Good morning.
I thought I would tell you more about precise spectroscopy.
My interest in that field dates back 40 years and one key motivation has been to do very precise spectroscopy in the simplest of the atoms, in atomic hydrogen.
As scientists of course we all want to discover something important but since any discovery of something new, of course, cannot be known before...
we never know whether what we discover will be important.
But at least we should have the ambition to discover something new that has not been previously known.
And how do you go about that?
One good strategy is to look where no one has looked before.
And, of course, a prime example is in particle physics to look at higher and higher energies.
And we are all eagerly awaiting the press conference at 11 where we will be told what evidence has accumulated for the existence of the Higgs boson.
But some other frontier where you also can look where no one has looked before is in making more and more precise measurements.
Precision measurements play an important role in many fields of science and technology.
And whenever you can measure something to more decimal digits than people before you, you have a chance to look where no one has looked before.
And in all likelihood you will see something surprising.
And if you are very lucky you might even find something fundamentally important.
Well, other examples of fundamentally important things that have been discovered by precision measurements:
Yes, Einstein's new framework for space and time, his special in general theory of relativity, are based on precise measurements.
Another area is quantum physics.
The simple Balmer spectrum of hydrogen has really been the Rosetta Stone for deciphering the strange rules of quantum physics.
And more than once seemingly tiny discrepancies between observation and theory have led to major conceptual breakthroughs including the relativistic quantum mechanics of Dirac
and the discovery of the Lamb shifts, the start of quantum field theories.
The invention of the laser some 50 years ago created tools that allow us to look even much more closely at hydrogen and to see whether this pursuit maybe we will find once more something unexpected.
The race to build the first laser was triggered by a seminal paper written by Art Schawlow and Charlie Townes.
Charlie Townes won the Nobel Prize for masers and lasers together with Basov and Prokhorov from Russia.
And since then there have been quite a few Nobel Prizes awarded for work around the laser.
Gabor for holography, Blumberg and Schawlow for laser spectroscopy.
Chu, Tannoudji and Philips for laser cooling.
Zewail for femtochemistry.
Alferov and Krömer for semiconductor lasers.
Cornell, Ketterle and Wieman for Bose-Einstein condensation of ultra cold gases.
Roy Glauber for quantum optics.
John Hall and myself for this precision work.
And most recently Kao for fibre optics.
And I'm pretty sure that this will continue and that some of you have a chance to be added to this record.
My own work on hydrogen started when I was postdoc with Art Schawlow at Stanford University.
At that time we had developed a widely tunable dye laser that was so highly monochromatic that you could apply Doppler free saturation spectroscopy.
And we applied it to the red Balmer alpha line of hydrogen.
And immediately we could resolve single fine structure components of the line that had never been resolved before optically.
Particularly we could see the Lamb shift directly resolved in the optical spectrum, that's a small energy difference between the S and the P state.
In the S state the electron comes close to the nucleus and it sees the nucleus smeared out because it jiggers around because of vacuum fluctuations.
That was an intuitive explanation given by Bethe.
So, this was the start of an adventure that continues today where we are trying to look at the simplest of all the atoms with ever higher resolution and ever higher measurement accuracy.
We have now reached 15 decimal digits of precision.
And have we discovered something new?
Maybe.
I will tell you in the end.
But I should say this journey over many decades has inspired quite a few new approaches to spectroscopy inventions.
For instance, the original idea of laser cooling of atomic gases was motivated by hydrogen spectroscopy.
And the frequency comb idea as well.
John Hall has shown you how clocks are improving.
Frequency combs make it possible to realise optical atomic clocks.
And these clocks are now so accurate that you can tell,
that you can observe a gravitational red shift if you elevate one clock just by 10 centimetres as shown in the work at NIST in Boulder by Jim Bergquist and co-workers.
In order to make very precise measurements in spectroscopy, of course, you need very sharp lines.
And there is a very sharp, 2 photon resonance in hydrogen, that you can excite with ultraviolet light.
That indeed is sharp.
It should have a natural line width of only 1.3 hertz compared to an optical frequency of 10 to the 15 hertz.
We started to look at this line back in '75 using Doppler free 2 photon spectroscopy, two laser beams coming from opposite directions so that the first of the Doppler shifts cancel.
In our current experiments we are still not using laser cooling.
There are not good convenient sources of Lyman-alpha radiation.
We are cooling atoms by collisions with a nozzle that is at the bottom of a helium cryostat.
These atoms are sent through an optical field in an optical resonator so that they can get excited and some 10 centimetres downstream we check by applying an electric quenching field.
