information is easily stored in them. They are readily manipulated, conveniently transported and not much disturbed by their surroundings. The photons from a strongly attenuated laser beam arrive one at a time, but this type of source is not practical to use in most quantum applications because we do not know when the photons will come. That problem is normally addressed by passing the laser light through a crystal which occasionally, and randomly, splits a photon into two. When a photon is needed, we wait for one of the two to be detected (and destroyed), then we know that the other photon is ready for use in our quantum application. The communication of secret messages is one famous application of this approach.

This works well if only one or two photons are needed at a time, but many applications require several identical photons (or even dozens of them), either simultaneously or with accurate time delays. Unfortunately, the random nature of the splitting does not allow that. We need a source that can deliver photons rapidly and reliably whenever they are needed for applications such as quantum information processing, or the simulation of complex quantum systems. The photons should be identical so that they can exhibit quantum interference - i.e. interference of one particle with another - which is the essential feature of quantum particles giving rise to the famous power of quantum mechanics. Such a source does not yet exist.

Here we propose to build a source that produces identical photons, rapidly and on demand. Our method is to embed individual molecules of an organic dye (known as DBT) inside a small optical resonator, coupled to an optical fibre. When a photon is required, the molecule is excited by a bright pulse of light, after which it emits a photon. The resonator forces the photon to be emitted at a specific frequency and into the specific direction that couples to the fibre, thereby making the photons indistinguishable from each other. This is sometimes called the Purcell effect after Ed Purcell, who first noticed it in the context of magnetic resonance. Other researchers have tried to use the Purcell effect to make a good photon source, with various emitters such as quantum dots or colour centres in a crystal, but these have not been able to produce a high yield of identical photons because of the inadequate optical properties of the emitter. The novel aspect of our proposal is the DBT molecule, which has near ideal properties for this application, as we have recently shown. By incorporating this molecule into one of our cavities, we expect to produce a photon essentially every time we ask for one, and we can expect these photons to be identical. We also plan to tune the molecule by applying an electric field inside the cavity. Our design will allow us to stack up several miniature photon sources and choose whether the photons are identical or are tuned to an array of different frequencies. These can then be used to make complex quantum states of light suitable for a range of applications in quantum technology where suitable sources are currently lacking.

Beyond the immediate application to quantum information processing, our DBT molecular light source provides a promising new element for nano-optics and nano-electronics more generally. With a little imagination, we can see that these molecules may one day be enhanced by the addition of chemical groups to turn them into custom sensors for specific molecules, with sensitive readout by the light, or perhaps directly through an organic semiconductor.

In short, we will use DBT molecules in cavities to make identical photons on demand, satisfying an immediate need of quantum technology. By working to utilise this new type of quantum emitter, we expect to make a fundamental advance in the science and technology of nano-optics and nano-electronics.

SATISFYING NATIONAL STRATEGIC NEED
Over the last several years we have developed methods to grow very thin, pure anthracene crystals, and to dope them with single organic dye molecules of DBT. We have demonstrated that these have exceptionally good optical properties for acting as single photon quantum emitters and show promise for incorporation into a range of photonic and electronic nanostructures. This gives us a world-leading opportunity to develop the use of DBT molecules as quantum emitters. The insertion of single functional molecules into photonic and electronic nanostructures will contribute to the advance of quantum devices - an important strategic capability that the UK can and should develop. Two immediate impacts of our photon source will be in quantum communication and quantum simulation. Quantum communication promises completely secure communication links which can offer huge advantages not only to large businesses but also to everyday people in securing online transactions. Quantum simulation is needed to understand the behaviour of complex quantum systems, e.g., the physics of low-dimensional materials or high-temperature superconductors - perhaps even the biochemistry of new medicines.

IMPACT ON THE FIELD OF NANOSCALE DEVICES
Our research has the potential to bring a significant new element into the important field of building nanoscale devices. The coupled molecule-cavity is a new solid-state system and we expect our research to advance the understanding of this system and to stimulate new ideas about incorporating single molecules into other optical and electronic nano-devices. In the longer term we see enormous promise for sensing using functionalised molecules coupled to optical nano-structures. We expect therefore that our research will have impact on the international academic community working on the physics and possible applications of aromatic dye molecules in single molecule optics, microscopy, nano-photonics and nano-electronics.

IMPACT ON QUANTUM TECHNOLOGY AND FUTURE UK ECONOMIC SUCESS
It is recognised throughout the world that quantum technology is a strategically important area for research and investment. The UK has been quick to act on this with a £270M Quantum Technologies Programme to develop prototypes, a supply chain and a commercial market. In order to propel this commercial activity forward in the future, it will be essential to press on with basic science research in the UK so that that key advances are ready to feed into the next generation of UK quantum devices. Our research project will focus on developing a fast, tuneable, source of photons, produced reliably on demand in an architecture that is readily scaled to many sources. The international community of researchers working to exploit quantum science and technology urgently needs that for a variety of applications such as quantum information processing, quantum communication, quantum simulation and quantum sensing. The proposed research will therefore contribute significantly to the health and future capability of quantum science and technology by delivering such a source. That will give us a unique, world-leading capability, with potential for contributing to the UK economy through commercialisation.

We will ensure maximum impact by publishing our findings in high-quality journals, by presenting our research at the leading international conferences, and by developing our personal interactions with leading groups throughout the world in these fields. We will maintain close ongoing interactions with the UK Quantum Hubs to ensure that our photon source is as useful as possible in satisfying the needs of quantum technology.