The quantum mirror was among the artifacts found on P3R-233. It may be made of naquadah, and touching it allows entrance to any number of alternate realities which can be randomly selected using a remote control device. According to quantum theory, for every possible universe there are infinite variations diverging at every choice we make like forks in the road. However, the remote control device for the quantum mirror does not dial up a specific reality. If it is left on, it will stay connected to the reality to which it was set, but if disconnected, one must randomly search to find the specific reality again.
Daniel was the first to enter the mirror during the mission to P3R-233, and he traveled to a reality in which Earth was destroyed by the Goa'uld. The mirror was then brought back to Earth and kept in a high security lock-up at Area 51 for study, until Carter and Kawalsky from a parallel universe used the mirror to enter our reality. Due to entropic cascade failure, they were unable to remain, and during the return mission through the mirror to save their world from the Goa'uld, the remote control device was left behind in the alternate reality. Afterward, at Hammond's order, the quantum mirror was destroyed for security reasons.
Cross Reference: Alternate Reality - P3R-233, Alternate Reality - Quantum Mirror, Area 51, Destroyers, Entropic Cascade Failure, Charles Kawalsky, P3R-233
Episode Reference: There But For the Grace of God, Point of View