Efficient universal quantum circuits

Universal circuits can be viewed as general-purpose simulators for central classes ofcircuits and can be used to capture the computational power of the circuit class beingsimulated. We define and construct quantum universal circuits which are efficient andhave very little overhead in simulation. For depth we construct universal circuits whosedepth is the same order as the circuits being simulated. For size, there is a log factorblow-up in the universal circuits constructed here which is nearly optimal.