Scientists Time Titanium-49 Atom's Magnetic Pulse Between Quantum States

Timing an atom's magnetic pulse

Scientists have measured, in the laboratory, the rhythm of an atom's magnetic core as it switches repeatedly between quantum states.

Using a scanning tunnelling microscope, physicists tracked electrons moving in step with the nucleus of a titanium-49 atom. This enabled them to calculate how long the isolated nucleus's magnetic pulse lasts.

"These findings," they write in their paper, "give an atomic-scale insight into the nature of nuclear spin relaxation and are relevant for the development of atomically assembled qubit platforms."

Nuclear spin and quantum computing

Physicists use the word spin for the quantum form of angular momentum. As well as being central to how magnets behave, spin commonly underpins quantum computing, where it can serve as an information 'bit' called a qubit.

A nucleus's total spin is produced by numerous subatomic particles in a quantum whirl. Yet the way these combined spins flip between configurations is readily affected by an atom's environment. Establishing the properties of this collective spin state before its surroundings disturb it could offer engineers another type of qubit to use.

However, measuring a nucleus's spin state without changing it presents a significant challenge. A group led by Delft University of Technology physicists Evert Stolte and Jinwon Lee, in the Netherlands, therefore considered whether an atom's electrons could act as an indirect indicator.

Several years earlier, scientists had established that the hyperfine interaction between electrons and their nucleus could provide such a guide, avoiding direct disruption of the nucleus's magnetic motion.

"The general idea had been demonstrated a few years ago, making use of the so-called hyperfine interaction between electron and nuclear spins," explains physicist Sander Otte of the Delft University of Technology. "However, these early measurements were too slow to capture the motion of the nuclear spin over time."

Titanium-49 measurements with a scanning tunnelling microscope

To address that limitation, the team devised a pulsed measurement method. Rather than examining an atom with a known nuclear spin continuously, their scanning tunnelling microscope took brief measurements separated by pauses.

For the experiment, they selected titanium-49, a stable isotope of titanium found naturally. It is widely used in nuclear physics studies because its nucleus has notable magnetically responsive properties and a strong spin, which researchers can manipulate to investigate how atomic nuclei behave.

With this pulsed approach, Stolte and Lee watched the atom switch in real time through the readout on their computer display. They found that each switch was separated by roughly five seconds - a measurement they could make more quickly than the nucleus oscillated.

"We were able to show that this switching corresponds to the nuclear spin flipping from one quantum state to another, and back again," Stolte says. "The first step in any new experimental frontier is being able to measure it, and that is what we were able to do for nuclear spins at the atomic scale."

The study has been published in Nature Communications.