Physicists Measure Quantum Effects in Energy Structure of Antihydrogen


In 1947, American physicist Willis Lamb and his colleagues observed an incredibly small shift in the energy levels of the hydrogen atom as the atom’s electron and proton interacted with vacuum. Under traditional physics theories of the day, the Lamb shift shouldn’t have occurred. 

The ‘nothing’ of vacuum shouldn’t influence the atomic behavior of hydrogen. The discovery spurred the development of a new quantum electrodynamics theory to explain the discrepancy, and won Lamb the Nobel Prize in Physics in 1955. Now, physicists with the ALPHA Collaboration at CERN have detected and measured the Lamb shift in antihydrogen, the antimatter counterpart of hydrogen.

ALPHA is an international antimatter research collaboration based at CERN, comprised of 50 scientists from eight countries.

It creates antihydrogen atoms by binding antiprotons delivered by CERN’s Antiproton Decelerator with positrons (antielectrons). It then confines them in a magnetic trap in an ultra-high vacuum, which prevents them from coming into contact with matter and annihilating. Laser light is then shone onto the trapped atoms to measure their spectral response.

This technique helps measure known quantum effects like the so-called fine structure and the Lamb shift.

“The observation of the Lamb shift in hydrogen was a landmark event in the history of modern physics,” said Dr. Makoto Fujiwara, a physicist with TRIUMF — Canada’s particle accelerator center and a member of the ALPHA Collaboration.

“I think every physicist who has worked in an antimatter experiment has dreamt of measuring this effect in antihydrogen. We’re thrilled that we’ve finally been able to do just that, thanks to new advanced laser technology.”

Both the fine structure and the Lamb shift are small splittings in certain energy levels (or spectral lines) of an atom, which can be studied with spectroscopy.

The fine-structure splitting of the second energy level of hydrogen is a separation between the so-called 2P3/2 and 2P1/2 levels in the absence of a magnetic field.

The splitting is caused by the interaction between the velocity of the atom’s electron and its intrinsic (quantum) rotation.

The ‘classic’ Lamb shift is the splitting between the 2S1/2 and 2P1/2 levels, also in the absence of a magnetic field. It is the result of the effect on the electron of quantum fluctuations associated with virtual photons popping in and out of existence in a vacuum.

The ALPHA team determined the fine-structure splitting and the Lamb shift by inducing and studying transitions between the lowest energy level of antihydrogen and the 2P3/2 and 2P1/2 levels in the presence of a magnetic field of 1 Tesla.

Using the value of the frequency of a transition that they had previously measured, the 1S-S transition, and assuming that certain quantum interactions were valid for antihydrogen, the scientists inferred from their results the values of the fine-structure splitting and the Lamb shift.

They found that the inferred values are consistent with theoretical predictions of the splittings in hydrogen, within the experimental uncertainty of 2% for the fine-structure splitting and of 11% for the Lamb shift.

“Within the precision of our experiment, the Lamb shift of antihydrogen is identical to that of a regular hydrogen atom,” said Dr. Takamasa Momose, a researcher at the University of British Columbia and a member of the ALPHA Collaboration.

“The work confirms that a key portion of quantum electrodynamics theory holds up in both matter and antimatter. It takes us a step closer to understanding the differences between matter and antimatter, and why so much antimatter vanished after the Big Bang.”

“Finding any difference between these two forms of matter would shake the foundations of the Standard Model of particle physics, and these new measurements probe aspects of antimatter interaction — such as the Lamb shift — that we have long looked forward to addressing,” said Dr. Jeffrey Hangst, spokesperson for the ALPHA experiment.

“Next on our list is chilling large samples of antihydrogen using state-of-the-art laser cooling techniques. These techniques will transform antimatter studies and will allow unprecedentedly high-precision comparisons between matter and antimatter.”


M. Ahmadi et al. Investigation of the fine structure of antihydrogen. Nature 578, 375-380; doi: 10.1038/s41586-020-2006-5