Message-Id: <v01540b26b0be37f3d875@[192.174.2.173]>
Date: Wed, 17 Dec 1997 18:18:19 -0800
To: pinhole@exploratorium.edu
From: pauld@exploratorium.edu (Paul Doherty)
Subject: counting the frequency
THE MOST PRECISE FREQUENCY MEASUREMENT ever
made in the visible or ultraviolet portion of the electromagnetic
spectrum has been carried out at the Max Planck Institute for
Quantum Optics near Munich. Measuring the frequency (or,
equivalently, the energy) of a light wave is easy in the microwave
region (around 10^9 Hz), where one can directly count oscillations
in an electronic circuit. This does not work for visible or ultraviolet
light, so Theodor Hansch (011-49-892-180-3212) steps down UV
waves by mixing them with light at lower frequencies, producing an
average or "beat" signal. After numerous stages the resultant signal
is amenable to high-precision counting methods (Physics Today,
Dec. 1997). In this way the frequency corresponding to the
important (for the study of quantum mechanics) interval between the
1S and 2S quantum states in hydrogen has been determined to be
2.466 061 413 187 34 x 10^15 Hz, with an uncertainty of only 3
parts in 10^13, an improvement by a factor of almost 100 over
previous work.
By the way the 2s to 1s transition is considered a "forbidden" transition,
this means the 2s state has a long lifetime and a very precise energy.
Paul Doherty