Chrono deflector

From Memory Alpha, the free Star Trek reference.

The chrono deflector was a device built to manipulate the spacetime continuum. Although it could allow someone to travel through time, it produced a great deal of tachyon radiation, making its use lethal to anyone who hadn't been inoculated with chronexaline. Further, the tachyon radiation levels meant that the device would probably burn itself out after one trip. (VOY: "Endgame")

In an alternate timeline, Korath had invented this device on or before 2404. Admiral Kathryn Janeway planned to use the device to correct what she felt was her greatest error, that is, not taking USS Voyager into the Borg transwarp network in 2378. She arranged to get Korath onto the Klingon High Council as payment for the deflector but eventually had to steal the device when Korath also demanded as payment her shuttlecraft's shield generator. With the help of then-Captain Harry Kim, Janeway installed the deflector on her shuttlecraft and successfully traveled back in time to the year 2378. (VOY: "Endgame")