If they are in the excited metastable state they emit Lyman-alpha photons which for a long time we have detected just using a solar blind photomultiplier tool.
If you chop the laser light and look for time delayed signals, you can essentially by velocity selection pick out extra slow atoms.
And so even though the atoms on average are not so cold, you still can reach temperatures, effective temperatures, of 50 millikelvin or so in these types of experiments.
And you can observe a resonance that is not yet 1 hertz but we are in the range of 100's of hertz, the resolution.
To appreciate it we should compare this spectrum with the visible Balmer spectrum that we looked at before.
And if you would display both on the same scale then this 2 photon spectrum could be wrapped around the equator of the earth.
And not only once but about 20,000 times at this resolution.
And so to do justice to such narrow resonances Art Schawlow had good advice.
He said: "You have to measure the frequency of light. Measuring wavelengths runs out of steam at 10 decimal digits."
And he went on to say:
"Whenever you want to measure anything very precisely, try to measure it as a frequency because measuring a frequency,
counting the number of cycles per second is intrinsically a digital procedure that is immune to many kinds of noise." But measuring the frequency of light was a challenge for a long time.
And we worked for about 10 years taking advantage of some large harmonic laser frequency chain at the Physikalisch-Technische Bundesanstalt in Braunschweig
until in the late '90s we actually had a measurement of this hydrogen resonance which at that time was the most accurate measurement of an ultra violet or visible optical frequency.
And from this frequency and other spectroscopic data in hydrogen we could derive a new value for the Rydberg constant.
You could measure the Lamb shift of the ground state of hydrogen and in this way provide a new test for bound state quantum electrodynamics.
And if you believe in quantum electrodynamic theory these spectroscopic measurements could be used to determine the charge radius of the proton and also the structure radius of the deuteron.
But as I said this was 10 years of work and we could have gone sailing in the Caribbean during all that time if we had simply waited for the arrival of the frequency comb.
That was an invention that made life suddenly very easy.
So, we now have this tool with which we can measure any optical frequency with mind boggling precision.
We have a phase coherent link between optical and radio frequencies and we have a clockwork mechanism for optical atomic clocks.
And if you want to find out about this development from my personal perspective you can read the Nobel lecture that has been published.
So, John Hall has already told us how a frequency comb works so I will briefly just remind you.
So, at the heart of it is a mode-locked laser, a femtosecond laser.
And in its most elementary form it would be an optical resonator consisting of 2 mirrors.
And you have a short flash of light bouncing back and forth so this is similar to the light clocks, the gedanken light clock of Einstein, except it's a laser.
So we have some amplifying medium, for instance a titanium sapphire crystal.
And so we can keep it going even though we couple out a train of femtosecond pulses.
The comb nature comes about if we look at the same phenomenon in a different way.
You can argue rather than this flash of light we have a laser that operates in a number of eigenmodes, of standing waves of the resonator.
They are all on all the time.
They differ by the number of half wavelengths and therefore by the frequency.
And whenever all these waves are in phase they interfere constructively and you get a bright flash of light.
And in the dark time between these flashes the modes are still on but they cancel each other through mutual interference.
And thanks to work in the "ultrafast-community" there are tricks that guarantee that you indeed produce short pulses.
And these non-linear optic streaks, such as Kerr-lens modelocking, ensure that the spacing between adjacent comb lines is very, very precisely equal to the pulse repetition frequency.
This has been experimentally confirmed to I think 21 decimals.
Here is an attempt to illustrate a frequency comb mechanically.
So imagine that you have an array of pendulums that at trimmed so that the first pendulum oscillates 30 times per minute, the next one 31, the next one 32 and so on.
And each pendulum corresponds to one of those modes of the laser.
And if they're all in phase, if they're all lined up, that's when the laser emits its short flash of light.
And if I let go then of course quickly they get out of phase and now they can cancel each other through interference.
So the pendulums are still oscillating but we don't see this constructive interference.
In a way from this perspective it looks like it might be chaotic or random.
Not quite, you see that there are patterns, correlations emerging.
And with some practice you could learn to read time from this type of pendulum array.
For instance, now half a minute has gone by.
So one could think of using it as an egg timer in the kitchen or you could use it for meditation in your office.
Or one could have a big one out doors as outdoor art.
And some laboratories have actually built mechanical pendulum arrays.
They have to be all trimmed carefully to eventually end up in phase again.
So now the minute has gone by and this is when the laser would emit an extra short flash of light.
So these are very simple principles but it was still surprising.
Applause.
It was still surprising to most experts how far these principles could be pushed.
That you could take one of these rainbow fibres, broaden the outward spectrum of a titanium sapphire laser to fill the entire visible spectrum and still have a comb.
