Breaking a century-old physics barrier: Scientists achieve perfect wave trapping with simple cylinders

A joint research team has successfully demonstrated the complete confinement of mechanical waves within a single resonator—something long thought to be theoretically impossible. Their findings, published on April 3 in Physical Review Letters, mark a major breakthrough in the century-old mystery of bound states in the continuum (BIC). The team is from POSTECH (Pohang University of Science and Technology) and Jeonbuk National University.

Many technologies around us—from smartphones and ultrasound devices to radios—rely on resonance, a phenomenon in which waves are amplified at specific frequencies. However, typical resonators gradually lose energy over time, requiring constant energy input to maintain their function.

Nearly a century ago, Nobel laureates John von Neumann and Eugene Wigner proposed a counterintuitive concept: under certain conditions, waves could be trapped indefinitely without any energy leakage. These so-called bound states in the continuum (BIC) are like whirlpools that remain in place even as a river flows around them. But for decades, scientists believed this phenomenon could not exist in a compact, single-particle system.

Now, the research team has broken this long-standing theoretical boundary by successfully realizing BIC in a single particle.

Using a system of cylindrical granular particles—small solid rods made of quartz—the researchers built a highly tunable mechanical platform. By precisely adjusting how the cylinders touch each other, they could control the way mechanical waves interact at the contact boundaries.

Under special alignment, a wave mode became fully confined within a single cylinder without any energy escaping into the surrounding structure. This so-called polarization-protected BIC was not just theoretical—it was observed in real experiments. Even more remarkably, the system achieved quality factors (Q-factors) of over 1,000, a measure of how efficiently a resonator stores energy with minimal loss.

What happens when many of these special cylinders are connected in a chain? The team discovered that the trapped wave modes could extend throughout the chain without dispersing—a phenomenon known as a flat band.

“It’s like tossing a stone into a still pond and seeing the ripples remain motionless, vibrating only in place,” said lead author Dr. Yeongtae Jang. “Even though the system allows wave motion, the energy doesn’t spread—it stays perfectly confined.”

This behavior is described as a bound band in the continuum (BBIC) and opens new possibilities for energy harvesting, ultra-sensitive sensors, and even advanced communications.

“We have broken a long-standing theoretical boundary,” said Professor Junsuk Rho, who leads the research. “While this is still in the fundamental research phase, the implications are significant—from low-loss energy devices to next-generation sensing and signal technologies.”