Atomic Fountain Clock

A geyser of atoms launched in slow motion through a microwave cavity, ticking out the SI second nine billion times per heartbeat.

An atomic fountain clock uses laser light to cool a small cloud of cesium-133 atoms to just above absolute zero, then tosses them upward through a microwave cavity. As the atoms coast through the cavity on the way up and again on the way down — a total interrogation time of roughly one second — microwaves tickle them at 9,192,631,770 Hz. This frequency matches the atom's ground-state hyperfine transition so precisely that the fraction of atoms flipped to the upper state peaks sharply. Electronics lock an oscillator to this peak, producing a signal that defines the SI second. NIST-F2, operating with a liquid-nitrogen-cooled interaction region to suppress blackbody radiation shifts, reached a fractional frequency uncertainty approaching 1 × 10^-16, meaning it would neither gain nor lose a second in 300 million years.

Six counter-propagating laser beams cool and trap a cloud of roughly 10^7 cesium atoms in a magneto-optical trap near the bottom of an evacuated tube. A brief frequency change launches the atom cloud upward at about 4 meters per second. On the way up, the atoms pass through a microwave cavity resonating at the cesium hyperfine frequency; they enter the cavity a second time on the way down, completing a Ramsey interference measurement whose narrow fringe width is inversely proportional to the total free-flight time of about one second. A detection laser measures the fraction of atoms in each state by observing fluorescence. Systematic effects — blackbody radiation from warm cavity walls, collisions between atoms, and magnetic field shifts — are painstakingly characterized and corrected. NIST-F2 suppressed the blackbody shift by cooling the microwave interaction region to approximately 77 K using liquid nitrogen. Multiple fountain clocks at national metrology institutes around the world contribute data to the Bureau International des Poids et Mesures (BIPM) to steer International Atomic Time (TAI).

In plain terms

Imagine you had a perfectly consistent metronome that beats exactly 9,192,631,770 times per second — no more, no less, ever. Cesium atoms are that metronome. A fountain clock tosses them gently upward, listens to their humming as they rise and fall, and counts every single beat to tell time more accurately than any pendulum or quartz crystal ever could.

Also called: cesium fountain, primary frequency standard, laser-cooled cesium clock.

Why it matters

Cesium fountain clocks define the SI second: the second is legally 9,192,631,770 cycles of the unperturbed cesium-133 ground-state hyperfine transition.
GPS satellites carry onboard atomic clocks synchronized against ground-based fountains; without regular corrections traceable to fountains, GPS position errors would accumulate at meters per day.
Fountain clocks calibrate optical atomic clocks, which must be measured against the existing cesium standard before any redefinition of the SI second can take effect.
Frequency-ratio measurements between different fountains at institutions worldwide test whether the fundamental constants of nature are truly constant — a key question in physics beyond the Standard Model.
International Atomic Time (TAI) and its derivative Coordinated Universal Time (UTC) are ensemble averages steered by contributions from more than 400 atomic clocks including about 10 primary cesium fountain standards.

Where it lives

NIST-F2 (NIST Boulder, USA) — cryogenic cesium fountain, fractional uncertainty ~1 × 10^-16, operated 2014–2015 as primary standard
SYRTE FO2 (Observatoire de Paris / LNE-SYRTE, France) — cesium fountain primary standard, contributes to TAI
PTB-CSF2 (Physikalisch-Technische Bundesanstalt, Germany) — cesium fountain contributing to UTC/TAI
NPL-CsF2 (National Physical Laboratory, UK) — co-holds first realization of the 10^-16 fractional uncertainty level
IT-CsF2 (INRIM, Italy) and SU-CsFO2 (VNIIFTRI, Russia) — additional TAI contributors listed in Wikipedia atomic clock article

Key numbers

Cesium hyperfine frequency: 9,192,631,770 Hz (Defines the SI second exactly, since 1967)
NIST-F2 fractional uncertainty: ~1 × 10^-16 (One second error per ~300 million years)
Cesium fountain height: ~1 m (Free-flight parabola in evacuated tube)
Interrogation time: ~1 second (Determined by launch velocity and tube height)
Optical clock advantage: ~100× (Best optical lattice clocks reach ~10^-18 fractional uncertainty)

Surprising facts

NIST-F2 would neither gain nor lose a second over 300 million years — yet optical lattice clocks have already surpassed it by more than 100-fold in accuracy.
The cesium atom oscillates 9,192,631,770 times each second; the fountain clock must count every one of those cycles precisely to define the second — a number chosen in 1967 to match the best mechanical and quartz-crystal clocks of the era.
NIST-F2 had to be cooled to near minus 200 °C using liquid nitrogen to tame a single systematic effect: the tiny but measurable energy imparted to atoms by blackbody radiation from the room-temperature cavity walls.
The interrogation time in a fountain — about one second of free flight in a roughly one-meter tube — exploits the same Ramsey separated-oscillatory-fields technique that Norman Ramsey invented in 1949 and won the Nobel Prize for in 1989.

Open questions

Collisional frequency shifts that depend on quantum statistics: When two cesium atoms in the same hyperfine state collide inside the fountain, their identical-boson quantum statistics suppress the collision cross section relative to atoms in different states. This 'cold-collision shift' depends on atom density in a way that had to be carefully characterized to avoid frequency errors, and is one of the main reasons fountain clocks must operate at dilute atomic densities rather than packing in more atoms for stronger signals.
Optical clocks already 100× better but still cannot replace cesium legally: Strontium and ytterbium optical lattice clocks have demonstrated fractional uncertainties at the 10^-18 level — roughly 100 times smaller than the best cesium fountains — yet the SI second is still legally defined by the cesium transition. Every optical clock's frequency must be verified by comparison against a cesium primary standard, meaning the 'better' clock is calibrated against the 'worse' one. The community is working toward a formal redefinition, expected within the current decade.

A short history

The cesium atomic clock was first demonstrated by Louis Essen and Jack Parry at the UK National Physical Laboratory in 1955, setting accuracy at 10^-9. The fountain geometry — launching atoms upward to extend interrogation time — was proposed by Zacharias at MIT in the 1950s but required laser cooling to become practical. The first operational laser-cooled cesium fountain, LPTF-FO1, was demonstrated at SYRTE (then LPTF) in Paris in 1991. NIST launched NIST-F1 in 1999 and NIST-F2 in 2014; both represented the state of the art for the primary realization of the SI second, achieving uncertainties 10,000 times better than Essen and Parry's original clock.

Sources

NIST: 'NIST's Cesium Fountain Atomic Clocks' (NIST PML Time and Frequency Division) (https://www.nist.gov/pml/time-and-frequency-division/time-realization/cesium-fountain-atomic-clocks)
NIST: 'A Brief History of Atomic Time' (https://www.nist.gov/atomic-clocks/brief-history-atomic-time)
NIST: 'The Second: Present' (SI second definition and NIST-F2) (https://www.nist.gov/si-redefinition/second-present)
Heavner et al., 'First accuracy evaluation of NIST-F2,' Metrologia 51, 174 (2014) (https://www.nist.gov/pml/time-and-frequency-division/time-realization/cesium-fountain-atomic-clocks)
Atomic fountains and optical clocks at SYRTE: Status and perspectives, Comptes Rendus Physique (2015) (https://www.sciencedirect.com/science/article/pii/S1631070515000614)