Our first frequency comb is now a museum piece.
It has been handed over to the Deutsche Museum in Munich.
Some of my students have a company that will sell you a frequency comb as a turnkey instrument.
You can turn it on and forget about it.
It will run continuously for months and gives you a very precise measuring tool.
And what you get is a rainbow of colours but not a continuous spectrum but maybe a million sharp spectra lines evenly spaced precisely by the pulse repetition rate.
And because there is this problem of the slippage of the phase of the electric field relative to the envelop that will move this entire comb by an in principle unknown frequency
which is called fCEO, the carrier envelop of that frequency.
But if you have a rainbow, an octave spanning comb, there are simple tricks, how you can measure this slippage frequency
and if you can measure it, you can also use several controls to make it go away.
In the future maybe we will see miniaturised frequency combs.
The cover of Nature Photonics of July of this year shows some toroidal microcavities ground out of some optical crystals.
And these can sustain optical modes and by cascaded four-wave mixing you can produce frequency combs.
The size of such a cavity can be millimetre or down to 10's of micron.
So we can expect that one day we have frequency combs in our wrist watch.
How do you use such a comb to measure the frequency of hydrogen?
There is a poster that was published on the occasion of the Nobel Prize that shows how you might go about it.
You have a tunable laser, you send some of this sharp laser line to the hydrogen atoms and see whether they find the light exciting.
The atoms of course only get excited if you hit this very sharp resonance.
And then you send the other part of the beam to the frequency comb and you look at the beat node between this laser frequency and the nearest comb line
whose frequency you can know precisely in terms of a caesium atomic clock.
And so from the beat node you know how far away are you from that precisely known reference line.
In reality the experiment looks a little bit more complicated.
What you see is a look into our lab that has evolved over decades now with the hydrogen atomic beam in the foreground, a laser table in the background.
And in the early work we used a dye laser, now we use all solid state semiconductor laser systems.
Of course we need a reference
and so far we've been lucky that colleagues from France have agreed to bring the world's most accurate transportable caesium fountain clock, a microwave clock, to Garching.
Here is Michel Abgrall from the observatory who is adjusting some diode lasers for laser cooling of caesium atoms.
Caesium atoms in a fountain thrown up and fall back through a microwave cavity.
Here is our most recent team: Christian Parthey, who might be here, I haven't seen him yet, Arthur Matveev, Nikolai Kolachevsky, Janis Alnis, postdocs.
And they have made a new measurement of this hydrogen frequency now with an uncertainty of 4.2 times 10^-15.
Again by comparing the hydrogen resonance to the caesium fountain clock published late last year.
The results of that experiment are compared to earlier measurements done in 1999.
The very first measurement of any optical frequency with a comb when an experiment in 2003 where we realised there are some troublesome systematic effects.
Because on a given day we get rather small statistical errors
but if we look at measurements from different days there are fluctuations that apparently are caused through some uncontrolled systematic errors.
So I was relieved to see that the new experiment doesn't suffer from these big errors.
And that we have almost, that we are almost within the error limits of the old measurement.
There is a yet unpublished, even more recent set of data shown here with even smaller statistical errors.
This was a measurement done in November of 2010.
At that time we no longer had the caesium clock from Paris.
But we had for the first time operating a frequency link, an optical fibre link between the Physikalisch-Technische Bundesanstalt in Braunschweig and our laboratory in Munich.
This is a link using existing communication fibres, altogether 920 kilometres.
We need to worry about the height difference of our 2 laboratories which amounts to a gravitational red shift of 4.4 times 10^-14.
Katherina Predehl just received her doctorate last month and she's first author of a paper that appeared in Science recently describing this link.
So in essence we have 2 clock lasers that we want to compare.
We use frequency combs to lock some single frequency lasers in the communications band at 1.5 micron to this optical standards.
And we send this light through the fibre.
We need amplifiers.
There are 9 amplifiers along the 900 kilometres.
And there are actually 2 independent bi directional links that are needed for characterisation.
And we were very pleased to see...
Here we look at the so called Allan deviation, the fractional instability versus integration time.
We were very pleased to see that we reach fractional instabilities of 10^-18 in measurement times of less than 20 minutes.
If you want to compare 2 distant optical clocks, I mean this will definitely be the method of choice.
Optical clocks of course have to be characterised for much longer times to reach this 10^-18 limit.
And now I am optimistic that we will be able to get even better results soon because after this experiment was completed people in our lab, Arthur Matveev, tried a new detector.
Rather than having the photomultiplier for looking at Lyman-alpha, he used graphite spray paint to coat the interior of a metal cylinder and used it as a photo cavern.
