For the first time, researchers have created a Bose-Einstein condensate based on quasi-particles. (Artist’s abstract concept.)
Physicists have created the first Bose-Einstein condensate – the mysterious “fifth state” of matter – made up of quasiparticles. These are entities that do not count as elementary particles, although they can still have elementary particle properties such as charge and spin.
For decades it was unknown whether quasiparticles could undergo Bose-Einstein condensation in the same way as real particles, and now it appears that they can. This discovery is expected to have a significant impact on the development of quantum technologies, including
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A paper describing the process of creation of the substance, which was achieved at temperatures just a tiny bit above 
A close-up picture of the apparatus in a cryogen-free dilution refrigerator. A dark red-colored cubic crystal in the center of the picture is cuprous oxide. A zinc selenide meniscus lens placed behind the crystal is an objective lens. A rod and a stage below the crystal are used for generation of an inhomogeneous strain field in the crystal that acts as a trap potential for excitons. Credit: Yusuke Morita, Kosuke Yoshioka and Makoto Kuwata-Gonokami, The University of Tokyo
BECs occur when a group of atoms is cooled to within billionths of a degree above absolute zero. Researchers commonly use lasers and “magnet traps” to steadily reduce the temperature of a gas, typically composed of rubidium atoms. At this ultracool 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 start to clump together, occupying the same volume as one indistinguishable “super
The cuprous oxide crystal (red cube) was placed on a sample stage at the center of the dilution refrigerator. Researchers attached windows to the shields of the refrigerator that allowed optical access to the sample stage in four directions. The windows in two directions allowed transmission of the excitation light (orange solid line) and luminescence from paraexcitons (yellow solid line) in the visible region. The windows in the other two directions allowed transmission of the probe light (blue solid line) for induced absorption imaging. To reduce incoming heat, researchers carefully designed the windows by minimizing the numerical aperture and using a specific window material. This specialized design for the windows and the high cooling power of the cryogen-free dilution refrigerator facilitated the realization of a 64 millikelvin minimum base temperature. Credit: Yusuke Morita, Kosuke Yoshioka and Makoto Kuwata-Gonokami, The University of Tokyo
An “exciton” is another such example. When light hits a semiconductor, the energy is sufficient to “excite” electrons to jump up from the valence level of an atom to its conduction level. These excited electrons then flow freely in an electric current — in essence transforming light energy into electrical energy. When the negatively charged electron performs this jump, the space left behind, or “hole,” can be treated as if it were a positively charged particle. The negative electron and positive hole are attracted and thus bound together.
Combined, this electron-hole pair is an electrically neutral “quasiparticle” called an exciton. A quasiparticle is a particle-like entity that does not count as one of the 17 elementary particles of the standard model of particle physics, but that can still have elementary-particle properties like charge and spin. The exciton quasiparticle can also be described as an exotic atom because it is in effect a hydrogen atom that has had its single positive proton replaced by a single positive hole.
Excitons come in two flavors: orthoexcitons, in which the spin of the electron is parallel to the spin of its hole, and paraexcitons, in which the electron spin is anti-parallel (parallel but in the opposite direction) to that of its hole.
Electron-hole systems have been used to create other phases of matter such as electron-hole 
Researchers applied inhomogeneous stress using a lens set under the sample (red cube). The inhomogeneous stress results in an inhomogeneous strain field that acts as a trap potential for excitons. The excitation beam (orange solid line) was focused on the bottom of the trap potential in the sample. An exciton (yellow sphere) consists of one electron (blue sphere) and one hole (red sphere). The team detected excitons by either luminescence (yellow shade) or the differential transmission of the probe light (blue shade). An objective lens set behind the sample collected luminescence from excitons. The probe beam also propagated through the objective lens. Credit: Yusuke Morita, Kosuke Yoshioka and Makoto Kuwata-Gonokami, The University of Tokyo