Sizing Up an Electron’s Shape

Some of the biggest questions about our universe may be solved by scientists using its tiniest particles. Since the 1960s, physicists have been looking at particle interactions to understand an observed imbalance of matter and antimatter in the universe. Much of the work has focused on interactions that violate charge and parity (CP) symmetry. This symmetry refers to a lack of change in our universe if all particles’ charges and orientations were inverted. “This charge and parity symmetry is the symmetry that high-energy physicists say needs to be violated to result in this imbalance between matter and antimatter,” explained JILA research associate Luke Caldwell. To try to find evidence of this violation of CP symmetry, JILA and NIST Fellows Jun Ye and Eric Cornell, and their teams, including Caldwell, collaborated to measure the electron electric dipole moment (eEDM), which is often used as a proxy measure for the CP symmetry violation. The eEDM is an asymmetric distortion of the electron’s charge distribution along the axis of its spin. To try to measure this distortion, the researchers used a complex setup of lasers and a novel ion trap. Their results, published in Science as the cover story and Physical Review A, leveraged a longer experiment time to improve the precision measurement by a factor of 2.4, setting new records. 

Measuring the eEDM

To understand how physicists measure the electron's electric dipole moment, it may be helpful to consider a clinical trial for a new medication. To ensure the trial is effective, doctors will run a study where half the sick participants take the drug in question and the other half take a placebo.  If the doctors see an improvement in patients that took the drug compared to the placebo group, they can conclude that their medication is effective. This approach helps to control for effects that impact both groups. Now imagine an (admittedly dystopian) world where the researchers have created an ‘anti-drug,’ shown to make sick patients worse by the same amount as the regular drug improves their health.  A new clinical trial could be organized, where half of the patients take the regular drug and the others take the anti-drug. The new trial would have all of the benefits of the previous trial but any effects of the drug would be even more clear. This drug and anti-drug analogy can then be applied to the electron symmetry.
 As Caldwell explained: “We look for the energy shift of an electron subject to an electric field in one direction [“aligned” electron] by comparing it to an electron subject to an electric field in the opposite direction [“anti-aligned” electron], where the energy shift caused by the eEDM is equal and opposite. By measuring both simultaneously we are protected from effects which shift the energy of both electrons in the same direction.”  In measuring the difference between the aligned electron and the anti-aligned electron for each energy oscillation between the particles, the researchers could determine a value for eEDM. 

To measure this energy difference, the researchers manipulated hafnium fluoride ions in an ion trap. The experiment began with a solid rod of hafnium in the experimental chamber.  A pulsed laser was then used to isolate hafnium in the presence of sulfur-hexafluoride gas, where the two react to create neutral hafnium-fluoride molecules.  Then the molecules flew down a tube where they enter the ion trap. “The entire cloud of gas enters the ion trap at about the same time,” JILA graduate student Trevor Wright stated. “When it reaches the center of the trap, we turn on the ionization lasers. These lasers each emit a pulse of light that overlaps with the cloud of gas and are tuned to certain frequencies which resonate with hafnium fluoride. So, the hafnium fluoride molecules flying through get ionized, and lose one of their electrons.  While this is happening, we turn on the voltages on our electrodes to stop the positively charged hafnium fluoride molecules, while the rest of the cloud will fly through the trap and out of the experiment.” Using this process, the researchers could prepare the system for further studying hafnium. 

New Records in Measurement

A new record was set for the length of “interrogation time” for the experiment—how long the researchers could trap and manipulate the electrons— at three seconds. While this may seem like a short amount of time, most quantum physics experiments run from femtoseconds (10⁻¹⁵ seconds) to nanoseconds (10-9 seconds), making three seconds seem like an eternity. Expanding on why the interrogation time is helpful in improving the measurement precision, Caldwell explained: “Think of a pendulum. If you wanted to measure the time period of a pendulum, you could just measure its swing once, and then stop its motion. But you’d have some error when you press stop. So, a better way to do it would be to let the pendulum swing 100 times, and then press stop, and then divide your answer by 100. Then, you get to divide your measurement error by 100, and you get a much better measurement of the pendulum’s period. Our experiment is kind of similar, we are looking for an oscillation that corresponds to the electrons EDM. In our case, the measurement error doesn't come from when we press stop, but the same ideas apply. We get to divide our ‘error bar’ by how many periods of oscillation we measure. Compared to the previous generation of this experiment, and, to our competitor experiments, we can keep our molecules trapped for a very long time. So, we can measure lots and lots of oscillations.” As previous experiments clocked interrogation times at three-quarters of a second, the expanded time of three seconds was a significant leap in advancing the interrogation time of this experiment and allowing for more flexibility in measurement. “We can hold on to our particles for a really long time as compared to previous experiments,” Wright said. “And we can vary the hold time because we can stop the experiment whenever we want.” Caldwell echoed this benefit: “Unlike other experiments, because our molecules are trapped rather than in a beam, we can control the length of the interrogation time. This allows us to better characterize and reject many types of systematic error that can affect the measurement.”

Thanks to this longer interrogation time, the researchers were able to make the most precise eEDM measurement yet. “Our result was consistent with zero and we used it to set an upper bound on the size of the eEDM,” Caldwell stated. “Previous experiments have also measured the electron EDM, but with less precision. Because our error bar is smaller, we can say, with more confidence, that its value is below a certain level. Our limit is 2.4 times smaller than the previous limit.” The researcher hope to continue pushing this measurement even further to reveal more about the quantum world. 

Written by Kenna Hughes-Castleberry