Hydrogen maser

A microwave oscillator that uses stimulated emission from hydrogen atoms to produce a highly stable signal. It serves as a frequency standard and clock.

Hydrogen hyperfine transition: The energy-level transition between two spin configurations of hydrogen’s ground state, corresponding to a 1,420.4 MHz photon. This atomic transition sets the maser’s microwave oscillation frequency.
Deep Space Network: NASA’s global system of large antennas for communicating with spacecraft beyond Earth orbit. Hydrogen masers provide stable frequency references for precise spacecraft tracking.
Cesium standard: A frequency standard based on the 9,192,631,770 Hz hyperfine transition of cesium-133. Cesium defines the SI second, while hydrogen masers excel chiefly at short-term stability.
Maser: A device that amplifies or generates microwave radiation through stimulated emission. The hydrogen maser is an atomic-beam maser using hydrogen atoms.
Stimulated emission: A process in which an incident photon prompts an excited atom to emit a matching photon. It amplifies microwave radiation in the hydrogen storage bulb.
Very Long Baseline Interferometry: A radio astronomy technique that combines observations from widely separated antennas. Hydrogen masers synchronize observatories whose signals are later combined.
Rubidium standard: An atomic frequency standard based on a microwave transition in rubidium vapor. Rubidium standards are generally compact and economical, unlike high-performance hydrogen masers.
Charles H. Townes: An American physicist who helped develop the maser and shared the 1964 Nobel Prize in Physics. His maser work established the technology from which hydrogen masers developed.
Atomic beam: A directed stream of neutral atoms traveling through a vacuum. A beam selects excited hydrogen atoms before they enter the storage bulb.
Global Positioning System: A satellite navigation system that determines position and time from precisely timed radio signals. Hydrogen masers have served as high-stability clocks on some GPS satellites.

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