One block which is laser emitter and another one named laser receiver. If placed on the same line horizontally or vertically, a laser appear between them (like the one emitted by beacon), if an entity or a player is passing through, the laser disappear and the laser receiver emit a redstone signal which one can be used to activate a command block, note block or whatever you want :D
I really think it's an awesome idea that can solve many things that I can't create currently like a lab, a security system a mob farm. Btw a second block called "powerful laser emitter" creating in this time a deadly laser with his receiver which kill the entity passing through the laser.
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