Researchers create first quasiparticle Bose-Einstein condensate

Newswise – Physicists have created the first Bose-Einstein condensate – the mysterious “fifth state” of matter – from quasiparticles, entities that are not considered elementary particles but can still have elementary particle properties such as charge and spin. For decades it was not known whether they could undergo Bose-Einstein condensation in the same way as real particles, and now it appears that they can. The discovery will have a significant impact on the development of quantum technologies, including quantum computing.

An article was published in the journal describing the formation process of the substance, which occurs at temperatures a hair’s breadth from absolute zero nature communication.

Bose-Einstein condensates are sometimes referred to as the fifth state of matter, alongside solids, liquids, gases, and plasmas. Theoretically predicted in the early 20th century, Bose-Einstein condensates, or BECs, were not made in a laboratory until 1995. They are also perhaps the strangest state of matter, and much about them remains unknown to science.

BECs form when a group of atoms is cooled to billionths of a degree above absolute zero. Researchers commonly use lasers and “magnetic traps” to steadily lower the temperature of a gas, typically composed of rubidium atoms. At this ultra-cold temperature, the atoms hardly move and behave very strangely. They experience the same quantum state – almost like coherent photons in a laser – and begin to cluster together, occupying the same volume as an indistinguishable “superatom”. The collection of atoms essentially behaves like a single particle.

Currently, BECs remain the subject of much basic research and for simulating condensed matter systems, but in principle they have applications in quantum information processing. Quantum computing, which is still in the early stages of development, uses a number of different systems. But they all depend on quantum bits or qubits being in the same quantum state.

Most BECs are made from rarefied gases of ordinary atoms. But until now, a BEC from exotic atoms has never been achieved.

Exotic atoms are atoms in which a subatomic particle, e.g. B. an electron or a proton, is replaced by another subatomic particle with the same charge. For example, positronium is an exotic atom composed of an electron and its positively charged antiparticle, a positron.

An “exciton” is another such example. When light strikes a semiconductor, the energy is sufficient to “excite” electrons to jump from an atom’s valence plane to its conduction plane. These excited electrons then flow freely in an electric current – essentially converting light energy into electrical energy. When the negatively charged electron makes this jump, the remaining space or “hole” can be treated as if it were a positively charged particle. The negative electron and the positive hole are attracted and thus bound to each other.

Taken together, this electron-hole pair is an electrically neutral “quasiparticle” called an exciton. A quasiparticle is a particle-like structure that does not belong to the 17 elementary particles of the Standard Model of particle physics, but can still have elementary particle properties such as charge and spin. The exciton quasiparticle can also be called an exotic atom because it is actually a hydrogen atom that has had its single positive proton replaced with a single positive hole.

There are two types of excitons: orthoexcitons, in which the electron’s spin is parallel to the spin of its hole, and paraexcitons, in which the electron’s spin is antiparallel (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 plasma and even exciton liquid droplets. The researchers wanted to see if they could make a BEC from excitons.

“The direct observation of an exciton condensate in a three-dimensional semiconductor has been in great demand since it was first theoretically proposed in 1962. Nobody knew if quasiparticles could undergo Bose-Einstein condensation in the same way as real particles,” said Makoto Kuwata-Gonokami, a physicist at the University of Tokyo and co-author of the paper. “It’s like the holy grail of cryogenic physics.”

The researchers thought that hydrogen-like paraexcitons found in copper oxide (Cu₂O), a compound of copper and oxygen, was one of the most promising candidates for fabricating exciton BECs in a bulk semiconductor because of its long lifetime. Attempts to generate paraexciton BECs at liquid helium temperatures of around 2 K were made in the 1990s, but failed because much lower temperatures are required to generate a BEC from exciton. Orthoexcitons cannot reach such a low temperature because they are too short-lived. However, it is known experimentally that paraexcitons have extremely long lifetimes of over several hundred nanoseconds, long enough to cool them down to the desired temperature of a BEC.

The team managed to capture paraexcitons in the bulk of Cu₂O below 400 millikelvin with a dilution refrigerator, a cryogenic device that cools by mixing two isotopes of helium and is often used by scientists trying to realize quantum computing. They then visualized the exciton BEC directly in real space using mid-infrared induced absorption imaging, a type of microscopy that uses mid-infrared light. This allowed the team to make precision measurements, including exciton density and temperature, which in turn allowed them to highlight the differences and similarities between exciton BEC and regular atomic BEC.

The group’s next step will be to study the dynamics of how the exciton BEC forms in the bulk semiconductor and study collective excitations of exciton BECs. Their ultimate goal is to build a platform based on a system of exciton BECs to further elucidate its quantum properties and develop a better understanding of the quantum mechanics of qubits that are strongly coupled to their environment.

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Financing:

This research was supported by MEXT, JSPS KAKENHI (Grant Nos. JP20104002, JP26247049, JP25707024, JP15H06131, JP17H06205); through the Photon Frontier Network Program, Quantum Leap Flagship Program (Q-LEAP) Grant No. JPMXS0118067246 from MEXT; and from JSPS via its FIRST program.

Researchers create first quasiparticle Bose-Einstein condensate

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