[LEAPSECS] The next primary frequency standard?

[Jonathan E. Hardis]

>http://www.wired.com/wiredscience/2010/02/quantum-logic-atomic-clock

>

>...

>

>There are some other misleading statements in that article.

>

>There's no need to change the definition of the second because the latest

>frequency standard is based on a different quantum transition. Magnesium,

>mercury, and ytterbium have been used in the past, though AFAIK all the

>current primary frequency standards are caesium.


The SI "second" is currently defined in terms of a microwave 
transition in cesium.  This allows the best atomic clocks to have 
accuracies on the order of 1 part in 10^15.  Atomic clocks based on 
optical transitions are thought to have  potential stability on the 
order of 1 part in 10^18.  However, as long as the definition of the 
second is based on cesium, they can never achieve better _accuracy_ 
than a cesium clock.

Because of this, many believe that one day the definition of the 
second will be changed to reference an optical transition.  Which 
system might be best for that purpose is still a subject of research 
and debate.


>The speed of light in a vacuum is a fixed constant that is used to define

>the metre in terms of the second, so I don't know what questions about it

>need to be resolved.


The speed of light was perhaps not the best example.  However, it is 
perhaps the best known "universal physical constant," which goes back 
to the point that the author was making.  The question being explored 
is how "constant" the physical constants might be.  At the top of the 
list is the value of the fine structure constant, alpha, which equals 
e^2/(h-bar c).  Today, "c" is a defined constant, and if the CGPM 
continues along its present path, in a few years "e" and "h-bar" 
might be defined constants as well.  Nonetheless, one can determine 
experimentally an upper limit to the rate-of-change, if non-zero, of 
alpha.  There are physical theories that predict that alpha does, in 
fact, change over time because of coupling between quantum physics 
and gravity.

For many years the best bound to "alpha-dot," as the issue is 
sometimes known, was based on astronomical observations of the early 
universe.  Today, the best bound comes from studies utilizing optical 
atomic clocks.  As the precision of these clocks increases (i.e., 
towards a part in 10^18), the bound on "alpha-dot" can be tightened. 
If, in fact, a non-zero alpha-dot were to be observed, it would be 
one of the great discoveries in physics in the 21st century.

Similarly, optical atomic clocks are being used to look for drifts in 
the electron/proton mass ratio, and the light quark mass (the average 
mass of up and down quarks).

    - Jonathan