What is a "cesium atomic clock"?


Since the 1950's, the NRC has used cesium atomic clocks, which are the world's best timekeepers. They use the exquisite reproducibility of spinning atoms of the element cesium. Pure cesium is a beautiful silver-gold coloured metal that melts just above room temperature. It is uncommon only because it combines so easily with other common elements.

The glass vial in this picture contains a gram of cesium: one year's supply for a typical atomic clock, which does not recirculate the cesium atoms. A gram of cesium could be found in about a cubic foot of ordinary granite. Natural cesium is pure cesium-133 (55 protons and 78 neutrons in the nucleus, 55+78=133): it is non-radioactive.


Cesium-133 atoms are sent from end to end in the vacuum tank of an atomic clock, as illustrated here.

In the clocks at the NRC, they travel up to 5 metres at about 250 m/s. The NRC's largest cesium clock is shown below.

Drs. Jean-Simon Boulanger and Rob Douglas, Research

Officers in the Frequency and Time group, make adjustments to one of the NRC-built cesium atomic clocks. The large aluminium tube is a vacuum vessel which contains the heart of the clock. Cesium atoms are emitted at one end of the tube and pass through two microwave cavities (the copper waveguide which feeds the microwaves to these cavities can be seen above the tube) before they are analyzed and detected at the other end. The clock is located in a copper room to isolate it from radio interference.

How does it all work?

Cesium is evaporated at the cesium source to form a beam of well-separated cesium atoms that travel without collisions at about 250 m/s, through a vacuum maintained by the vacuum pump.

The A magnet selects cesium atoms with their atomic magnets pointing one way (those in the f=3 level of the ground state of the cesium-133 atom), and sends other atoms to be absorbed by a carbon getter.

Some atoms have their magnets set spinning by microwaves in the Ramsey cavity. Allowing for tiny corrections, their magnetization spins at 9 192 631 770 rotations per second in a very uniform magnetic field, the C field of less than 1/10 the Earth's magnetic field. Magnetic shielding isolates the atoms from outside magnetic fields. (Quantum mechanics describe these cesium-133 atoms as an oscillating combination of the two hyperfine levels, f=4 and f=3.)

The spinning is stopped by the microwaves at the other end of the Ramsey cavity.

The B magnet collects the cesium atoms that stayed in step with the microwaves, and which now have their magnetization pointing the other way (the cesium-133 atoms in the f=4 level). The B magnet deflects the in-step atoms towards a detector, the hot wire cesium ionizer and ion collector. The other atoms are absorbed by another carbon getter.

The quartz oscillator is adjusted automatically by the servo control to maximize the number of cesium ions collected, keeping the microwaves in step with the spinning of the cesium atoms. After the small remaining biases are measured and eliminated, the output frequency is a very accurate 10000 000 Hz, accurate to about 5 parts in one hundred thousand billion when averaged over a day. This is a frequency standard, suitable for use in metrology, communications, and many other applications in engineering or science.

A cesium atomic clock needs a few other parts. Simple electronics counts the output cycles of the quartz oscillator, and issues a pulse every 10 million cycles - exactly 1 second apart. When first started, the atomic clock's time is set with respect to International Atomic Time (TAI, Temps Atomique International) - which has been kept by generations of atomic clocks since 1958 when it was set relative to astronomical time. Other circuits count the atomic clock's minutes, hours, days, years, decades, centuries, millennia...