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702 whenever a whole number of wavelengths of the incident light fits into the resonator. Moreover, destructive interference occurs between light emerging from two coupled resonators. Early microresonators were simply aerosols containing particles of varying sizes, but modern fabrication procedures produce individual resonators with highly controllable resonance frequencies. These resonators come in various forms: ring shapes; disks or spheres, in which the light skims across the outer surface as sound does in a ‘whispering gallery’; or less obvious forms, such as defects in an otherwise perfect photonic crystal. Their small size makes them ideally suited to perform operations on light analogous to those performed by components of silicon chips on electrons, and numerous applications have been proposed for them. Several methods for combining EIT and optical microresonators to obtain EIT-like resonances in integrated optical systems were proposed in 2004, and a laboratory demonstration of such an effect was published last year. In that experiment, EIT-like transmission spectra were observed in two interacting microresonators in the form of glass spheres approximately 400 micrometres in diameter that supported resonances of the whisperinggallery type. Xu et al. take this advance a stage further. Using modern nanofabrication procedures, they manufactured a pair of micro-ring resonators coupled to parallel waveguides on a silicon-on-insulator substrate typical of an electronic chip. The refractive index of the silicon that made up the waveguides and the rings (3.45) was significantly greater than that of the silicon oxide that surrounded them on all sides (1.46). This meant that light could be confined in rings of a very small diameter through the process known as total internal reflection. EIT-like behaviour was demonstrated by measuring the transmission at different frequencies of a laser beam injected into the system. The authors constructed several such devices, where the distance between the micro-rings was varied from device to device, and thus the spectral location of the EIT-like transmission peak could be adjusted. So what good is all this? As the name indicates, EIT is primarily a way of rendering transparent an otherwise highly absorbing material, and so lends itself to applications such as the construction of long-distance optical transmission lines. The dispersion characteristic that results from EIT has already allowed ‘slow light’ propagation to be achieved at speeds just a small fraction of the normal speed of light. Optical ‘buffers’ to temporarily store light are an obvious application of such slow-light technology. Perhaps most importantly, EIT resonances can be much narrower than those of individual resonators, whether these are atoms or microresonators. As a resonance frequency can typically be measured only to an accuracy given by some fixed fraction of the resonance linewidth, narrowed linewidths could be useful for performing precise optical measurements of quantities such as magnetic field strength. The ability to synthesize such components on an optical chip suggested by Xu and colleagues’ innovation is a crucial step towards the development of such integrated optical devices. ■