OIST team moves a levitated centimeter-wide diamond with electron spin force alone

"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," said Professor Jason Twamley of OIST's Quantum Machines Unit ^[1]. The key word is "classical." The force comes from electron spins trapped in the diamond's nitrogen-vacancy centers ^[4]. The diamond responds by moving the way any pushed object moves ^[1].

The mass gap is large, a factor of 100 million to a billion ^[16]. Twamley measured it against other spin-mechanical experiments ^[1]. Tests of quantum mechanics itself use a different yardstick. "There have been many efforts to test whether quantum mechanics holds for anything larger than a few tens of nanometers, so far without success," Twamley said ^[7].

That gap sets how much the result says about gravity. Positional superposition, in which an object sits in distinctly different places until measured, has only been demonstrated at microscopic scale ^[8]. "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," said first author Anshuman Nayak, a PhD student in the unit ^[9]. The OIST rig supplies a heavy levitated object whose motion answers to spins ^[2]. Putting that motion into superposition is future work, by the team's own account ^[15].

The design runs opposite to the usual route. "The typical approach has been to start with extremely small, levitated objects and gradually increase their mass until the effects of gravity become relevant," Nayak said ^[10]. "But levitating macroscale objects using conventional techniques, such as optical traps, has proven extremely challenging. That's why we're in the opposite camp," he said ^[11]. His group uses diamagnetic levitation, the effect that can lift maglev trains ^[12]. Big objects are much harder than nanoscopic particles to isolate from heat and vibration ^[13].

The rig combines devices the team had already demonstrated separately ^[14]. A levitated graphite plate carries a mirror. A carbon rod runs from it through magnetic shielding to the diamond, which hangs above a magnet ^[4]. Green laser pulses polarize the spins, and the diamond is pushed down ^[5]. A second laser reflected off the mirror into an interferometer reads distance changes to picometer precision ^[6].

The press account does not report how far the diamond moved, what the assembly weighs, or how the team separated the spin force from other effects of laser light on the diamond, such as heating. Those figures decide the sensor case. OIST says the work opens a route to sensors for dark matter, gravitational waves and other exotic phenomena ^[18]. Until a force sensitivity is published, that claim cannot be set against existing detectors.

I think the fair reading is narrower than OIST's framing, and it is still a good experiment. The group has a working coupling between quantum spins and a centimeter-scale levitated mass ^[2]. It also chose its spin system with the next step in mind. "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," said co-author Daehee Kim, also a PhD student in the unit ^[15].