A tachyon detection grid
A tachyon detection grid was a technique developed by Starfleet as a means of detecting cloaked Romulan warships. The grid used active tachyon beams transmitted between different starships or sentry posts to create a tachyon field; any cloaked ship entering this field would be exposed. The technique was developed by USS Enterprise-D chief engineer Geordi LaForge in early 2368.
The tachyon detection was first seen in practical use during the Starfleet blockade of the Klingon-Romulan border during the Klingon Civil War. Twenty-three starships successfully prevented a convoy of Romulan supply ships from sending further aid to the forces loyal to the House of Duras, tilting the balance in favor of Gowron, the legally appointed leader of the Klingon High Council. (TNG: "Redemption, Part II")
Following this success, the Federation began deploying the tachyon detection grid to supplement its gravitic sensor net along the border of the Romulan Neutral Zone. (TNG: "Face of the Enemy")
By 2373, the Klingons had deployed a tachyon detection grid around the perimeter of their central command base on Ty'Gokor. (DS9: "Apocalypse Rising")
In the Delta Quadrant, the Swarm used a similar technique to monitor the borders of their own territory. (VGR: "The Swarm")
See also: Antiproton beam