Levitation has long been pursued by stage magicians and physicists alike. For audiences, the sight of objects floating midair is wondrous. For scientists, it's a powerful way of isolating objects from external disturbances. This is particularly useful in case of rotors, as their torque and angular momentum, used to measure gravity, gas pressure, momentum, among other phenomena in both classical and quantum physics, can be strongly influenced by friction. Freely suspending the rotor could drastically reduce these disturbances – and now, researchers from the Okinawa Institute of Science and Technology (OIST) have designed, created, and analyzed such a macroscopic device, bringing the magic of near-frictionless levitation down to earth through precision engineering.
Microscale devices utilizing optical or electrical levitation require extremely sophisticated setups and are highly sensitive to environmental factors. By contrast, macroscale systems using room-temperature magnetic levitation are both simpler and much more resistant to the environment, and because they are subject to gravity unlike the atomic particles suspended in microscale devices, they are interesting both for practical gravimetry and foundational research in the boundary between quantum and classical physics. However, these setups have long been hindered by so-called eddy-current damping.
In a study now published in Communication Physics , members of the Quantum Machines Unit at OIST have come up with an elegant solution. Daehee Kim, PhD student in the unit and first author on the paper, explains: "With a one-centimeter graphite disk and a few rare earth magnets, we have demonstrated experimentally and proved analytically how to create a diamagnetically levitating rotor that experiences no eddy-current damping at all thanks to axial symmetry. If we can slow its rotation enough, its motion will enter the quantum regime, which could open up an entirely new platform for quantum research."
Turning the tables on eddy currents
When a conductive material changes its position in a non-uniform magnetic field, such as by moving closer to or further away from the magnets, circulating currents – or eddies – of electrons form inside the material, creating opposing magnetic fields that resist motion a bit like frictional drag. When desired, eddy-current damping has many practical applications, like in efficient brakes in power tools and Shinkansen bullet trains. But if you want to measure physical phenomena through the movement of a rotor, this friction is problematic.
Last year, the unit researchers addressed this challenge by fabricating a square plate from graphite powder coated in silica and embedded in wax, which confined eddy currents to the individual grains of powder rather than the whole plate, dramatically reducing eddy-current damping. The development of this levitating plate paved the way for precise accelerometers, which are extremely sensitive to physical phenomena like gravity. A device directly based on an earlier version of the unit's design recently went into space as a proof-of-concept for future space-based levitation experiments , such as for studying dark matter interactions and gravimetric waves among other fundamental physics questions.
However, the wax used to combine the silica-coated graphite powder significantly reduced the levitative power of the system, making it less suitable for integration into other systems as added weight – such as from a mirror used to track its rotation – may disrupt it.
The new rotor disk design is made purely from graphite, retaining strong levitative force, and does away with eddy-current damping entirely in an ideal system. "The plate design experiences slight eddy-current damping when moving up and down, because the magnetic strength – or flux – changes, forming eddy currents inside the silica-coated graphite grains," explains Professor Jason Twamley, head of the unit and senior author on the study. "But a rotor remains in the same magnetic field when rotating around its central axis above magnets. It does not experience a change in flux – and this therefore eliminates eddy-current damping."
Modelled through simulations, proved mathematically, and demonstrated experimentally, the precision of the system is now solely dependent on the machining of the graphite plate and magnets to achieve ideal axial symmetry, and the reduction of air friction by getting as close to a perfect vacuum as possible. "With practical improvements to the manufacturing process, our levitating rotor is perfect for extremely precise sensors operating at the scale of milli- instead of nanometers," summarizes Prof. Twamley. "It can be spun up to serve as precise and reliable gyroscopes or spun down – cooled – into the quantum regime. We're particularly interested in the latter, as it's a very promising platform for the study of quantum phenomena like vacuum gravity and rotational superposition at a macroscopic level."
