So the topic for today is... in a moment will appear.
I'm interested in issues about precision and sometimes these tools that we have, let us have a little bit sharper view,
microscopes with good resolution.
You've heard what amazing things show up when you have twice more resolution or something.
And some dimensions are...
Aha!
It's working I...
It might last that long.
We'll do an experiment.
So the issue is a collective kind of issue, but some places are special.
Here's a place where Nobel is not respected.
You can get it simply by making a purchase and that's in a university town that's not far from Boulder.
So another topic is that one encounters people who deserve the absolute entire respect.
Charles Townes, as you know, we lost earlier this year.
He was just months from the 100 mark.
So, I'm not trying to say that I'm a famous pal of Charlie Townes,
but, in fact, I had a modest start, at high school being slightly frightened of girls, and not very social.
So that continues.
So, the subject which I like is Metrology.
It's just fascinating to see how well can you measure something.
And not everything is worth measuring exactly perfectly.
For example, a piece made out of wood will change its dimensions with humidity.
But things that are made, and thoughtfully made, can really teach one a lot.
So we've had these ideas about measuring length, that was a good enterprise and we have a possibility and definition of length
from the people in Crete, all the way to the contribution my community made of effectively defining away one of the parameters
that people thought were necessary.
And now, one plays in frequency.
As you probably know, the choice of making measurements by frequency is the right choice.
It gives you the highest possible resolution.
So, in fact, the resolution is now in this 18th place.
So 18th place, Steve Chu already mentioned what that means.
The time from the beginning of the universe to now divided into 10 to the 18th parts gives you a second or a half a second.
So, what can it depend on.
Well, I used to work, before I retired for this National Institute of Standards and Technology.
And at that time of the slide there were still seven fundamental units.
These were made by wise men in 1875.
Every person at that time thought they understood the universe.
And they thought that there were seven fundamental parameters that would be enough for metric, uh Metrology, of everything which people could do.
Well, my own group, among other equivalent guys in different labs have been working.
This kind of picture I'll use several times.
Typically going on this axis is the time and here is the imprecision.
So down is good on this axis and you see from the first introduction of the atomic clock in 1950,
it was developed and finally chosen as a standard in 1967 and the improvements over time.
Laser cooling from Steve Chu and Bill Philips and others came in in this 1995 epoch
and made a huge impact especially on us optics guys, that could see with these higher frequencies.
Higher by a tenth, a hundred thousandth as a factor.
Then there are more clicks in the second and so we have the chance to get the resolution corresponding to that if we can only
learn to do the technology better.
So that's hard. Takes a long time and we didn't have a very good learning curve at first.
But then the integration of laser cooling.
So that brought us then into this place, and with trapped single ion that can't vibrate or move around.
And strontium atoms, one of my favourites, and at this point we are in this 17th digit.
So, it's really a good one to study.
So, the kind of questions that one can get at with this and there's all kind of practical issues and each, probably nearly every person here
has one of the uses for atomic clocks just in a pocket which is to find out where you are with the aid of the GPS.
But there are lots of other things.
For example, this issue about laser line width.
Charles Townes' theory said that the line width could be millihertz or so, and the frequency is about 10^15.
So that's 18 digits.
That really seems like it's nice.
And now one realizes that the line width actually can be absolutely less.
The issue about whether the fundamental constants are stable that really comes into my brain when we hear this business about
negative pressures, dark energy and though gonna be, I don't know what the time is, but 10 years , 20 years,
you guys are gonna be in for a really treat because one is either gonna find out
that oh the fundamental constants changed over time or that space is not the way that we think it is or whatsoever.
So, it's going to be a good story.
It is a good story now.
It's, we're in the same kind of place as this young lady who had first encountered neodymium magnets.
And we know this technology is marvellous and we really have some fun.
This young lady who sensed that there's some kind of conservation principle.
You'll see she has the first non simple mode where two active particles crash against this.
My wife's dream is to get all kids interested in science.
These little joyous young people have been part of our travels.
