Scientists Edge Closer to Overcoming a Fundamental Barrier in Quantum Physics
Scientists Edge Closer to Overcoming a Fundamental Barrier in Quantum Physics
In a breakthrough that challenges decades of assumptions in wave physics, researchers from POSTECH (Pohang University of Science and Technology) and Jeonbuk National University have demonstrated that mechanical waves can be completely confined within a single solid resonator — a feat many believed impossible in compact systems.
Published on April 3 in Physical Review Letters, the study centers on a phenomenon known as Bound States in the Continuum (BIC) — a rare condition where waves remain trapped indefinitely without losing energy, even though the surrounding environment offers pathways for them to escape.
For years, BICs were considered theoretical curiosities, mostly limited to complex, extended systems. Many physicists assumed they couldn’t exist in small, standalone structures. This new work proves otherwise.
Resonance Without Leakage: A Step Toward Perfect Confinement
Resonance is a fundamental principle behind countless modern technologies — from smartphone filters and ultrasound imaging to radio transmitters. In most cases, resonators amplify waves at specific frequencies, but they inevitably leak energy over time, requiring constant input to sustain the signal.
Back in the 1920s, mathematicians John von Neumann and Eugene Wigner proposed that under very specific conditions, waves could be perfectly confined — never decaying, never escaping. These “bound states” remained largely theoretical for nearly a century, especially in mechanical systems with simple geometries.
Now, this joint South Korean team has not only predicted such states theoretically but also observed them experimentally in a compact, tunable setup.
They built a system using cylindrical granular crystals — tiny rods made of quartz — arranged so that their contact points act like adjustable knobs. By fine-tuning the pressure and alignment at these junctions, the researchers could control how vibrations move through the structure.
At a precise configuration, they observed a mechanical wave mode that vibrated entirely within a single cylinder, with no detectable energy leakage to neighboring parts.
Using a laser Doppler vibrometer — a tool that measures microscopic surface motions — they confirmed the wave was fully localized. The resonator achieved a quality factor (Q) above 1,000, meaning it sustained oscillations with minimal damping. In practical terms: the energy stayed put, rather than dissipating.
From Single Traps to Collective Bands: The Rise of BBICs
What makes this discovery even more powerful is its scalability. When multiple such resonators are linked together, their trapped modes can interact and form extended states.
The team assembled a finite chain of these cylinders, deliberately breaking symmetry to create what they call a Bound Band in the Continuum (BBIC). In this setup, vibrations form a flat band — a state where the group velocity of the wave drops to zero at a specific frequency, effectively freezing the energy in place across the entire chain.
All units in the chain showed high-Q, dispersionless resonance, meaning the wave didn’t spread or weaken — a rare achievement in solid-state systems.
“It’s like tossing a stone into a still pond and seeing the ripples stay frozen in place,” said Dr. Yeongtae Jang, lead author of the study. “The system allows motion, but the energy doesn’t travel — it vibrates perfectly in confinement.”
Toward Real-World Applications
While still in the realm of fundamental research, the implications are promising. Systems that can trap mechanical energy with near-zero loss could revolutionize:
- Ultra-sensitive sensors (detecting tiny mass or force changes),
- Efficient energy harvesting devices,
- Low-loss signal processing in communications,
- And even components for quantum mechanical systems where coherence is critical.
“We’ve overcome a long-standing theoretical barrier,” said Professor Junsuk Rho, who led the research. “This isn’t just about trapping waves — it’s about opening a new path to design compact, high-performance devices based on perfect wave confinement.”
A New Path for Solid-State Wave Control
In simple terms, the team created a system where the contact points between quartz rods act as precision controls. With the right adjustment, a vibration can be locked inside one rod. Connect several rods, and the trapped mode extends across the chain — without spreading.
This work provides a clear blueprint for engineering strong wave confinement in simple, solid materials — something once thought out of reach.
As research progresses, these principles could find their way into next-generation technologies that demand extreme efficiency and precision — all starting from a single, perfectly still vibration.
Sources: POSTECH, American Physical Society
