Symmetry is a splendid thing in nature. It can help animals appear more attractive to potential mates, and it makes diamonds especially brilliant. In the quantum world, symmetry represents something a little different: the ability of a particle or atom’s properties to remain the same despite various exchanges, rotations, or reflections. But it’s no less important; symmetry is crucial for scientists trying to build and understand exotic futuristic materials.
Now, an international team of researchers say they've observed the first direct evidence of symmetry in the magnetic properties -- or nuclear “spins” -- of atoms, a feat that physicists around the world have been pursuing. Their findings are described in Science Express this week.
The key to the discovery was an incredibly stable laser, the kind scientists use to measure the properties of the world’s most precise and stable atomic clock. Unlike a wristwatch from a brand of ill repute, these clocks keep perfect time because they’re regulated by vibrations at the level of atoms and molecules. For this study, the researchers used an atomic clock made of 600 to 3,000 strontium atoms trapped by the laser light. They watched what happens when the strontium atoms within the clock collide, in hopes of finding a hint of spin symmetry.
Strontium atoms have 10 possible nuclear spin configurations that influence magnetic behavior. The team analyzed how atom interactions (or collisions) at the two electronic energy levels used as the clock’s “ticks” were affected by the spin state of the atoms’ nuclei. Electronic and nuclear states are coupled in most atoms, but in strontium, this coupling vanishes and collisions are independent of nuclear spin states, making the element a useful test subject.
Using lasers and magnetic fields to manipulate nuclear spins, the team observed that when two strontium atoms have different nuclear spin states -- no matter which of the 10 states they have -- they will collide strongly, and with the same strength (pictured above, yellow with green). On the other hand, when two atoms have the same nuclear spin state -- regardless of what that state is -- they will interact much more weakly (pictured above, yellow with yellow).
"Spin symmetry here means atom interactions, at their most basic level, are independent of their nuclear spin states,” says Jun Ye from the University of Colorado Boulder in a news release. “However, the intriguing part is that while the nuclear spin does not participate directly in the electronic-mediated interaction process, it still controls how atoms approach each other physically. This means that, by controlling the nuclear spins of two atoms to be the same or different, we can control interactions, or collisions.”