Lasers trap radioactive radium

25 May 2007

A team of researchers in the US say they have optically trapped radium for the first time.

Researchers in the US have come up with just the right optical set-up to laser cool and trap radium atoms in a magneto-optical trap. Having overcome a list of problems that makes the radioactive element so difficult to trap, the team now hopes its result will aid fundamental physics studies – such as why there is more matter than antimatter in the universe (Physical Review Letters 98 093001).

"Radium's atomic structure is not that favorable for laser cooling," Jeffrey Guest from the Physics Division of Argonne National Laboratory told optics.org. "We were forced to use an inefficient laser-cooling transition and a novel repumping scheme to capture these atoms. Because radium is radioactive, there is never much of it around and it has no stable isotopes. We used only a few tens of nanograms of radium-225."

Guest and colleagues used a Ti:sapphire ring laser referenced to molecular iodine lines to laser cool their radium atoms. The set-up also included an external cavity diode laser emitting at 1429 nm to repump and excite the atoms back into the desired atomic state.

The researchers also found that blackbody radiation helped to repump their radium atoms. "This mechanism may be helpful in trapping other atoms with a complex structure," added Guest. "It extended the lifetime of the trap from milliseconds to seconds. We think that this is the first demonstration of blackbody radiation being used an effective repump source."

But why go to all this trouble to trap radium? "The primary application is to test time-reversal symmetry," explained Guest. Time-reversal symmetry is needed to explain why there is more matter than antimatter in the universe, but evidence for this effect has not yet been detected.

"We can use the optical apparatus to look for electric dipole moments (EDMs). The presence of an EDM would violate time-reversal symmetry," said Guest.

Although there are other ways of searching for EDMs, Guest explains that these have all consistently yielded the same result: no EDMs. The Argonne team is now hoping to expand its optical approach.

"In order to make an EDM measurement, we need to transfer the cold atoms from the magneto-optical trap (where we can capture them now) to an optical dipole trap (optics tweezers) and move them into a region designed for this precision measurement," explained Guest. "This transfer is the next step."

But Guest is under no illusions that there are technical challenges ahead. "In over 50 years of EDM measurements, no-one has attempted to measure an EDM in an optical trap," he concluded. "We expect to wrestle with new systematic effects which have not been encountered before."