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GS: How does that compare with the prior standard accepted for atomic clocks.
SC: This is the short-term stability and that was comparable and maybe better than
the existing time standards. Then in 1996 Andre Clairon came back with a
comparable short-term frequency. They were now one times 10 [to the] minus 13,
so instead of 12 1/2 decimal places they got 13 decimal places. But more
important than that, they are also beginning to claim an absolute accuracy. And
that's very important because short-term stability is different. Absolute
accuracy says that they now understand the systematics at that level. And that
was now at the level of 2 times 10 [to the] minus 13. That is to say the
relative uncertainty was 2 time 10 [to the] minus 15. That was done in 1996.
At the time the absolute standards were at the 10 to the minus 14 range, so
that's a factor of five and then 10 better. That was in '96. We're talking
seven years after the first atomic fountain. You have to understand that atomic
clocks have had literally 50 years of pretty hardcore engineering time, so in 7
years one was able to achieve a half a century of engineering development
because the atoms were going so much slower.
GS: Was this improvement based on the techniques of cooling atoms that you
developed?
SC: It was based on the concept of cooling atoms and the concept of once they're
cold you can make what's called an atomic fountain.
GS: They were building on the concept or the ability you perfected?
SC: Perfected is not the word. They're perfecting it.
GS: They're perfecting it in that particular application but they're using a
technique that you had developed.
SC: They're using techniques and ideas that we first demonstrated. And that's the
way it should be going. The professional clockbuilders are really the ones who
should be doing this. The people in the university should be trying to figure
out the next big thing that's going to take hold. There have been other
applications of cooling and trapping. One of them is -- and this is again
unexpected in the early middle 1980 -- it turns out you can quantum mechanically
split an atom so that an atom simultaneously takes two pathways. And you can
bring them back together again.
See, atoms have a wavelike character because they're quantum mechanical. So you
think of the atomic waves as interfering with one another. In 1991 there were
four groups around the world. We were one of them, that showed you can split an
atom apart and bring them back together and you can see the inteference between
these two matter waves.
GS: How do you split the atoms apart?
SC: There are two ways to split them apart. Two groups took the path that
essentially since there's really a very close similarity between atoms and
electromagnetic radiation, photons, in that they really were matter waves.
Therefore using electron lithographic techniques, people made tiny tiny
diffraction gratings or slits that if they were in a larger scale you could use
them to make interference patterns with light. And they just made them smaller
so you could use the atomic quantum particle nature of the atom to make
interference patterns.
GS: Essentially an atom is being forced to split apart as it goes through the grating?
SC: That's right.
GS: At least on a wave level. How do you explain it on a particle level?
SC: That's the heart of quantum mechanics. What quantum mechanics says is that you
can't tell. Let's say you've got ten slits. You can't tell which slit the atom
went through so you have to assume it went through all of them. Then you'll
naturally get interference patterns. If you try to do an experiment to decide
which slit it went through, then you destroy the interference pattern. That's
the very heart of quantum mechanics.
GS: That's the Heinsenberg Uncertainty Principle?
SC: No, that's something different. The Heisenberg Uncertainty Principle tries to
tells you that there are certain pairs of quantities which you can't know
precisely simultaneously. These quantities are postion and momentum. You can't
know precisely where a particle is and know precisely its velocity.
GS: So it's not necessarily linked to quantum mechanics.
SC: No, it's an inherent part of quantum mechanics. It's really one of the
foundations of quantum mechanics. But this other thing I'm telling you about is
something a bit different. Let's say you have two slits and you can pass an
atom through both the slits at the same time under certain conditions. Under
those conditions, if you don't make a measurement to tell which slit it went
through, quantum mechanics insists that it goes through both of them at the same
time. Therefore you have to get interference effects. If it makes it any
easier, think about it in terms of electromagnetic waves -- light that goes
through two slits. You know you're going to get some interference patterns in
the back.
GS: So there's physical reality to this, it's not just theory.
SC: Very physical reality! You can decrease the intensity of light so there's only
one particle of light coming through at the same time and what you find on the
back side if you add up the arrival of millions of particles of light, you end
up getting an interference pattern.
GS: So this is exactly the same.
SC: Exactly the same. Except it's now done with atoms.
GS: How does laser cooling improve that technique?
SC: For the same reason that they made better clocks.
If you have slow atoms, you can split them apart in such a way... For example,
if you accelerate while these atoms are in free flight, you can pick up the
interference between one path and another path. You can measure acceleration
very accurately. If you rotate during the travel time of the atom, because the
atoms go so much slower, you are giving the laboratory the chance to rotate
further in terms of more angle, in the transit time the atoms take as they
traverse these two paths. If you have a laser gyroscope... A laser gyroscope
is divides each particle of light, photon into two paths, one traveling
clockwise and other traveling counterclockwise. Then in the transit time of the
gyroscope if the apparatus or the airplane rotates under you, you can pick this
up in the difference in the phase interference in the two beams of light.
You can do the same with an atom. If a platform rotates underneath an atom as
they're going along these separate paths, you pick up the difference in the
phase just like a laser gyro. And you can do a similar thing with acceleration.
For example, what we did in the early to middle '90s, up until very recently, we
could measure the acceleration due to gravity on an atom with higher precision
and accuracy than by measuring the acceleration due to gravity on any object. A
graduate student who did that work in my laboratory, then went on to became a
professor and did the equivalent gyroscope work and his gyroscopes are now more
precise than the best laboratory gyroscopes. The only gyroscopes that come
close to the gyroscopes that we can make with atoms, are gyroscopes now being
put into space to test general relativity. And there may be mechanical
gyroscopes built in the military that are of comparable precision, but the
performance of those are classified. But his gyroscopes are a hundred times
better than the gyroscopes that you put on a 747.
GS: How does a physicist make any implement to that degree of precision? Wouldn't that require machining resources?
SC: If you were going to try to imitate the existing technology and try to do
better than an aerospace company that's been making gyrosopes for 30 years,
there's no way because you're working with grad studens and post-docs who are
not trained engineers. But that's not the point. The point is you come up with
a completely new technology. Actually my former student who is now a professor
at Stanford is pushing the technology in a very hard way.
GS: There would be a lot of room to push.
SC: He wants to put this in a robust package that you can actually put in a
submarine. So he's got a contract with the Navy. He's going to plot a full six
axis, three acceleration directions, three rotation directions into a nice neat
tidy little package that Navy people can operate, and he'll stick it in a
trident submarine and he'll compare it to their best inertial guidance systems
they got in the submarines. So it's gotten to that stage already.
GS: It all starts with using cooled atoms.
SC: It uses all the tricks of cold atoms. It uses all the tricks that we learned
during this whole process. We can also look for oil. We can look for diamond
mines. Those are other examples of completely unexpected applications. At the
time we were trying to first cool the atoms, 1983, '84, '85, I wasn't thinking
you could look for oil this way. The other thing I wasn't thinking about...
GS: How do you look for oil and diamonds. Is it because of fluctuations in the gravitational field that can be detected by the gyroscrope?
SC: Suppose you had a big hollow spot in the ground and there was no rock there,
that it was of lower density, so if you sit above the hollow spot, the
acceleration of gravity would be less. Of course it depends on how big the
hollow spot is but it would be less.
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“The only gyroscopes that come
close to the gyroscopes that we can make with atoms, are gyroscopes now being
put into space to test general relativity.”
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