In the laboratory of the Okinawa Institute of Science and Technology (OIST), a quantum effect has for the first time directly moved an object visible to the eye. Scientists made a centimetre-scale diamond suspended above a magnet move up and down under the action of a force generated by the spins of electrons in its atomic lattice.
The experiment relies on nitrogen-vacancy centres — defects in the diamond crystal where a carbon atom is replaced by a nitrogen atom and a vacancy. These centres hold unpaired electrons whose spins can be polarised by a green laser. Under pulsed illumination, the spins align in such a way that their tiny magnetic moments create a weak but measurable force.
The diamond is suspended on a carbon rod connected to a graphite plate that levitates above magnets thanks to diamagnetism. When the laser is off, the movement is minimal; when the laser is switched on, the diamond is displaced by tens of picometres — a distance registered by an interferometer with picometre precision.
The mass of the object is eight to nine orders of magnitude greater than in previous spin-mechanical experiments. This is the first case in which a purely quantum spin force has caused a classical mechanical response in an object subject to gravity.
The result opens the way to testing the quantum nature of gravity. To observe macroscopic superposition, it is necessary to levitate objects in a vacuum and gradually increase their mass until gravitational effects become noticeable.
Even now the setup makes it possible to create ultra-precise sensors for the search for dark matter and gravitational waves. The next step is another order of magnitude in mass, and then the experimenters hope to approach a real embodiment of "Schrödinger's cat".
Thus, tiny quantum fluctuations of spins have for the first time touched the world of visible things and outlined a bridge between quantum mechanics and the general theory of relativity.
