The Bose-Einstein condensate is an exotic state of matter, sometimes referred to as the fifth state of matter, considering that the other four are solid, liquid, gas, and plasma. Quasiparticles are also exotic, since they cannot be counted as elementary particles but have characteristics of elementary particles, such as charge and spin.
For decades, it has been unknown whether it would be possible to achieve Bose-Einstein condensation with just quasiparticles in the same way that it is achieved with real particles, and now it is clear that it is possible.
The pioneering experiment in which it has been demonstrated is set to have a significant influence on the development of quantum technologies, including quantum computing.
The achievement is the work of Makoto Kuwata-Gonokami, Yusuke Morita, and Kosuke Yoshioka, all from the University of Tokyo in Japan.
Although Bose-Einstein condensates were predicted theoretically in the early 20th century, they weren’t created in a laboratory until 1995. And since then, not much has been learned about them by the scientific community.
Bose-Einstein condensates form when a group of atoms is cooled to temperatures a billionth of a degree above the lowest temperature allowed by the laws of physics, minus 273.15 degrees Celsius, called “zero.” absolute”. Researchers often use lasers and “magnetic traps” to reduce the temperature of a gas to near absolute zero. Normally the gas used is composed of rubidium atoms. At this ultracold temperature, the atoms hardly move and begin to exhibit very strange behavior. They all experience the same quantum state (almost like coherent photons in a laser) and begin to clump together as a “superatom.” Atoms behave essentially as if they were all a single particle.
Currently, Bose-Einstein condensates are still the subject of much basic research, but it is already becoming clear that they will have many applications in quantum computing.
Some of the machinery used to create the first Bose-Einstein condensate made from quasiparticles. (Photo: Yusuke Morita, Kosuke Yoshioka, Makoto Kuwata-Gonokami, University of Tokyo)
Most Bose-Einstein condensates are made from diluted gases of ordinary atoms. But until now, a Bose-Einstein condensate made from exotic atoms had never been achieved.
Exotic atoms are atoms in which a subatomic particle, such as an electron or proton, is replaced by another subatomic particle that has the same charge. Positronium, for example, is an exotic atom made up of an electron and its positively charged antiparticle, a positron.
An “exciton” is another example. When light strikes a semiconductor, the energy can “excite” electrons to jump from an atom’s valence level to its conduction level. These excited electrons then flow freely in an electrical current, essentially transforming light energy into electrical energy. When the negatively charged electron makes this jump, the space it leaves, or “hole,” can be treated as if it were a positively charged particle. The negative electron and the positive hole attract each other and stick together.
This combined electron-hole pair is an electrically neutral quasiparticle called an exciton. A quasiparticle is similar to a particle but is not one of the 17 elementary particles of the standard model of particle physics. Still, it can have elementary particle properties, like charge and spin. The exciton quasiparticle can also be described as an exotic atom, since it is a hydrogen atom that has had its single positive proton replaced by a single positive hole.
Makoto Kuwata-Gonokami and colleagues present the technical details of their achievement in the academic journal Nature Communications, under the title “Observation of Bose-Einstein condensates of excitons in a bulk semiconductor.” (Font: NCYT by Amazings)