Recent progress in quantum and nonlinear optical lithography

We review the status of the field of quantum lithography, that is, the use of quantum-mechanical effects to write lithographic features with resolution finer than that achievable according to the Rayleigh criterion. In particular, we first review the original quantum lithography proposal by Boto et al., and we then describe the status of research aimed at realizing this process. Several proposals have been put forward in recent years for methods to increase the spatial resolution of lithographic systems. These methods are of enormous potential practical importance because, for instance, they could be used to increase the density of electronic gates on a silicon chip. Several years ago Boto et al. [1] proposed to use quantum effects to increase the resolution of the lithographic process. They showed theoretically that a resolution N times greater than the Rayleigh resolution limit [2] could be achieved by making use of N entangled photons in the incident quantum state. This idea is illustrated in figure 1 for the case N 2. Here a laser beam falls on to a nonlinear mixing crystal in which parametric downconversion occurs, producing two entangled photons. This photo