"NV diamonds are well understood and easy to control," said co-author Daehee Kim, a Ph.D. student in the unit. "NV centers have some of the longest known coherence times, allowing them to maintain quantum superposition at room temperature much longer than other systems."

How do you keep a centimeter diamond floating?

Holding the diamond in place required magnetic levitation. The team suspended a graphite plate above a magnet using diamagnetic repulsion — the same principle that lifts maglev trains. A carbon rod ran from the graphite through magnetic shielding up to the diamond, which hovered above a separate magnet.

A laser interferometer reflected light off a small mirror on the graphite plate and measured the diamond's motion with picometer precision — displacements smaller than the diameter of a hydrogen atom. Tracking the position so precisely confirmed that electron spins, not stray vibrations, caused the movement.

Why does scale matter for quantum physics?

Quantum mechanics and general relativity each describe the universe well — but they clash when objects grow heavy. To probe whether gravity itself is quantum, scientists must place massive objects into superposition, meaning the objects exist in two places at once, and hold them still enough to measure.

First author Anshuman Nayak, a Ph.D. student, said the typical approach has been to start small and work up. Optical traps, which use focused beams of light to hold particles, struggle with anything heavier than a speck.

"To test the quantum nature of gravity, we ultimately need to put objects with large enough masses into quantum superposition," Nayak said. "These objects need to be levitated in a vacuum to minimize the influence of environmental noise."

The OIST team took the opposite route: start with a centimeter-scale object and work down. Diamagnetic levitation can hold a diamond weighing 100 milligrams, where gravity is strong but quantum effects have not yet been seen.

What could this lead to?

The platform could host sensors for dark matter, gravitational waves, and other phenomena too faint for today's instruments. Twamley framed the result as a stepping stone toward a long-standing dream: placing a macroscopic object into quantum superposition under ordinary gravity.

"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 to achieve quantum superposition within the regime of Einstein's general relativity."

On the way, the team is developing a new class of extremely precise sensors. As Twamley put it: "We're pushing the bar from nanometers to centimeters. All we need is another order of magnitude, and we can finally observe Schrödinger's cat in real life."

The team now aims to push a heavier diamond into a genuine quantum state, where the crystal itself exists in two positions at once.