Testing ground for QED calculations'

Interview with Physics Nobel Laureate Theodore Wolfgang Hnsch.

PRECISION spectroscopy of atoms underwent a kind of revolution after Theodore Wolfgang Hnsch of the Max Planck Institute for Quantum Optics, at Garching near Munich in Germany, invented the laser-frequency comb technique. For this major advance in the laser spectroscopy technique, Hnsch was awarded the Nobel Prize in Physics in 2005 along with two others ( Frontline, December 2, 2005). The technique basically uses mode-locked lasers to release a train of (as many as a million) coherent extreme short-duration (femtosecond, or 10 -15) pulses.

Hnsch had realised in the late 1990s that if these pulses could be made to interfere with one another, the interference fringes would be in the form of equally spaced, extremely sharp laser-frequency lines (much like a comb) that could serve as a ruler for comparing and measuring frequencies with high precision.

The hydrogen atom, with an electron orbiting around a proton, is the simplest of all atoms. So Hnsch's group at Garching undertook some very precise measurements of the hydrogen spectrum. One of the very important achievements of the group was the precise measurement of the so-called two-photon 1S-2S resonance transition in hydrogen (as well as deuterium) where the atom absorbs photons and is excited to a higher energy level (2S) from its ground-state energy level (1S). The precision achieved was up to 14 decimal places (accuracy to within 1x10 -14).

In an interaction with a visiting group of Indian journalists in October 2009, Hnsch revealed the essential findings of their surprising measurement of the proton radius that was significantly less than the earlier known value. However, in deference to his request, this finding was not reported then by the visiting journalists, as the results were yet to be double-checked and published as a paper. Excerpts from an interview he gave Frontline:

Using the frequency-comb measurement technique, what is the best limit that you have achieved so far?

We have measured this particular two-photon transition in hydrogen, the 1S-2S resonance, to about 14 decimal places. A new measurement is under way and we hope to get [accuracy up to] 15 digits. Hydrogen is much more difficult than other transitions. The atoms are very light. They cannot be trapped easily. So transit timeline and [line broadening due to] second-order Doppler effects make it very difficult to get very sharp [spectral] lines. So 15 digits is the answer for [the accuracy in the measurement of] frequency measurements.

What about precision tests for quantum electrodynamics (QED), say, through Lamb shift measurements?

If you ask how well we can test QED, then I have to admit that not so well because of the proton. Proton is a composite system made up of quarks and gluons, and the charge radius of the proton or the proton size which affects the energy levels of the hydrogen atom to some extent is only known to be within a few per cent. That was true until very recently. Muon is 200 times heavier than the electron and comes 200 times closer to the proton and so the energy levels depend much more on the size. The hope is that from this Lamb shift, which we have measured, one can determine the size to, say, within one per thousand [1x103 accuracy] and then we can test the higher-order QED calculations much better. It will also give an immediate sixfold improvement of the Rydberg Constant. We are not ready yet [in October 2009] to make an announcement because one has to test everything.

Is any of your work on Lamb shift measurements published?

There is a whole series of publications on the Lamb shift, starting way back in 1975. This muonic Lamb shift, however, is not published yet. One difficulty is that the [proton] radius that we find is not what was expected. That would be a small sensation if that were true but we have to be sure that the theoretical calculations on the radius-Lamb shift relation [in muonic hydrogen] are correct.

How are muonic hydrogen atoms produced and how do you observe the Lamb shift?

In order to observe the Lamb shift, muons are slowed down and captured in a low-pressure gas of molecular hydrogen. We know that when a muon enters a target chamber there would be at most one muon at a time in this target chamber and if it gets stopped and it gets captured by a hydrogen atom, most of these atoms quickly decay to the ground state and they are not useful. But about 1 per cent of the atoms are in metastable excited [2S] state with a lifetime of about of 1 microsecond. Then we have a laser that induces transitions from the metastable state to the nearby 2P state [like the Lamb shift in hydrogen] except that it is not a radio frequency signal but in the mid-infrared, 6 micron.

So we have to run the laser during that short period. We have five, six or seven good events per hour if all goes well. We were successful this time after ten years of effort. Now it is a question of making sure that our calibration is right and theoretical calculations are right, and then we will write a paper.

How much is the difference in the radius?

It is five standard deviations. Now, I think I don't want to say it right now.

What would it imply for QED and the general atomic theory itself?

I think it would make hydrogen a more interesting testing ground for quantum electrodynamic calculations because these uncertainties are out of the way.

If we believe that maybe QED is ultimately correct, we don't know whether the methods used to calculate the levels approximately are correct. In the past, these calculations were done by very few groups. Sometimes, a single person and it is easy to overlook something.

So, to have some real experimental data, to test against the calculations, is good. But if theorists are trying to make more accurate calculations, they have to either go to higher and higher orders in this ordinary perturbation theory expansion in terms of the fine-structure constant or they can try totally new approaches using clever algorithms. Either way, they can make progress developing these calculational techniques with better data.

Does it also have implications for the actual formation of proton as a bound-state quark?

Probably. People who are interested in quantum chromodynamics (QCD) [the theory that describes the interactions between quarks and gluons] would try to understand such a system better. If they know the radius better they can also test their models, but at present they are quite far from that.