A cesium clock operates by exposing cesium atoms to microwaves at one of their transition frequencies and then counting the corresponding cycles as a measure of time. The frequency involved is determined by the energy of the incident microwave photons when they excite hyperfine transitions in the atom. An atom exists in certain discrete energy states determined by the electromagnetic interactions between its electrons and nucleus.
 The cesium beam tube is a self-contained device that performs all the tasks required for continuous, extended operation of the clock. The length of a cesium beam tube may vary from about 30 centimeters for a compact unit to several meters for primary frequency standards used in metrology laboratories. |
Transitions at many different energies are possible; those discussed here occur at very low energy and refer to changes in the magnetic interaction between electron and nuclear spins (hyperfine interaction) in the atomic ground state.
Cesium was selected as the best candidate "working atom" for an atomic beam clock for a number of reasons.
 Atomic beam current (Ibeam) from which the clock signal is derived depends upon the difference between the applied microwaves and the atomic transition frequency |
The cesium atomic beam source, which resides inside a vacuum, is an oven with a small exit hole, heated to about 90 degrees Celsius. A ribbon of cesium atoms emerges from the oven, forming the atomic beam. The atoms are detected at the other end of the beam tube when they hit a hot wire that strips their single valence electron, imparting a positive charge. The positively charged atoms are then accelerated in an electric field and detected by a charged particle detector. Between the oven and the detector, the atoms pass through two magnets and a device called a Ramsey cavity, named for physicist Norman Ramsey, who introduced the device in the late 1940s. The two magnets, A and B, produce two strong, highly inhomogeneous magnetic fields (tens of thousands of Gauss per centimeter gradients).
As the cesium atoms effuse from the oven, they are equally likely to be in one of 16 different quantum magnetic states. Half of these 16 magnetic states have a negative polarity and half a positive polarity with respect to an external magnetic field.
Atoms in a "+" polarity state are deflected toward the beam tube axis by the A-magnetic field, allowing the atoms to pass through the Ramsey cavity. Atoms in a "‒" state are deflected in the opposite direction by the same magnetic field, away from the beam tube axis, and therefore blocked from entering the Ramsey cavity.
The B-magnetic field is set to exactly oppose the action of the A-magnetic field. Atoms deflected by the A-magnetic field into the B-magnetic field are now deflected in the opposite direction, away from the hot-wire detector.
As long as no other outside influences act, all the atoms would be deflected and none would be detected at the other end. However, microwaves at (or near) the clock atomic transition frequency of 9.192631770 gigahertz are injected into the Ramsey cavity. When the microwave frequency is tuned to exactly match the atomic resonance, atoms in the Ramsey cavity, and in the proper magnetic state, are able to absorb the microwave energy and undergo a transition that flips the polarity in such a way that the atoms are deflected by the B-magnetic field toward the hot wire, where they are detected.
Detection of atoms is a signal that the correct microwave frequency is being delivered to the cavity, thus allowing locking of that "clock" frequency to the atomic transition frequency. This locking of the microwave frequency to the atomic transition frequency is the heart of the extreme stability of the atomic clock.
-