The Tachyon Ring is a device invented by Adrian Veidt.
The Tachyon Ring was invented by Adrian Veidt in 1979; intended as a contingency plan to use against Doctor Manhattan as part of his master plan to "save the world". Veidt called the ring "Plan A" (the 'A' standing for amnesia). However, he never used it because he would have had to find a way to force it right into Manhattan's head, which would be impossible unless he allowed it to happen.
On December 13, 2009, Manhattan, now under the guise of Calvin Jelani, visited Veidt in Karnak, Antarctica and asked for his help in rendering his powers to become human again. Veidt offers him the Tachyon Ring he had created three decades prior, and in exchange, Manhattan offers to teleport Veidt to his Europa ecosystem, where the Phillips and Crookshanks clones will serve and adore him as he so desires; Veidt accepts his offer.
Manhattan returns to Saigon, Vietnam and gives the Tachyon Ring to Angela Abar after explaining the purpose of the device. He gets down on knee and Abar kisses him on the forehead, and says that she loves him. Manhattan says that he loves her too as she puts the device into his brain. The device proves to be successful as it inhibits Manhattan's memory, rendering him unable to use his powers except in life-or-death circumstances. He leaves Vietnam with Angela and Manhattan officially adopts the identity of Calvin Abar.
In 2019, Angela returns home after escaping the Trieu Industries facility. Cal finds her at home rummaging for a hammer. Angela tells him that she has always loved him as a husband, but now, calling him "Jon", that they are in trouble, and proceeds to bash his head in. She extracts a Tachyon Ring from his head, and watches as a blue glow emits from Cal's body.