Scientists have observed an extraordinary quantum phenomenon inside tiny semiconductor crystals, revealing a synchronized “dance” between two different quantum particles.
The discovery provides new insight into how quantum systems behave and could help pave the way for future quantum technologies.
The international research team reported the findings in Nature Communications, where they described directly observing how excitons and phonons move together in perovskite nanocrystals.
The results show that these two quantum entities can remain linked and evolve together in a highly coordinated way.
To understand the discovery, it helps to know what excitons and phonons are.
An exciton forms when light strikes a semiconductor and excites an electron. The energized electron leaves behind a positively charged “hole,” and the electron and hole become bound together, behaving as a single quantum particle.
A phonon is something entirely different. It is a quantum unit of vibration within a crystal lattice. In simple terms, phonons represent tiny vibrations of the atoms that make up a solid material.
Although excitons and phonons are very different, the researchers found that in perovskite nanocrystals they become strongly connected and move together as a single quantum system.
The crystals used in the study are incredibly small—only a few nanometers across, thousands of times thinner than a human hair. Their tiny size traps both excitons and phonons inside a confined space, strengthening the interaction between them.
When a short laser pulse creates an exciton, it also slightly disturbs the crystal structure around it. This disturbance generates phonons, and the two become linked in a combined quantum state known as an exciton-polaron.
Normally, maintaining delicate quantum states is extremely difficult. In most materials, vibrations from countless atoms quickly destroy quantum coherence, causing the system to lose its quantum properties.
However, the researchers discovered an unusual exception.
At a temperature of just 2 Kelvin, or about -271 degrees Celsius, the crystal vibrations remained remarkably stable. This allowed the coupled exciton-phonon system to maintain quantum coherence for around 10 picoseconds. While this may sound incredibly brief, it is long enough for many cycles of quantum motion to occur.
Using ultrafast laser pulses lasting only about one hundred femtoseconds, the team was able to track the evolution of the system in real time.
They observed strong quantum beats—rhythmic oscillations that occur when a quantum system exists in multiple states simultaneously. As the states evolve at slightly different energies, their quantum waves interfere with one another, producing a regular pattern of oscillation.
These quantum beats revealed how excitons and crystal vibrations exchange energy and move together in a coordinated fashion.
The researchers also found that the effect can be controlled simply by changing the size of the nanocrystals. Smaller crystals create stronger interactions between excitons and phonons, while larger crystals allow the oscillations to remain coherent for longer periods.
This ability to tune the behavior of the system could be highly valuable for future technologies.
The findings suggest that perovskite nanocrystals may become an important platform for quantum computing, quantum communication, and advanced light-based devices. They may even help scientists generate individual phonons, opening new possibilities for controlling quantum information.
Perhaps most importantly, the study challenges a common assumption in physics. Crystal vibrations are usually viewed as a source of noise that destroys quantum effects. This work shows that, under the right conditions, those vibrations can instead become a useful quantum resource, helping quantum systems perform in ways previously thought impossible.
