For 10 points, namethese extremely accurate timekeepers.
Optical frequency combs are used to improve the accuracy of these devices. Norman Ramseyproposed using a second microwave pulse to cancel out Doppler shifts in a precursor to these devicesdesigned by Harold Lyons that placed ammonia molecules in a waveguide absorption cell. Thesedevices were placed aboard airliners in the Hafele-Keating experiment, which was a test of (*) specialrelativity. Modern examples of these devices typically use cold Cesium-133 or Rubidium-87, and theywork by exploiting the frequency of transition between states of atoms. The US Naval Observatory has anensemble of these devices, which are required for GPS satellites and NTP servers. For 10 points, namethese extremely accurate timekeepers.
Optical frequency combs are used to improve the accuracy of these devices.
atomic clocks [prompt on "clock" or similar answers implying timekeeping]