So, in this way we collect not a small solid angle but essentially Lyman-alpha photons emitted in any direction.
And you can see that the statistical errors can be very much reduced.
Which means in the future we will be able to work at much lower laser intensities.
No longer big uncertain light shifts.
We will be able to select velocities of our atoms to have better control of second order Doppler shifts.
And it's not inconceivable that one can redefine the second in the future with the help of a hydrogen clock.
Of course also comparing clocks based on different transitions allow us to ask questions whether fundamental constants are constant.
In the next couple of years I expect to see the first spectroscopy of antihydrogen.
2 teams at CERN have now succeeded in magnetically trapping antihydrogen atoms made up of antiprotons and positrons and holding them in a magnetic trap.
Of course, it will be interesting to see if there are any differences between the 2.
But there is one aspect that is not so satisfying.
If we want to measure fundamental constants, in particular the Rydberg constant, we see that this seems to be stuck.
This is the NIST web page of the CODATA value.
It seems to be stuck with relative uncertainty of 6 times 10^-12.
And the reason for that is: In order to extract this constant from our hydrogen frequency, we need to know the charge radius of the proton.
And this is unfortunately not very well known.
So for more than 10 years we have been participants in the collaboration, trying to find out how small is a proton by looking not at ordinary hydrogen
but at special exotic man made hydrogen atoms where the electron is replaced by a muon, 200 times heavier lepton.
And in the summer of 2009 part of that collaboration had something to celebrate
because for the first time the Lamb shift between the 2 F and the 2 P state in this muonic hydrogen atom had been observed in experiments at the Paul Scherrer Institute in Switzerland.
This is an international collaboration that has grown to more than 30 scientists from 12 different institutions.
Randolf Pohl from our laboratory is co spokesman together with Franz Kottmann from the ETH.
And the results are shown here.
So you tune a lasers, the Lamb shift is in the infrared region.
All the energy levels are scaled up because the muon comes much closer to the proton.
And the Lamb shift is dominated by vacuum fluctuations.
And it's in the wavelength range of 6 micron or in the frequency range of 49 terahertz.
And we had made calculations where the resonance should occur if you believe in the proton radius as determined by electron scattering experiments.
Or if you believe in the official CODATA proton radius which is heavily dominated by spectroscopy in hydrogen.
And the result was that the resonance is not where expected, but right outside.
And if you translate that into the charge radius of the proton, this is the official CODATA value from this Lamb shift measurement,
we get a radius known to better than 1 per mil, if the theory is correct.
But way outside the error bars.
The proton has shrunk by something like 4%.
If we look at the Rydberg constant that we get from this new proton radius and our hydrogen frequency, we find also something more precise but outside the error limit.
This measurement has not been taken into account in the most recent adjustments of the fundamental constants because people don't trust the muonic hydrogen spectroscopy.
This proton size puzzle has stirred the attention of journalists, cartoonists.
In 2010 Physics World counted this measurement as one of the 10 top breakthroughs, but in place number 9.
La Recherche also counted it among the top 10 but in 7th place.
And in Scientific American there was some comment which summarises the situation that we are in now.
The puzzle is not solved and "...Whatever the answer physicists will most likely have plenty to keep scratching their heads about for years to come".
I have one minute left so let me very briefly say that frequency combs and hydrogen play a role not only at the scale of the proton but also at the scale of the universe.
Because in the universe we have hydrogen, atomic hydrogen, gas clouds between the galaxies.
We can observe them by looking at a forest of Lyman-alpha lines in the light of distant quasars.
So there is a dense forest of lines.
All the same line, all the vacuum ultraviolet Lyman-alpha line but shifted by different amounts because of different distances of these clouds.
And so now image if I try to measure the centres of gravities of these lines today and come back say in 10 years and see are they still there or have they shifted.
And if we can calibrate astronomical spectrographs well enough we should have a chance to see a shift at the rates compatible with the models of cosmologists.
And frequency combs are now being recognised as interesting calibration tools for astronomical spectrographs.
Tobias Wilken from our lab has very recently published a paper demonstrating the performance of such an astrocom that might be enough to detect earth like planets around sun like stars
and maybe eventually also evidence for the expansion of the universe.
Here, my last slide I will skip.
I will conclude with some words from Charlie Townes, since we started with him.
In his book "How the Laser Happens" he says:
"There is much that we don't understand and in many cases we don't understand that we don't.
And the really surprising discoveries will probably depend primarily on individuals, not teams or committees."
So with this plead for curiosity driven research I've reached the end and I thank you very much.
Applause.