First observation of quantum spins shifting a centimeter-scale object in the lab

Modern technological breakthroughs like lasers, MRI scanners, semiconductors and quantum computers rest on the study of quantum mechanics. However, the field has predominantly focused on (sub)atomic phenomena far removed from our human senses.

Demonstrating quantum phenomena in objects massive enough to be meaningfully influenced by gravity has proven difficult, as large objects are significantly harder to isolate from environmental factors, such as heat and vibration, than nanoscopic particles are.

But now, researchers from the Okinawa Institute of Science and Technology (OIST) have moved a levitating, centimeter-wide diamond using the force generated by electron spin alone, the first time a quantum effect has been observed directly manipulating an object subject to gravity.

Published today in Science Advances, their findings set a new baseline for studying the boundary between classical and quantum physics, opening new avenues for exploring the fundamental nature of our universe as well as developing a new class of extremely accurate sensors for detecting dark matter, gravitational waves and other exotic phenomena.

Professor Jason Twamley of the OIST Quantum Machines Unit explains, “There have been many efforts to test whether quantum mechanics holds for anything larger than a few tens of nanometers, so far without success. 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.”

Unifying physics with magnets, an impure diamond and a bit of graphite

Classical and quantum physics have long been divided over questions of scale. Gravity is central to the theory of general relativity in classical physics, yet its effects are extremely weak at the nanometer scale, where quantum phenomena have been observed. Conversely, positional superposition—the phenomenon in which a particle exists in distinctly different places simultaneously until measured—has only been experimentally demonstrated at the microscopic level.

“To test the quantum nature of gravity, we ultimately need to put objects with large enough masses into quantum superposition. And these objects need to be levitated in a vacuum to minimize the influence of environmental noise,” explains first author Anshuman Nayak, a Ph.D. student in the unit. “The typical approach has been to start with extremely small, levitated objects and gradually increase their mass until the effects of gravity become relevant.

“But levitating macroscale objects using conventional techniques, such as optical traps, has proven extremely challenging. That’s why we’re in the opposite camp—going from large to small. Just as diamagnetic levitation can lift maglev trains, it can also be used to levitate centimeter-wide objects holding diamonds, where the effect of gravity can be extremely strong, but quantum effects have not been observed.”

The key was to combine smaller devices recently demonstrated experimentally by the same team: a diamagnetically levitated graphite plate fitted with a mirror, connected by a carbon rod that passes through magnetic shielding to a diamond hanging above a magnet. This diamond has billions of so-called nitrogen-vacancy (NV) centers, which trap unpaired electrons and act as tiny, controllable quantum magnets via their quantum spin force.

Periodically illuminating the diamond with a green laser polarizes these centers into a predefined spin state, generating tiny magnetic fluctuations that push the diamond down. The researchers tracked this motion using an interferometer that reflects a laser off the small mirror on the graphite plate to measure distance changes with picometer precision.

“NV diamonds are well understood and easy to control. That, and the fact that NV centers have some of the longest known coherence times, allowing them to maintain quantum superposition at room temperature much longer than other systems, makes them particularly attractive for generating macroscopic superposition of motion or an object in future research,” adds co-author Daehee Kim, a Ph.D. student in the unit.

From theoretical particles to experimental cats

Einstein struggled with quantum mechanics during the last decades of his life, not because it is difficult, but because it poses a series of as-yet-unanswerable questions about general relativity. Is gravity quantum or entirely classical? What happens to information at the center of a black hole? What are dark matter and dark energy? Can an object with mass really be in two places at once?

These questions have eluded science largely due to a fundamental disconnect among the mathematical, theoretical and experimental frameworks of quantum and classical physics. But now, by demonstrating a mechanical effect arising solely from quantum spin, the OIST team has created an experimental platform to test exotic theories of physics.

Twamley concludes, “We’ve shown a large classical response from a small quantum effect. 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. And on the way, we’re developing a new class of extremely precise sensors. 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.”

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Gaby Clark

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Robert Egan

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