I haven't looked at the article in Science yet, but I can mention a
thing or two that might help see what they are getting at.
The problem being tackled is the precision in the measurement of the
frequency of the signal, in this case a photon in or near the visible
region. (I'll look for more details later on this.) I would imagine that
the basic technique for measuring this photon's frequency is to use it
to "lock in" the frequency of a counting circuit's oscillator.
Here it gets a bit more technical, though I am almost certainly
oversimplifying what is done. Once an oscillator has been "frequency
locked", it's output signal can be shaped to produce a square wave (as
opposed to sine wave) and this can be fed to a counting circuit--more
likely, a cascade or chain of circuits that step down the frequency at
precise ratios and then into a counting circuit. Much of the research in
the last decade or so has dealt with devising these stepdown chains (aka
"frequency dividers") because electronic circuits just cannot count fast
enough to keep up with the frequency of light. By the way, these
oscillators nowadays are all lasers or masers, I believe, and are thus
quite stable themselves.
The precision of the measurement might then be seen as being the period
of oscillation (in the original photon). Say I have a source that
produces 1 kHz signals. My counting precision is then � 1 ms since I can
count to the nearest cycle, which takes 1 ms (= 1/f = 1/(1000 Hz)). If I
move up to a 1 MHz source, my counting precision then becomes � 1 �s
(=1/(1 000 000 Hz)).
Another factor that comes into play is the sensitivity of the phase-lock
loop or whatever that "frequency locks" the oscillator. Higher
frequencies lead to less wobbling (in terms of the amount of time
deviation) about the base frequency since the waves' instantaneous
magnitudes rise and fall faster.
You're right, Jim. The accuracy and stability of the source must be
considered. That has been worked in parallel with the development of
chain-counters. The latest improvement I've seen mentioned in public is
NIST's "fountain" clock. If this uses a single atom of mercury, it
sounds like the fountain clock is involved and I think we've mentioned
and described that in the past. USMA's Metric Today had a article on
that as I recall. Here we are taking advantage of quantum mechanical
effects related to the energy of the atom. "Cooler" atoms produce more
stable emission lines than "hotter" atoms. Single atoms produce more
stable emission lines (albeit at the expense of signal intensity) than
many atoms acting as a canonical ensemble (technical term used in
statistical mechanics for "a whole herd of 'em"). The latter bump into
each other and the atoms are not all at the same energy level, so the
emission lines are broadened, producing a spectrum of many frequencies
centered about the main one.
That's probably a whole lot more than you wanted to know. But I'll check
up on this with Science Online just in case you were expecting more....
;-)
Jim
Jim Elwell wrote:
>
> The article below talks about a new clock being more "precise,"
> but everything in it refers to a smaller "tick" interval. What
> use is a finer tick interval if its fundamental accuracy is no
> better, or perhaps even worse?
>
> Jim
....
--
Metric Methods(SM) "Don't be late to metricate!"
James R. Frysinger, CAMS http://www.metricmethods.com/
10 Captiva Row e-mail: [EMAIL PROTECTED]
Charleston, SC 29407 phone/FAX: 843.225.6789
