Newswise — Physicists have created the first Bose-Einstein condensate — the mysterious ‘fifth state’ of matter — made up of quasiparticles, entities that don’t count as elementary particles but can still have elementary particle properties like charge and spin. For decades it was unknown whether they could undergo Bose-Einstein condensation in the same way as real particles, and now it appears they can. The discovery is expected to have a significant impact on the development of quantum technologies, including quantum computing.
An article describing the process of creating the substance, carried out at temperatures close to absolute zero, was published in the journal Nature Communication.
Bose-Einstein condensates are sometimes described as the fifth state of matter, alongside solids, liquids, gases, and plasmas. Theoretically predicted at the turn of the 20th century, Bose-Einstein condensates, or BECs, weren’t created in a lab until 1995. They are also perhaps the strangest state of matter, and many ‘among them remain unknown to science.
BECs occur when a group of atoms is cooled to a few billionths of a degree above absolute zero. Researchers commonly use lasers and “magnetic traps” to steadily reduce the temperature of a gas, usually made up of rubidium atoms. At this ultracold temperature, the atoms barely move and begin to exhibit very strange behavior. 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”. The collection of atoms essentially behaves like a single particle.
Currently, BECs are the subject of much fundamental research and for simulating condensed matter systems, but in principle they have applications in quantum information processing. Quantum computing, still in its infancy, uses a number of different systems. But they all depend on quantum bits, or qubits, that are in the same quantum state.
Most BECs are made from dilute gases of ordinary atoms. But until now, a BEC composed of exotic atoms has never been realized.
Exotic atoms are atoms in which a subatomic particle, such as an electron or a proton, is replaced by another subatomic particle that has the same charge. Positronium, for example, is an exotic atom composed of an electron and its positively charged antiparticle, a positron.
An “exciton” is another example. When light strikes a semiconductor, the energy is sufficient to “excite” the electrons to jump from an atom’s valence level to its conduction level. These excited electrons then flow freely in an electric current, essentially transforming light energy into electrical energy. When the negatively charged electron makes this jump, the space left behind, or “hole”, can be treated as if it were a positively charged particle. The negative electron and the positive hole are attracted and therefore bound together.
Combined, this electron-hole pair is an electrically neutral “quasi-particle” called an exciton. A quasiparticle is a particle-like entity that does not count as one of the 17 elementary particles in the Standard Model of Particle Physics, but can still have elementary particle properties like charge and spin. 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.
Excitons come in two forms: orthoexcitons, in which the spin of the electron is parallel to the spin of its hole, and paraexcitons, in which the spin of the electron is antiparallel (parallel but in the opposite direction) to that of his hole.
Electron-hole systems have been used to create other phases of matter such as electron-hole plasma and even exciton liquid droplets. The researchers wanted to see if they could make a BEC out of excitons.
“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,” said Makoto Kuwata-Gonokami, a physicist at the University of Tokyo and co-author of the paper. “It’s sort of the holy grail of low-temperature physics.”
The researchers thought that the hydrogen-like paraexcitons created in cuprous oxide (Cu2O), a compound of copper and oxygen, were one of the most promising candidates for fabricating exciton BECs in a bulk semiconductor due to their long lifetime. Attempts to create a paraexciton BEC at liquid helium temperatures of around 2 K had been made in the 1990s, but had failed because, to create a BEC from excitons, temperatures much lower than those these were needed. Orthoexcitons cannot reach such a low temperature because their lifetime is too short. Paraexcitons, however, are experimentally well known to have extremely long lifetimes of several hundred nanoseconds, long enough to cool them to the desired temperature of a BEC.
The team succeeded in trapping the paraexcitons in the mass of Cu2O below 400 millikelvins using a dilution refrigerator, a cryogenic device that cools by mixing two isotopes of helium together and is commonly used by scientists trying to make quantum computers. They then directly visualized the BEC exciton in real space using mid-infrared induced absorption imaging, a type of microscopy using light in the mid-infrared range. This allowed the team to take precision measurements, including exciton density and temperature, which in turn allowed them to mark the differences and similarities between the exciton BEC and the regular atomic BEC.
The next step of the group will be to study the dynamics of BEC exciton formation in the bulk semiconductor and to 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.
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Funding:
This research was supported by MEXT, JSPS KAKENHI (Grant Nos. JP20104002, JP26247049, JP25707024, JP15H06131, JP17H06205); by the Photon Frontier Network Program, Quantum Leap Flagship Program (Q-LEAP) Grant No. JPMXS0118067246 of MEXT; and by JSPS through its FIRST program.
Related links:
Gonokami Group: http://www.gono.tu-tokyo.ac.jp/e_index.html
Graduate School of Science: https://www.su-tokyo.ac.jp/en/
Graduate School of Engineering: https://www.tu-tokyo.ac.jp/en/soe
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