Bose-Einstein condensates are sometimes described as the fifth state of matter. They weren’t created in a lab until 1995. They experience the same quantum state – almost like coherent photons in a laser – and begin to clump together, occupying the same volume as an indistinguishable super atom.
Currently, BECs are the subject of much fundamental research for simulating condensed matter systems, but in principle they have applications in quantum information processing. Most BECs are made from dilute gases of ordinary atoms. But until now, a BEC composed of exotic atoms has never been realized.
Scientists at the University of Tokyo wanted to see if they could make a BEC from excitons. Using quasiparticles, they created the first Bose-Einstein condensate, the mysterious “fifth state” of matter. The discovery is expected to have a significant impact on the development of quantum technologies, including quantum computing.
The combined electron-hole pair is an electrically neutral “quasi-particle” called an exciton. The exciton quasiparticle can also be described as an exotic atom because it is actually a hydrogen atom whose single positive proton has been replaced by a single positive hole.
Makoto Kuwata-Gonokami, a physicist at the University of Tokyo and co-author of the paper, said: “The direct observation of an exciton condensate in a three-dimensional semiconductor has been much sought after since its first theoretical proposal in 1962. No one knew whether quasiparticles could undergo Bose-Einstein condensation in the same way as particles. real. It’s sort of the holy grail of low-temperature physics.
Due to their extended lifetime, paraexcitons produced in cuprous oxide (Cu2O), a mixture of copper and oxygen, were considered one of the most promising possibilities for generating exciton BECs in a solid semiconductor. In the 1990s, attempts to produce BEC paraexciton at liquid helium temperatures of around 2 K had been made. Yet they had failed because much lower temperatures are needed to produce a BEC from excitons. Because they are too transient, orthoexcitons cannot reach such a low temperature. However, it is known from experience that paraexcitons have a very long lifetime of more than a few hundred nanoseconds, which is sufficient to cool them to the necessary temperature of a BEC.
The team used a dilution refrigerator, a cryogenic device that cools by combining two isotopes of helium and is frequently used by scientists trying to develop quantum computers, to trap paraexcitons in the majority of Cu2O below 400 millikelvins. Next, they used mid-infrared induced absorption imaging, a kind of microscopy that uses light in the mid-infrared range, to directly visualize the BEC exciton in real space.
As a result, the team was able to obtain accurate measurements of exciton density and temperature, which allowed them to identify differences and similarities between the exciton BEC and the conventional atomic BEC.
The scientists further want to study the dynamics of BEC exciton formation in the bulk semiconductor and study the collective excitations of BEC excitons. Their ultimate goal is to build a platform based on an exciton BEC system to further elucidate its quantum properties and develop a better understanding of the quantum mechanics of qubits strongly coupled to their environment.
Journal reference:
- Yusuke Morita, Kosuke Yoshioka, and Makoto Kuwata-Gonokami, “Observation of Bose-Einstein condensates of excitons in a bulk semiconductor,” Nature Communications: September 14, 2022. DOI: 10.1038/s41467-022-33103-4
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