Quantum information science addresses how uniquely quantum mechanical phenomena such as superposition and entanglement can enhance communication, information processing, and precision measurement. Photons are appealing for their low-noise, light-speed transmission and ease of manipulation using conventional optical components. However, it has been very difficult to achieve the necessary two-qubit operations since the physical interaction between photons is much too small. In a breakthrough, Knill, Laflamme, and Milburn (KLM) showed that effective nonlinear interactions can be achieved using only linear optical elements, auxiliary photons, and measurement. Inspired by the KLM approach, a number of quantum logic gates using heralded photons and event postselection have been proposed and demonstrated. Furthermore, optical quantum circuits combining these gates have been demonstrated . We experimentally demonstrate a two photon quantum gate (controlled-NOT gate) based on the KLM approach. This result confirms the first step in the original KLM “recipe” for all-optical quantum computation, and should be useful for on-demand entanglement generation and purification. Our other recent progress on linear optics quantum circuit will also be introduced.