So, 1875 treated in meter, there's no good idea for what mass is, except that we'll hear later in this session
how people currently think.
At that time the definition of meter was some fraction of the distance around the earth, goes through the pole and it goes through Paris.
Those are the two places that you could pretty easily guess.
But we need a representative in the lab, and so here is one of these meter bars.
40 were made.
Each country got two and that was a really, a wonderful thing.
When I joined the National Bureau Standard measuring the scratches on this that were the end point of the meter
was something that was still a research topic.
At the same year, this business about lasers... Here's a Doppler width.
Here is the width of a saturated absorption resonance which is really strong and really narrow.
You don't need to be a genius to realize that we could better by some factor here which is 300 or 500 just on the very first day.
We'd like to measure the frequency of that.
Here's National Bureau Standards' team heading to measure the last measurement of the speed of light in 1972.
So that led to the possibility to redefine the meter.
In 1960 it was defined to be incoherent light from a krypton discharge.
1960 was the same year in which the laser was announced.
So that shows that these communities don't always know about the other progress.
Then it was redefined in 1983.
I don't think we need to give a quiz anymore because you all know this number by heart.
It's surely as important as the telephone number.
Then, Ted Hänsch has an idea, maybe a hundred years after Mr. Fourier
same time as Chebotayev, and that was if you had an array of equally spaced frequency
that would really be a lovely measuring thing.
It's kind of like a meter stick with all these scratches you could line up the frequencies and make the comparison.
So that was a very good idea and some combination of competition and cooperation between München and Boulder lead to
absolute glorious success on this.
So the frequency standards are getting better, applying new technologies and ease of making measurements.
So this community going down, that's the cesium clock and around in here someplace
the idea of having cold atoms that are coming up and then making a shower fountain down.
Exciting by radio frequency that lets you have this clock working well.
But it is very hard to get long enough interaction time because a second is a meter scale, right.
So, and in a second you only have 10^10 oscillations.
So now we're in the 18th digit so how are you going to get eight digits out of this?
That just doesn't seem possible.
However, if you start with an optical frequency again, which is something times 10^14,
it's pretty easy to make some progress.
And what you can see is some points that are marked in green here.
There's going to be a problem because these new technologies are going to produce frequencies that you can't state in hertz
or in metric units because the standard is not as good.
These new technologies are based on either having a trapped ion on the laser interrogation
or my favourite is to use multiple laser beams to have overlapping grids of standing waves.
As Steve explained, the atoms like to go where the wave is the brightest.
And a very clever invention of Katori in Japan was that you could have a trap that didn't hurt
the optical transition you wanted to use if the shift due to the light, that's what, that's gonna make a force,
it shifts the energy levels.
And if it was the same on the ground state and on the excited state that you wanted to use
then the difference that is the optical clock wouldn't be hurt.
So that seemed like a really grand idea and a lot of energy is gone into it.
I'd like to just say, where are we now?
Well for 60 years cesium has been the best thing and it served extremely well as the international reference.
That's the best reference that mankind has to make any kind of measurements.
Only that exceed that is simply enumerating the numbers.
With clever work that business improved by six orders of magnitude in 60 years to 16 digits where the brick wall is
and it ain't going any more.
However, in 2008, with these new optical methods, three clocks became more potentially accurate than time itself.
And that's clearly something that a person might want to explore.
It seems like there's new opportunities there.
It falls into the category of searching where somebody else isn't.
And in trapped ions there's already some candidates, lots of candidates and neutral atoms.
And so then, you don't need even one beer to get to the place of saying: Do all these clocks really keep time at the same rate?
Let's turn the crank on the universe, go forward another year and see how that is.
So, the name of that topic is Cosmology.
We're doing Cosmology experiments in the lab.
Low cost thing that the people of Germany do in Braunschweig in the PTB, people in the United States do in Boulder,
National Bureau Standards, now called NIST.
It's really cool stuff.
So here's an experiment by one of my students that I thought was outstandingly nice.
