An international team of the researchers from the Technical University of Dortmund, Saint Petersburg State University, ITMO University, the St. Petersburg Institute of Technology, and Jackson State University in the U.S. has taken an important step in the development of quantum technologies through direct observation, for the first time, of long-lasting quantum beats in CsPbI₃ perovskite nanocrystals. The research shows that such materials are able to maintain a quantum state for an unusually long time, opening up new possibilities for quantum communication, photonics, and future quantum computing systems.
In recent years, perovskites have been considered among the most promising materials in energy and optoelectronics. They are already used for creation of highly efficient solar cells, LEDs, and lasers. But in addition to their applied properties, scientists are also interested in their unusual quantum physics. Unlike traditional semiconductors, in perovskites, electrons interact very strongly with vibrations of the atomic lattice, known as phonons. Because of this, electronic excitations begin to behave not as separate particles, but as hybrid states coupled to the crystal vibrations. Such states are called exciton-polarons.
It is this interaction with lattice vibrations that is generally considered one of the main challenges for quantum technologies. Any external disturbances quickly destroy quantum coherence – a special state in which a quantum system can exist in multiple states simultaneously. In many materials, such coherence disappears in just a few picoseconds. However, in perovskite nanocrystals, the researchers have discovered an unusual regime in which interaction between excitons and phonons does not destroy coherence instantly but allows it to persist for much longer.
To study this phenomenon, the scientists used ultrashort laser pulses lasting about 120 femtoseconds and the photon echo method, which is one of the most sensitive tools in modern quantum spectroscopy. The experiments were conducted at a temperature of about 2 Kelvin, that is, nearly absolute zero. Under these conditions, the researchers were able to observe how different exciton-polaron states begin to interfere with one another, creating characteristic rapid signal oscillations – quantum beats. In fact, the scientists directly tracked the coherent dynamics of such states in perovskite nanocrystals for the first time.
A key finding was unusually long coherence time. In the nanocrystals studied, the quantum state persisted for approximately 300 picoseconds, which is a record duration for such perovskite structures. By comparison, in many other materials, coherence breaks down tens or hundreds of times faster due to thermal noise and interaction with the environment.
The researchers have also discovered that two low-energy lattice vibration modes with the energies of 3.2 and 5.1 meV play a key role. It is these modes that determine the observed quantum beats. At the same time, it turned out that the properties of the system can be tuned by changing the size of nanocrystals. The smaller the particle, the stronger the interaction between excitons and lattice vibrations. This means that the material’s quantum properties can potentially be ‘tuned’ for specific tasks, for example, by changing the speed and nature of quantum processes even at the stage of nanocrystal synthesis.
The scientists believe that these results are significant for several areas of research. On the one hand, they help to better understand the fundamental physics of perovskites, which remains an active research area. On the other hand, they pave the way for creating controllable quantum states in solid-state materials. It is precisely these states that are considered one of the foundations of future quantum communications, ultra-sensitive sensors, and the components of quantum computing systems.
