Physicists have moved a macroscopic diamond sample using only the force produced by the electron spins inside it. The result shows that a small quantum effect can produce a large classical response, and it could point toward a new class of sensors precise enough to probe the nature of gravity and test the limits of quantum mechanics.
The work was led by physicist Jason Twamley at the Okinawa Institute of Science and Technology (OIST) in Japan. "There is a lot of interest in this spin force because the force it exerts can be a quantum force," Twamley said. "I think it's amazing that tiny spins can exert so much force and displace a macroscopic object that weighs so much." The team reported its findings in the journal Science Advances.
A central principle of quantum mechanics is that objects can exist in a superposition of two or more states. Such superpositions are fragile, and interactions with the surrounding environment tend to destroy them. The larger an object is, the harder it becomes to shield it from those interactions, which is why Schrödinger's famous cat, a macroscopic object, cannot be both dead and alive at once. For researchers hoping to study the quantum nature of gravity, this poses a problem, because objects that interact strongly through gravity are by definition massive. Massive objects will not stay in quantum superposition unless thermal vibrations and other noise are tightly controlled. Levitating the object in a vacuum is one strategy, but conventional techniques such as optical traps have proved difficult to apply at this scale.
The team took a different approach. Their trapping system consists of a diamagnetically levitated graphite plate fitted with a mirror and connected to the diamond, a centimetre-scale sample weighing 128 milligrams, through a carbon rod that passes through magnetic shielding. The OIST researchers engineered the diamond to contain billions of nitrogen-vacancy (NV) centres, defects formed when two neighbouring carbon atoms are replaced by a nitrogen atom and an empty lattice site. These centres trap unpaired electrons, and their electronic spins are well isolated from the surrounding lattice. If placed in a particular spin state, they remain in it for a relatively long time, even at room temperature.
When the researchers shine a green laser on the diamond in pulses, the NV centres polarise into a predefined spin state. That polarisation generates tiny magnetic fluctuations that push the diamond upward. The team tracked the motion with an interferometer that reflects laser light off the mirror on the graphite plate. Using a 50-milliwatt green laser, they found they could controllably move the sample by as much as about 100 nanometres in air and up to about 1.5 micrometres in vacuum. The movements were much larger than those observed when the laser was off.
Twamley said the longer-term goal is more ambitious. "This means that under the right conditions, one can put the spin in a superposition of states and so create a superposition of forces that can displace the diamond into a superposition of positions," he explained, while acknowledging that reaching those conditions will be extremely difficult. Spatial superpositions of objects larger than tens of nanometres have not yet been achieved, and Twamley said demonstrating one would allow researchers to explore the fragile border between the quantum and classical worlds. One possibility he describes is a "spatial Schrödinger's cat", an object whose centre of mass sits in superposition across two locations. "Our experiment shows that tiny spins can lead to a large displacement, which is one step along the road towards taming this cat," he said.
Previous work in this area focused mainly on levitated nanodiamonds containing a single NV spin, according to Twamley, so the OIST team chose to study a macroscopic sample instead. The researchers hope their results will encourage other groups to investigate macroscopic diamond NV ensembles and ways of driving them in a magnetic field gradient. Twamley also noted a practical lesson: the low-frequency levitated system is highly sensitive to vibrational noise. "We are now discovering the many joys of vibration isolation and hopefully will be able to put these oscillators in a nice quiet box where they can be super-protected from the world," he said.
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