He's got mercury ions and mercury is interesting because it has so much charge, sucks the inner electrons really in.
They have huge speed.
Huge speed for a charge means huge magnetic field.
So this fine structure splitting is actually half of the optical energy for mercury.
In the case of aluminium, has different colours, it's not the same
fraction, in fact is a very weak fraction, only something in a percent.
So by using the comb to measure these two frequencies you have the possibility for a really interesting comparison.
So if you measure the frequencies,
so the ratio of frequencies of aluminium and hydrogen, uh mercury, on this axis, and we're in the 10th to the minus 16,
and you see it goes from four to five 10 the minus 16 and this is a year or more on the horizontal axis here.
And sensibly one would say that we probably should put a straight line through that
because if you say, with a sloping line on that, that the fundamental constants are changing,
you're going to get a lot of newspaper people coming to your door and a lot of scientist asking you,
well that's really not quite enough data to support that.
However, if you notice the dates, 2007, do you think these people are resting?
Possibly it's running now.
So, it's not up to me to say how this curve looks now, but it might be interesting.
One way to discuss this, this is frequency ratios, so that then I told you was an analogue of this fine structure constants.
This famous one over 137.059999
and some more digits measured by Gerald Gabrielse most precisely recently.
In the other axis is magnetism.
So what you can see on this curve is that there is one place where all the different experiments settle in on.
And so, so is the fine structure constantly changing?
Well, if it's changing, the change would have been less than a part in three million
across the age of the universe.
So if you're betting, I'm gonna say probably one to pick
this physical constant doesn't change.
But what about in the next digit?
So that's left for exercise for you young people to see how it's going to turn out.
One thing which is unbelievable is that you can create a new case by transmitting information.
We heard a talk about entanglement already.
And the group in Innsbruck have recreated atoms in states which were the states at a different place
in different atoms and the propagation is by information.
Another thing which is a little bit surprising initially
is that if you put your two clocks on two tables then you go to the rental store and get a jack
and you turn the handle, lift the table up, lift the table up 35 centimetres and what happens?
The frequency changes.
Well no that's not quite what was anticipated by our friends in Paris is 1875.
They thought that the constants were universal and that we would be able to do Metrology across the world.
And then 100 years ago there was this
total revolution that was introduced by Einstein and now we understand that there's an issue about
hidden in our discussion, is something about the framework we're in, but it would be very ugly if just changing
the framework would happen.
So what we believe, I think, is that, if that you stay within one gravitational frame, you won't get any surprises.
But if you happen to use a better clock in which this easy resolution you've got a new frequency now only corresponds to two centimetres
and the latest clock, which Jun Ye's group is playing with.
In fact, here is a picture that summarizes what their work is.
So now you see we're at one second, one is talking about something times 10^-16.
And it goes down and this measurement shows that there's nothing funny going on.
There's a nice paper that's just come out during this year.
In fact, most of the noise that's in this comes from the oscillator that you try to find out what the frequency is.
That you can't measure much better than the instability of that oscillator.
And that's fixed by the thermal motion of the atoms that are in the coatings on this.
So now there's progress in that because every time you can understand what the problem is you have a chance to make it better.
So now people make the coatings out of a crystalline material and the thermal fluctuations in the low loss
crystalline material are less.
So, I come back to this evolution of clocks.
We go down the cesium highway here and it's getting better.
We're zooming down this highway and, in fact, here is the latest evaluation of the strontium at minus 18 in the year 2000.
Again, as I said, the work of Jun Ye.
There's no better joy in life, I think, than for an old professor to find the right guy to replace him.
So that is the case here.
Jun Ye is doing stuff better and quicker than me.
In the future, it's obvious that a new standard is needed, but the real question is when.
Well, one criterion is that, if we're going to change the standard that everything depends on
this better not be having any mistakes on it.
So progress should have slowed down and we should have had lots of systems that have been evaluated
and checked which is the best one.
And the part which I come to now, which will be the fun for your guys, if you choose to play in this game,
is about remote comparisons because that's how we do science.
