Technology · Specialized
The second, defined by an atom
Between 1967 and 2019 the second was defined by a transition in cesium-133, because every atom of it is identical and the frequency is not a property of any apparatus.
Explore the supporting references at the foot of this article.
THE MATERIAL THAT DOES THE WORK
Chemical form
The reference is neutral cesium-133 atoms in the gas — a thermal beam in older standards, or a cloud of laser-cooled atoms launched upward in a fountain. The relevant quantity is the frequency of the hyperfine transition in the ground state, fixed by definition at 9,192,631,770 hertz.
- 01
Prepare the atoms
Cesium vapor is laser-cooled and launched as a slow cloud, so the atoms are nearly at rest and identically prepared.
- 02
Interrogate the transition
Microwaves probe the ground-state hyperfine transition twice during the flight, producing a narrow resonance.
- 03
Steer an oscillator and distribute the time
A servo loop locks a quartz oscillator to the atomic resonance, transferring the atom's stability to a usable signal. Locked oscillators feed national timescales and satellite systems, where a nanosecond of timing error becomes about thirty centimeters of position error.
Why it works
A standard has to be reproducible somewhere else without shipping the artifact. Atoms satisfy this because they carry no manufacturing history: a cesium atom in Tokyo is identical to one in Paris. Cesium was chosen because it has a single stable , a convenient hyperfine splitting in the microwave range and a vapor pressure that makes a beam easy to produce.
How it is made & used
In a fountain clock, cesium atoms are cooled by laser light to microkelvin temperatures, launched upward, and pass twice through a microwave cavity as they rise and fall. The interrogation time set by that flight determines the width of the resonance, and a servo loop steers a quartz oscillator to keep the microwave frequency centered on it.
Inside the application
Redefining the second in 1967 moved timekeeping from astronomy to atomic physics, and the consequences reach ordinary life through satellite navigation, where a position fix is a timing measurement and a nanosecond of clock error is about thirty centimeters of range error.
Limits & alternatives
Cesium standards are now outperformed by optical clocks based on strontium, ytterbium or aluminum , whose higher transition frequencies give proportionally finer ; a redefinition of the second is under discussion. Every clock also needs corrections for the gravitational potential at its location, and at the best accuracies a height difference of a few centimeters is measurable.
Part of a larger system
This connects atomic structure to metrology and then to infrastructure: a hyperfine splitting in one element underwrites the timestamps in navigation, financial records and telecommunications.
Sources & further reading
- NIST — Cesium ↗National Institute of Standards and TechnologyReference explanation · Source checked
- USGS — Cesium ↗United States Geological SurveyReference explanation · Source checked
- RSC Periodic Table — Cesium ↗Royal Society of ChemistryReference explanation · Source checked
- NIST — Cesium ↗National Institute of Standards and TechnologyReference explanation · Source checked
- NIST — Cesium ↗National Institute of Standards and TechnologyReference explanation · Source checked
Sources and further reading. What the review labels mean