Electron Spins Alone Push a Centimeter-Wide Levitating Diamond, a First, Okinawa Physicists Report

Researchers at the Okinawa Institute of Science and Technology have moved a levitating, centimeter-wide diamond using nothing but the force generated by electron spin. They say it is the first time a quantum effect has been seen directly manipulating an object subject to gravity. The findings were published in Science Advances.

Quantum mechanics underpins lasers, MRI scanners, semiconductors and quantum computers, but the field has mostly dealt with phenomena at or below the atomic scale. Showing quantum effects in objects massive enough to be meaningfully influenced by gravity has been hard, because large objects are much harder to shield from heat and vibration than nanoscopic particles are.

"There have been many efforts to test whether quantum mechanics holds for anything larger than a few tens of nanometers, so far without success," said Professor Jason Twamley of the OIST Quantum Machines Unit. "Now, we have observed a classical mechanical response to a quantum force on an object that is eight to nine orders of magnitude more massive than the current state-of-the-art spin-mechanical experiments."

The setup combines smaller devices the same team demonstrated recently. A diamagnetically levitated graphite plate fitted with a mirror connects through a carbon rod, which passes through magnetic shielding, to a diamond hanging above a magnet. The diamond contains billions of nitrogen-vacancy centers, defects that trap unpaired electrons and act as tiny, controllable quantum magnets.

Periodically shining a green laser on the diamond polarizes those centers into a set spin state. That generates small magnetic fluctuations that push the diamond downward toward the magnet. The researchers tracked the motion with an interferometer that bounces a laser off the small mirror on the graphite plate and measures changes in distance with picometer precision.

First author Anshuman Nayak, a Ph.D. student in the unit, said most groups start with very small levitated objects and add mass until gravity matters, but optical traps struggle to hold macroscale objects. His team works in the opposite direction, going from large to small, because diamagnetic levitation, the same principle behind maglev trains, can lift centimeter-wide objects. Co-author Daehee Kim noted that nitrogen-vacancy centers have some of the longest known coherence times, letting them keep quantum superposition at room temperature, which makes them attractive for creating superposition of motion in a large object.

The team says the result sets a new baseline for studying the boundary between classical and quantum physics and could lead to extremely accurate sensors for dark matter, gravitational waves and other exotic phenomena. The long-term goal is to put a massive object into quantum superposition and test whether gravity is quantum or entirely classical. "We've shown a large classical response from a small quantum effect," Twamley said. "It's no longer a question of whether such technology is possible, but of how we can refine experimental conditions."