You do the measurement in one place and you want the same result as the measurements in another one.
Well, I'm sorry to tell you, but that is not gonna be possible.
Take a look at this.
If we had a sun and if we had an earth and there was 93 million miles, I apologize for those units,
but there's a little bit of wobble you know that we're not in an exactly circular orbit.
So the general relativistic red shift is modulated yearly by 1.6 parts in 10^18.
So actually we could see it except we don't know the reference because everything is moving.
However, if we look in this case that we've got a lab that's in the bright sunshine and in the noon
and another lab that's in the slight off set between these, at midnight there's a significant gravitational shift between those.
In fact, it amounts to a 180 hertz and, as I mentioned, the measuring precision is millihertz now.
So here's a chance to check general relativity just perfectly.
If we check everything in our own lab, the GR framework is the same.
We have gravity and okay we can take account of the slow gradient part, 10^17 meter,
but wouldn't it be absolutely stunning if we had a way to go from one, from the midnight to the noon labs
and compare because then we could really look at this issue about universal time.
Well, there ain't no universal time.
There's only frame dependent times.
And this is what has happened to the concept of time.
In us it's quite clear everything is serially ordered and that's a good indexer, but if you want to get it exactly right
you have to pay attention and gravity is now part of it.
Well how do we compare times and frequencies?
We have this GPS system that works extremely well and it will let you enjoy something into 10^-14 for one day.
It would be better if you used a satellite that you pay for and then you have the fastest lengths
and it turns out in that you have to cut down a factor 350 between the one measurement and where you need to get to.
It takes a year to do that.
So there is one other way, and I am advertising for this as a,
I hope that we still have some of those industrial people because this is in their domain as a possibility.
We've learned how to send a light in a fibre.
Of course the light comes out, but the fibre jiggles and that changes the phase and so, in order to have a success,
you have to send it to the other end, have then mark it someway, comes back to you.
You can see this is what I sent.
This is what I got back.
So half of that is how much noise there is going that way.
So we put up a little paper in 1994 that shows how to do that.
Divide the phase by half, by two and then you can just...
Go ahead and now this is used in lots of interesting places.
The radio telescope array in Chile is all synced up with this femtosecond comb.
And, of course, the tiny community,
there's not very many people, but they really care intensely about clock comparisons, atomic frequencies.
If you want to build a bigger accelerator or even one that's not very far away from here, you have to get it synchronized
and there's nothing more powerful than doing that with light.
So, the numbers are that you can do 10^-17 at a second or something and transfer millihertz for the line width.
Well this idea, of course, resonates everyplace and here's the dreams that some French colleagues
have put up about how Europe could get organized to go from Braunschweig in the north
and then the University of Innsbruck, and of course in their plan in Paris will be some kind of hub for that.
In fact, it's tried out already on a smaller scale.
This was a 150 kilometres link.
This paper's already multiple years old and the most amazing thing is that they got this
delicious result, far better than the GPS limit here at the top.
They got this result on a channel of the fibre,
a colour that's in the fibre and it's just for their use and the rest of the fibre's bandwidth
was being used for commercial traffic.
So that sounds like the beginning of a place where we could have an agreement with some companies
and so now, you can see what my dream is.
Here's where I'd like to end up.
I'd like to be with a competitive Strontium clock in 2015, which is this 10^-18 kind of domain, and it's only going to get better.
So what do we want?
Well, we'd like to have a fibre.
They don't talk about how many dark fibres they have.
They talk about 60,000 kilometres of lit fibre, but they, of course, turn my light into electrons,
but I still wonder if maybe they have a dark fibre that we could use.
Here's a different company.
They've got a global network and I wonder if these guys have a dark fibre that we could use.
So... And here's goobers of fibre.
And this, we've just lost in the last couple of years, the person that dreamed this underseas fibre stuff at AT&T.
One of Steve Chu's friends.
It was totally marvelous when I went to visit Steve because T. D. Lee was there with a whole bunch of his team
and they were really excited.
