This is an introduction to the quantum computing mod for Minecraft. It is a simulator of real logical quantum circuits implemented in Minecraft (a virtual 3d world). The mod is free (and sources are included), although Minecraft itself costs money.
The wiki can be found here: http://qcmod.wikia.com/
The official Minecraft forum thread can be found here: http://www.minecraftforum.net/topic/1946878-quantum-circuits-computers-redstone-compatible/