A phase-conjugate graviton beam is a type of graviton beam devised in the 2370s by Doctor Lenara Kahn of the Trill Science Ministry to seal the Bajoran wormhole without causing damaging to it or harming the Prophets. It was intended to be used by the crew of Deep Space 9 to prevent a Dominion invasion of the Alpha Quadrant.
Miles O'Brien and Jadzia Dax were able to rig the phase-conjugate beam by remodulating the stations deflector grid frequencies and projecting it through its graviton emitters. (DS9: "In Purgatory's Shadow")
Unfortunately when executed, it was discovered that the emitters were sabotaged and, as a result, the phase-conjugate graviton beam had the exact opposite effect on the wormhole by making the wormhole's spatial matrix more stable. So stable, in fact, that not even trilithium explosives could destroy it.