Lunch time was discussion time.
So surely someplace there is an unused link, actually we need it a little special
because every 200 kilometres you need an amplifier.
They need to be Raman amplifiers and we need to say what frequency should we use as the Raman offset.
So we're gonna use one of their channels to transmit the 11 gigahertz for the reference.
It simply could be done if you'd get a catalogue and try to figure it out you'd need about 200 millions to do it,
but who wants to take the trouble to do that if it's probably already existing.
These guys that are concerned about security, I guess they have dark fibres because they want redundancy,
but probably since these are the bankers they don't want us messing around with their fibre network.
So, another idea might be to follow these Russian dreamers with this Molniya orbit.
It goes out quite a way and it's a 12 hours loop.
And if you were at this place the earth looks like this so you can go from Boulder to someplace in Europe just in one shot.
But the velocity of this is incredible.
It's going to do 30,000 kilometres in six hours.
So there's going to be some first order Doppler effects that are hard to get rid of.
Remember, we're going to transmit an optical frequency, we're gonna use a Doppler effect on that,
and you need to do it so accurately that you are in millihertz.
Well that sounds like it's really hard.
So, another idea for this would be to do the experiment deliberately that we wanted to do by getting these linked.
So that STAR Mission is something which has been talked about since 10 years almost.
And the idea is to put two clocks...
No, I need to do it here.
Two clocks in orbit.
One of them is an optical atomic clock and you know, you saw from on the screen, that if you lift up one of the clocks
then the gravitational potential's frequency changes.
But, now what would happen if you use one of Ted Hänsch's light pulses, then this light pulse is going back and forth
and the crystal is laid out.
It's got these lattice planes, and it's really stiff, even if you put it in earth gravity and put it on the end it hardly compresses.
So, I'm of the option that we need some theoretical help about this because there's no doubt
that the general relativity says that length changes, but this is not really length.
This is lattice constants that I want to use as my reference.
There's no gravity effect on the optical field in satellites inertial space, right?
So, I think it would be a really cool thing to do.
Here's the suggestion in my domain of expertise is to send a bunch of light beams around in this mirror
as multi bounce so I get a longer absorbing time.
It's not necessary to talk about all this, but I have
now worked out how, for those of you who have tried to work in the lab with phase modulators,
you discovered that, in addition to phase modulators, they steer the beam and they amplitude modulate the beam.
And I'm here to tell you that there's a good answer for that now.
So, I made a theory for how these clocks would work, and the main thing to look at is that in this column,
about how much power you'd need, it's something in milliwatts.
And how unstable is the frequency in this orange column.
Those are in hertz.
It's something completely less than one hertz.
That means, I solved all the technical problems, but that's what you would get if you dealt with solenoids in an optimal way.
So, that actually works.
Here's a Doppler absorption peak and the saturate absorption peak.
This nice little sharp guy is the same kind of peak as I showed you in the first experimental picture.
This is coming together, people in Bob Byers group in Stanford are working on it.
So, killing off this residual amplitude modulation, the title of the patent application is RAM-Buster,
and figured out a way based on symmetry to do it.
It's not little tweaky things, it's automated, and it uses symmetry just to get a zero as the result.
So, the claim about that is pretty nice, and what I would like to do is to start on a size scale like my lunchbox.
Here are the mirrors ready to be optically contacted onto the cavity.
Here's an optical fibre and the microwave fibre and some little self soldered electronics there.
I could built that whole clock in this box.
No only could. I am building.
And so we're gonna see.
Another important thing which I would like to insist on for people who are just building experience
is to talk to smart people and try your ideas out.
Once in 500 times you'll find a physicist or scientist who is a bad guy and goes and runs with your experiment, idea.
Most of the time they will help you understand more clearly what the good thing is and so I've had a half a lifetime of joy
of interacting with Ted and his...
I've never seen Thomas Udem in a shirt that wasn't red, but anyway, that's his choice.
So we were doing some research, here's my group from a few years ago.
And thank you for paying attention.
(audience applause)