SPACE - How Powerful Would A Laser Need To Be In Order To Send A Signal To Another Star System?

Theoretically, any laser could be used, because a single photon could travel the distance unimpeded. But a single photon is difficult to detect. So, the real limit is based on the sensitivity of the receiver in the other star system. For example, in just the human eye, 9 photons hitting a rod/cone cell in the retina in under 0.1 seconds is enough for the brain to take notice.

http://www.forbes.com/sites/quora/2013/09/04/how-powerful-would-a-lase