Scientists from the National Institute of Atomic and Molecular Sciences in the Netherlands, in collaboration with partners from Germany, Switzerland, and Austria, have created a novel metamaterial where sound waves can flow in unprecedented ways. It presents a new form of mechanical vibration amplification, holding potential for enhanced sensor technologies and information processing devices. This metamaterial marks the first example of a "Bosonic Kitaev chain," deriving its unique properties from its topological nature. The findings were published in the March 27th issue of the journal Nature.
The Bose-Kitaev chain (artistic representation): Multiple mechanical string resonators are connected into a chain through light. Vibrations (sound waves) propagate and amplify along the chain. Image source: National Institute for Atomic and Molecular Physics, Netherlands.
The Bose-Kitaev chain is a theoretical model used to describe the physical phenomena of electrons in superconducting materials, particularly nanowires. The model is renowned for predicting the existence of special excitations at the ends of nanowires, known as Majorana zero modes. In 2018, it was speculated that the Bose-Kitaev chain could exhibit previously unknown behaviors in natural or engineered metamaterials.
Essentially, the Bose-Kitaev chain consists of a series of coupled harmonic oscillators. It is a type of metamaterial, a synthetic material with engineered properties. The oscillators can be thought of as the "atoms" of the material, and the way they are coupled together controls the collective behavior of the metamaterial, in this case, the propagation of sound waves along the chain.
Researchers indicate that the chain links of the coupler must be made of special springs. They create the desired connections between nanomechanical resonators by applying forces with light, coupling them together to create "optical" springs. By adjusting the intensity of the laser, researchers can connect five resonators and achieve the Bose-Kitaev chain.
Studies have found that this optical coupling amplifies the vibrations of nanomechanical oscillators, with sound waves (i.e., mechanical vibrations propagated through the array) exponentially amplified from one end to the other, but vibrations cannot be transmitted in the opposite direction. If the wave is delayed slightly (such as 1/4 oscillation period), the behavior is completely reversed. Thus, the Bose-Kitaev chain acts as a unique type of directional amplifier, holding significant potential for signal manipulation, particularly in quantum technology.
Further research has also demonstrated that the Bose-Kitaev chain is actually a new topological phase of matter. As predicted in 2018, researchers have shown unique experimental features of the topological properties of metamaterials: if the chain is closed, it forms a "necklace," with amplified sound waves continuously circulating within the resonator loop, reaching extremely high intensities, similar to the strong beams of light produced in lasers.
(Original Title: First Example of Bose-Kitaev Chain: New Topological Metamaterial Exponentially Amplifies Sound Waves)
