|
|
|
The A magnet selects one of the states, say
the kind 2 state, which is deflected downward by the magnets A and B. (Atoms in the kind 1 state would be
deflected upward.) As long as the
atoms selected remain in the kind 2 state, none of them reach the hot-wire
ionizer detector. However, if
microwaves of frequency 9.192,631,770 GHz are applied to the Cs beam, the
atoms that absorb this frequency undergo a transition and become a kind 1
atom. The B magnet deflects these
atoms upward to the ionizer. The
electrical current generated in the ionizer is proportional to the number of
atoms that make the microwave induced transitions. Thus the microwave frequency can be locked
to the value that produces the maximum current.
|
|
The oven is at ~100°C, the Cs pressure in
the oven is ~10-3 torr, the cavity is at ~10-9 torr;
the typical average atom speed is 100 m/s; the typical cavity length in
commercial standards is 10 to 20 cm; the interaction time is ~1 to 2 x 10-3
s; the linewidth is ~0.5 to 1 kHz; the Q ~ 107; in standard
laboratories, the cavity length is ~4 meters and the Q ~ 108.
|
|
Cs standards are more accurate than Rb
standards because the Cs atoms pass through a high-vacuum region without
collisions with buffer gas molecules or walls which cause frequency
instabilities in Rb standards. Instead
of confining the atoms to a small cell, as is done in Rb standards, the Cs
atoms travel through a relatively long microwave cavity. Cs standards also use the Ramsey separated
field method which further narrows the atomic resonance linewidth - see the
references for details.
|
|
|
|
|
|
H. Hellwig,
"Frequency Standards and Clocks: A Tutorial Introduction," NBS
Technical Note 616, 1977, Time and Frequency Division, NIST, 325 Broadway,
Boulder, Colorado, 80303.
|
|
|
|
H. Hellwig,
"Microwave Frequency and Time Standards," in E. A. Gerber and A.
Ballato, Precision Frequency Control, Vol. 2, pp. 113-176, Academic
Press, 1985.
|
|
|
|
L. L. Lewis, “An
Introduction to Frequency Standards,” Proc. IEEE, vol. 79, pp. 927-935, 1991.
|
