A quantum discriminator was a technology used for determining the precise moment in time to which a signal was to be sent when attempting to communicate through time. In the 31st century, quantum discriminators were located in students' desks in high school, for use in learning to construct such a communications device.
When the temporal agent named Daniels accidentally changed history by removing Jonathan Archer from the timeline, he managed to construct a device with which Archer could communicate with his crew in 2152, but experienced difficulty in contacting them on the same day which Archer initially left, due to the absence of a quantum discriminator. (ENT: "Shockwave, Part II")