Giant optical nonlinearity and photon production using single molecules coupled to a waveguide

We propose to build a light circuit , similar to an electronic circuit, but operated with single molecules and guided light beams instead of transistors and electricity. Densely packaged on a microchip, these circuits could be the basis of a future generation of extremely powerful computers. A key problem is that one light beam barely affects another when they cross and this limits the capability of optical circuits to compute. Even special nonlinear optical materials cannot induce enough of an interaction. Ten years ago, theorists discovered a way to perform simple quantum computations without interactions between the beams provided the photons are identical and therefore indistinguishable. Recently this method was demonstrated on a small silicon chip. However, the lack of interactions between photons still makes it impossible to scale this up to complex calculations and therefore a new, strongly nonlinear material is still required in practice. In this proposal we show how the problem may be solved by squeezing the photons into very tiny waveguides on the chip and placing single organic dye molecules within these waveguides. At a temperature below 2K, each molecule has a large enough interaction with the light to make the passage of one photon very sensitive to the presence of a second photon. Thus the molecule causes a light-light interaction that will solve the central practical problem for expanding quantum computation on an optical chip. This same approach offers a second important practical advance towards scaling up. At present, the source of identical photons - the raw material of the computation - is a nonlinear crystal external to the chip, which produces pairs of photons at random. One of the pair is detected in order to herald the arrival of the other photon, which is sent to the chip. It is difficult with this approach to obtain several photons at once on the chip. In this proposal we show how a molecule attached to a waveguide can serve as an on-board source of identical single photons, which can be dispensed as needed. There can be many such integrated sources on the same chip, producing many photons simultaneously for more complex calculations. Our project is to optimise the design of the waveguide and the placement of the molecules in order to achieve the best light-light coupling and the best on-chip photon sources. We will use this knowledge to make circuits that are suitable building blocks for complex applications.

THE BENEFICIARIES AND HOW THEY WILL BENEFIT Commercial private sector Organic dye molecules are multi-functional materials widely used in the industrial sector. They are used for solar cells, displays, electronic active devices, biological markers, optical gain materials and nano-scale light sources. Since the development and the mass production of organic dyes are relatively straightforward, the possibility for cross-disciplinary invention and commercialization is quite high. The proposed research project will provide detailed new information on the interaction of the single dye molecules with very faint light via photonic structures, and this is closely related to the photonic application of these dyes. Since the devices we develop will operate at cryogenic temperatures, they are unlikely to have immediate commercial impact. However, the insights we obtain will provide important clues for the R&D of sensors and active devices that can operate with only a few quanta at room temperature. Such sensors have a good prospect of leading to commercial applications. Wider public in general Photons are superior to electrons for high-rate low-loss data transmission: the fastest internet cables operate with light. As the data rate increases, optical communication is progressing not only in long-distance applications but also short-range interconnects. Ultimately, one can envisage chip-scale optical interconnects and quantum optical circuits replacing their electronic counterparts. Our research aims to improve both the scalability and the miniaturisation of quantum-optical circuitry. These are both key steps towards making quantum information processing a practical and effective part of mainstream technology, just as they were for conventional electronics. PLANS TO ENSURE BENEFITS Communication and engagement Our results will be disseminated primarily through the CCM website, refereed journals and conferences. In addition our past results have been reported by the popular media, such as the BBC, New Scientist, Science et Vie, Optics and Photonics News, and MIT Technology Review. Since our proposal concerns the two most important next steps towards practical devices - miniaturising and scaling up - the popular interest is likely to intensify and we will take every opportunity to encourage that. Collaboration We interact with the London Centre for Nanotechnology, where most of our fabrication will be done. Imperial also has a formal collaborative link with NPL, through which we have joint students and frequent discussions about applications of our ideas. We interact with the Centre for Integrated Photonics (CIP), a company that has helped us build complex atom chips in the past. We also plan to begin a close collaboration with the group of J. O'Brien at Bristol, who is a world leader in the optical processing of quantum information. Exploitation and application We will exploit and protect the outputs from our research by regular consultation with Imperial Innovations, a company founded to protect and maximise commercial opportunities arising from research at Imperial College. We will also interact with Business Development, the College's interface with industry, and with Imperial Consultants Limited which provides academic expertise from Imperial to serve the needs of industry. Capability The impact activities will be carried out by the PI through collaboration with LCN, NPL and Bristol, interactions with the rest of the scientific community, scientific publications, popular articles, public lectures, and interviews with media and press. Named researcher Jaesuk Hwang plans to disseminate our research through the CCM website and through talks at local schools.