The term "laser harp" and the first working laser harp were invented and made by Geoffrey Rose in 1976.The first model was constructed with a matrix of 5 X 5 laser beams in an octagonal frame. It has subsequently been used in a number of different designs, including a MIDI version invented by Philippe Guerre, and a recent one created by Yan Terrien. They have also been used in public art installations such as those created by Jen Lewin on display at Lincoln Center in 2000 and Burning Man 2005.
A laser harp is generally built using a single laser, splitting its beam into an array of beams in parallel or fan arrangement. When a beam is blocked, this is detected by a photodiode or photoresistor, which is connected to electronics activating the relevant note. Playing the actual sound is usually handled by connecting the laser harp to a synthesizer, sampler or computer.
Here is a video of Jean Michel Jarre performing his Rendez Vous 2 on a laser harp. And if you want to build one yourself - here is a guide. And watch your eyes ;)