A new type of topological material has been created by using laser light to couple together the mechanical vibrations of five nanomechanical resonators. [Image: Ella Maru Studio]
Researchers in Europe have reportedly created an optomechanical network that transmits and amplifies sound in ways that have never been observed before (Nature, doi: 10.1038/s41586-024-07174-w). Using laser light to couple together a chain of five nanoresonators, they say their experiment demonstrates a new type of topological matter that could yield more powerful sensing technologies as well as novel devices for processing quantum information.
Amplifying waves
The aim of the experiment was to realize an acoustic analog of the so-called “Kitaev chain,” a theoretical model that describes the topological behavior of electrons in a superconducting nanowire. While phonons and other bosons do not interact in the same way as electrons, such bosonic Kitaev chains (BKCs) are predicted to support a series of extraordinary transmission phenomena for both light and sound waves.
In the experiment, the five nodes in the chain are made from a pair of silicon nanobeams patterned with a sliced photonic-crystal geometry that traps light in the nanoscale gap between the beams. The mechanical vibrations of these nanoresonators are coupled together using the optical forces exerted by a laser tuned to the resonant frequency of the nanocavity, which allowed the researchers to control the collective behavior of the chain by carefully modulating the laser intensity.
The team’s results show that the optomechanical chain amplifies the sound waves exponentially as they travel from one end to the other, while blocking transmission in the opposite direction. Shifting the phase of the laser light inverts the behavior, amplifying the backward signal while blocking it the other way. “The bosonic Kitaev chain thus acts like a unique type of directional amplifier, which could have interesting applications for signal manipulation, in particular in quantum technology,” explained team leader Ewold Verhagen, AMOLF, Netherlands.
The topological nature of the chain protects this directional amplification from perturbations or defects, creating a dramatic effect when the ends of the chain are connected together to form a ring. The exponential growth in the intensity of the circulating sound waves is accompanied by a narrowing of the frequency spectrum, indicating the emergence of a dynamic phase transition that resembles a lasing threshold.
The team’s results show that the optomechanical chain amplifies the sound waves exponentially as they travel from one end to the other, while blocking transmission in the opposite direction.
Toward sensing technologies
The researchers also found that the open chain is extremely sensitive to any disturbance affecting the last resonator in the sequence, causing signals traveling along the chain to change direction and be amplified a second time. Such perturbations could be caused by a single molecule touching the resonator or by interactions with the qubits in a quantum computer—an effect that could be exploited in next-generation sensor technologies.
“We have seen the first indications of the sensing capabilities in our experiments, which is very exciting,” commented Verhagen. “We now need to investigate in more detail how these topological sensors work, whether the sensitivity is boosted in the presence of various types of noise sources, and which interesting sensor technologies can benefit from these principles.”