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ScienceWise - Autumn 2010

QED. We Think?

Article Illustration
Atoms can be given a small momentum kick when they absorb a laser photon in an electron energy transition. If the laser is tuned slightly away from the transition energy the laser photons will not be strongly absorbed. If however, the atoms are moving towards the laser, the transition energy will be slightly Doppler shifted towards the photon energy. This increases absorption and thus momentum transfer to the atom in effect slowing it down or cooling it. Once the atom has slowed, the Doppler effect is removed and absorption ceases.
Article Illustration
Members of the research team with an atom trap

Putting Quantum Electrodynamics to the Test

Quantum Electrodynamics (QED) is a complex theoretical description of the way collections of electrically charged particles interact with photons. For example when an electron decays from an excited level in an atom, QED can predict with amazing accuracy the energy of the photon that will be emitted.

QED is remarkable in that it has been one of the most successful theories in the history of physics. Since it’s development in the 1920s it has also been one of the most extensively tested with literally thousands of diverse experiments probing the various predictions of QED. And in every case QED’s predictions turned out to be spot on.
For this reason it came as a huge surprise to scientists when recent experiments showed a small discrepancy between the measured fine structure of the 23P triplet state of helium and the predictions of QED.  As other researchers confirmed that the measured energy level is slightly, but significantly different to the predictions, the scientific mystery became more and more intriguing.

Amongst the international scientists taking note of this was Professor Ken Baldwin of The Australian National University. Professor Baldwin is part of an experimental laboratory led by Dr. Andrew Truscott, comprising research fellow Dr. Robert Dall and postgraduate students Sean Hodgman and Lesa Byron.  Dr. Truscott  and his team are one of only four world-wide that have succeeded in cooling metastable helium into the quantum realm where Bose-Einstein condensation (BEC) occurs.  Since BEC techniques employing laser cooling and magneto-optic trapping are ideal for confining and isolating a cloud of helium atoms in ultra-high vacuum for extended periods, the team decided to use the facility to investigate the anomalous 23P state.

This particular experiment aims to make precise measurements – for the first time - of the lifetimes (as opposed to energies) of the 23P1 and 23P2 states as they decay to the helium ground state.  In addition, the measurements set an upper bound on the decay of the 23P0 state which is predicted to be completely forbidden. 

The same technique also enabled the most accurate measurement yet of the lifetime of the 23S1 state, which at ~8000 s is the longest lived neutral atomic state thus far discovered.  This ‘metastable’ atomic state  is important in wide range of technological devices and naturally occurring phenomena, since like the 23P states it is highly energetically excited (containing some 20 electron volts of energy as well as being very long lived). Professor Steve Buckman, an electron physicist who was involved in the metastable lifetime measurements, comments that “metastable helium is important to the physics of the ionosphere, gas discharges and gas lasers as it enables the ‘storage’ of large amounts of energy for long times.”

The tremendous advantage of the ANU facility is that it can routinely trap an atomic cloud for several minutes – a comparative eternity on the atomic scale. The decay of these states to the ground state results in the emission of a characteristic extreme ultraviolet (XUV) photon. So if you can hold your helium atoms for a long enough time in a small space under ultra high vacuum, you can count the emitted XUV photons and deduce the decay time. The longer you can hold the atoms in an undisturbed environment, the more accurate the measurements will be.

The lifetime results measured at ANU were in precise agreement with the predictions of QED.  “These lifetime measurements are a reassuring validation of QED theory,” Professor Baldwin says, “But the discrepancies in the helium 23P energies measured elsewhere by other groups remains. And because those measurements have been done by different people using different methods it would appear that the anomaly is real.

“Both experimentalists and theorists need to work hard to resolve this.”

Theoretical calculations of the energy levels of multi-electron atoms are hugely complex and often expressed in terms of the summation of an infinite sequence. It’s possible that assumptions about the convergence of these series or the cancellation of higher-order terms are not correct. Alternatively it’s possible that this small anomaly is our first glimpse at something new and exciting. Observations of tiny discrepancies in the orbit of Mercury in the nineteenth century that couldn’t be explained by Newtonian mechanics turned out to be our first glimpse of relativity, although no one knew it at the time.

No one is quite sure of what’s going on yet, but Professor Baldwin believes there’s a possibility that it might be something exciting. “The energy level measurements enable physicists to measure the atomic fine structure constant (alpha) with unbelievable precision. One thing physicists are interested in doing is making these measurements several years apart to determine if alpha has changed. One of the great questions in physics is, ‘Are the fundamental constants of the universe really constant?’ And this may well be a good test of that.”

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