A phase-conjugate graviton beam was a graviton beam that was intended to close the Bajoran wormhole without damaging it or harming the Prophets. The beam was devised by Lenara Kahn of the Trill Science Ministry.
In 2373, the crew of Deep Space 9 planned to use a phase-conjugate graviton beam to seal the wormhole after they discovered that a Dominion fleet was preparing to invade the Alpha Quadrant from the Gamma Quadrant. Deep Space 9 emitted the beam, but sabotage to the graviton emitters by a Changeling posing as Julian Bashir caused the spatial matrix of the wormhole to strengthen, preventing it from ever being closed. (DS9 episodes: "In Purgatory's Shadow", "By Inferno's Light")
In 2374, William T. Riker suggested that the crew of the USS Enterprise use a phase-conjugate graviton beam to close a wormhole created by Lem Faal that the entity 0 was using to enter the galaxy. (TNG - The Q Continuum novel: Q-Strike)