Measuring a fundamental property that the Standard Model of particle physics says should be zero might seem like the ultimate waste of time. But if the electron does have a non-zero electric dipole moment (EDM), it would have profound implications and point to new physics. Now, Jony Hudson and colleagues at Imperial College London have made the most precise measurement of the EDM yet, reducing its known upper limit by about 50% – and providing more evidence that it is either zero or extremely small.

The Standard Model, in its simplest form, prohibits the electron from having an EDM because this would violate time-reversal symmetry. While more sophisticated versions of the Standard Model do allow for an EDM, they nevertheless suggest it would be much too small to measure in the lab. Although Hudson's team has only been able to put an upper limit on the EDM, it claims the new technique could be refined to search for an EDM 100 times smaller still.

Polarized molecules

In their method, the researchers studied the outer (or valence) electrons in ytterbium monofluoride (YbF) molecules. The molecules are exposed to an electric field, which polarizes the molecules. This polarization creates a very large local electric field in the vicinity of the valence electrons. If the electrons have an EDM, then they too would be polarized by this large local field.

But instead of seeking to measure a tiny EDM directly, Hudson and colleagues tried instead to measure the effect that the polarization would have on the electron energy states of the molecules. They began with a pulse of ultracold molecules that had been set into a superposition of two quantum states. The molecules were passed between two parallel plates where electric and magnetic fields can be applied. The molecules are then detected as they emerge from the plates.

In the presence of just a magnetic field, the relative phase of the two quantum states is rotated. Varying the strength of the magnetic field causes quantum interference between the two states and the result is a series of interference fringes at the detector.

Switching the electric field on should only affect this interference pattern if the electron has an EDM because this would introduce a separate phase rotation. To test for this, the team looked for changes in the interference pattern that were correlated to changes in the applied electric field. This was done for 25 million pulses of YbF and found no evidence of a phase shift related to an EDM.

Less than a hair's width

This allowed the team to place an upper limit on the EDM of 10.5 × 10–28 e cm with 90% confidence. According to the researchers, this means that if the electron were magnified to the size of the solar system, its EDM would be no bigger than the width of a human hair.

This is about 50% better than previous measurements using thallium atoms and the team believes that it could soon improve the result by as much as a factor of 100. The researchers are currently trying to cool the YbF molecules to even lower temperatures and gain better control of the pulses as they pass through the experiment.

The research is reported in Nature 473 493.