A quantum singularity is min-black hole that connects different worlds and parallel dimensions. People in different universes can travel between the universes through a quantum singularity.
There are two known ways to open a quantum singularity. The first is to fire a phased polaron cannon simultaneously from both worlds, from the same location and into the same black hole.
As only a single phased polaron cannon exists in the world, this is generally difficult. The second way is to create a Quantum Singularity Machine, which is designed to duplicate the effects of the first way.
Both ways were used in the course of "The Palindrome Reversal Palindrome", the first by Wendy Watson and the Middleman, the second by the good and evil versions of Ivan Avi.