One of the most mind-bending revelations of modern science is that particles—the building blocks of matter—are not simply solid, tiny billiard balls. Instead, they have a dual nature. Depending on how they are observed, they can behave like particles or like waves. This strange duality is at the heart of quantum mechanics, the theory that describes the universe on its smallest scales.
Quantum theory predicts that not only electrons and atoms but also larger particles, perhaps even microscopic bits of dust, can display wave-like behavior. This wave behavior is described mathematically through something called the wave function, which represents both the motion of the particle and the probability of finding it in a particular place.
But while experiments have repeatedly demonstrated wave-like behavior in small particles, extending this strange property to larger objects has remained one of the great challenges of modern physics. It is as if the universe is teasing us: giving us glimpses of its strangeness at the tiniest levels, while making it harder to see at scales closer to the human world.
The Challenge of Scaling Quantum Effects
The difficulty lies in the fact that as particles get larger, their delicate wave-like properties become increasingly fragile. Even the smallest interactions with the surrounding environment—such as collisions with stray photons or molecules—can destroy the coherence of the wave. When this happens, the particle returns to behaving like an ordinary object in classical physics, losing its quantum strangeness.
Physicists call this process decoherence, and it has been one of the main obstacles preventing researchers from observing quantum effects in larger particles. To protect wave-like behavior, particles need to be carefully isolated from all disturbances, cooled to extremely low temperatures, and manipulated with precise experimental control.
Despite these difficulties, scientists have been steadily pushing the boundaries, developing new tools and techniques to capture the elusive wave functions of larger particles.
A Leap Forward in Quantum Research
Recently, researchers at ETH Zurich and the Barcelona Institute of Photonic Sciences introduced a breakthrough method that may finally make it possible to explore the quantum wave functions of much larger particles. Their work, published in Physical Review Letters, builds on an elegant principle: if you cannot shrink the tools to match the scale of quantum behavior, why not expand the quantum behavior itself?
To do this, they used a concept known as quantum squeezing, a technique that manipulates the uncertainties inherent in quantum systems. By applying this method to optically levitated nanoparticles, they were able to extend the coherence length—the distance over which a particle’s wave-like properties remain intact.
This innovation could bring physicists a step closer to observing matter-wave interference with massive objects, a phenomenon that would offer one of the most striking demonstrations of quantum mechanics.
From Textbook Theory to Experimental Reality
Matter-wave interference is one of the most beautiful demonstrations of quantum physics. It shows that even massive objects—like nanoparticles—can behave as if they are ripples on a pond rather than solid specks of matter. In principle, the laws of physics say this wave-like behavior applies to everything, from electrons to entire planets. But in practice, observing it has been nearly impossible for anything larger than atoms or molecules.
The team led by Massimiliano Rossi decided to tackle the problem in a new way. They focused on a single nanoparticle held in place with an optical tweezer, a beam of light that can trap and manipulate microscopic objects. Under normal conditions, the quantum wavepacket describing the nanoparticle’s motion is extremely narrow—just a few picometers across, far smaller than anything we can easily manipulate with conventional tools.
Building an experimental apparatus tiny enough to probe this scale would be virtually impossible. So Rossi and his colleagues asked a different question: instead of making the experimental tools smaller, could they make the quantum wavepacket itself larger?
Stretching the Quantum Wavepacket
Their method was deceptively simple in principle, though demanding in execution. By carefully weakening the optical trap that held the nanoparticle, they allowed the wavepacket to expand. Then, before it recompressed back to its original narrow size, they tightened the trap again.
This clever timing trick allowed the nanoparticle to retain its expanded wavepacket. As a result, the team managed to double the coherence length—from about 30 picometers to roughly 70 picometers. While still minuscule in absolute terms, this expansion represents a dramatic proof-of-concept: wave-like behavior in larger particles can indeed be amplified and controlled.
Rossi explained that if this process is repeated with multiple pulses, the delocalization—the spread of the wave function—can increase exponentially, potentially reaching the size of the nanoparticle itself. Achieving that milestone would mark a turning point in quantum research, enabling true interference experiments with macroscopic particles.
The Next Frontier: Suppressing Decoherence
Even as the team celebrates this achievement, they remain keenly aware of the next big challenge: decoherence. At present, the main source of decoherence in their setup comes from photons scattered by the optical tweezer itself. These stray interactions disrupt the wave-like behavior they are trying so carefully to preserve.
To address this, the researchers are now developing a hybrid trapping system that combines optical tweezers with an electrical quadrupole trap. This approach, similar to techniques used in ion trapping, could confine particles with far less decoherence, allowing the quantum wavepacket to expand even further without collapsing.
If successful, this could bring physicists to the brink of observing wave interference in objects approaching macroscopic scales—a feat that would blur the boundary between the quantum and classical worlds.
Why This Matters
At first glance, doubling the coherence length of a nanoparticle may seem like a small, highly specialized result. But in the grander scheme, it represents a profound step toward answering some of the deepest questions in science.
How far does quantum mechanics extend? Is there a size limit at which wave-particle duality breaks down, or can even macroscopic objects display quantum behavior if isolated well enough? Could experiments like this eventually help explain the mystery of why the everyday world appears so classical when its foundations are entirely quantum?
The work at ETH Zurich and the Barcelona Institute of Photonic Sciences does not answer these questions outright, but it gives us a tool to explore them more directly than ever before.
A Glimpse Into the Quantum Future
The dream of seeing a dust particle, or perhaps even a virus-sized object, behave like a wave is no longer science fiction. Each advance brings us closer to bridging the gap between the quantum world of atoms and the familiar world of objects we can see and touch.
Beyond pure curiosity, this research could also fuel future technologies. Improved methods of quantum control may feed into advances in quantum computing, ultrasensitive detectors, and even new approaches to materials science.
But perhaps most importantly, these experiments remind us that the universe is far stranger and more wondrous than it appears. The idea that a nanoparticle—a speck so small it is invisible to the naked eye—can be stretched into behaving like a wave is a vivid demonstration of how reality resists our simple categories of “particle” or “object.”
Quantum mechanics continues to reveal a universe that is fluid, dynamic, and filled with hidden possibilities. And thanks to the creativity of researchers like Rossi and his team, we are learning not just to observe these possibilities, but to shape them.
More information: M. Rossi et al, Quantum Delocalization of a Levitated Nanoparticle, Physical Review Letters (2025). DOI: 10.1103/2yzc-fsm3. On arXiv: DOI: 10.48550/arxiv.2408.01264
