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

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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.