6-5
Prepare Atomic
State
Apply
Microwaves
Detect Atomic
State Change
Tune Microwave Frequency
For Maximum State Change
B
A
Generalized Microwave Atomic Resonator
·   Let A and B be two possible energy states of an atom, separated by energy ho; then o
     is the frequency of the electromagnetic radiation required to convert the atoms from A to
     B, or from B to A;  o is in the microwave range for all currently manufactured atomic
     standards.

·   Population difference between energy states, when ho << kT, is near zero.  Therefore, in
     a natural ensemble of atoms, when o is applied, about half the atoms absorb ho and
     half emit ho; the net effect is zero.

·   A nonthermal distribution is prepared, i.e., one of the states is "selected," by optical excita-
     tion from one of the levels to a third level or by magnetic deflection of an atomic beam.

·   Microwave energy is absorbed in the process of converting the selected atoms to the
     other energy state, e.g., from A to B.  Thus, the applied microwave frequency can be
     "locked" to the frequency corresponding to the atomic transition.

The microwave signals that interrogate the atoms are generally modulated at audio frequencies.  Phase sensitive detection of the atomic signal is used to adjust the frequency of the crystal oscillator to the frequency that produces the maximum atomic signal.


   L. L. Lewis, “An Introduction to Frequency Standards,” Proc. IEEE, vol. 79, pp. 927-935, 1991.