  It has been understood since 1897 that accelerating charges must emit
electromagnetic radiation. Cyclotron radiation, the particular form of
radiation emitted by an electron orbiting in a magnetic field, was first
derived in 1904. Despite the simplicity of this concept, and the enormous
utility of electron spectroscopy in nuclear and particle physics,
single-electron cyclotron radiation has never been observed directly. Here we
demonstrate single-electron detection in a novel radiofrequency spec- trometer.
We observe the cyclotron radiation emitted by individual magnetically-trapped
electrons that are produced with mildly-relativistic energies by a gaseous
radioactive source. The relativistic shift in the cyclotron frequency permits a
precise electron energy measurement. Precise beta elec- tron spectroscopy from
gaseous radiation sources is a key technique in modern efforts to measure the
neutrino mass via the tritium decay endpoint, and this work demonstrates a
fundamentally new approach to precision beta spectroscopy for future neutrino
mass experiments.
