Researchers have created the first Bose-Einstein quasi-condensate

Close-up of the device in a refrigeration-free dilution refrigerator. The dark red cubic crystal in the center of the image is copper oxide. A zinc selenide meniscus lens placed behind the crystal is an objective lens. A rod and phase down the crystal are used to generate an inhomogeneous stress field in the crystal that acts as a potential trap for excitons. Credit: Yosuke Morita, Kosuke Yoshioka, and Makoto Kawata-Junokami, University of Tokyo

Physicists have created the first Bose-Einstein condensates – the mysterious fifth state of matter – made of quasiparticles, entities that are not considered elementary particles but can still have properties of elementary particles such as charge and spin. For decades, it was not known if they could undergo Bose-Einstein condensation in the same way as real particles, and now it appears that they can. The result is set to have a significant impact on the development of quantum technologies including quantum computing.

A paper was published in the journal describing the process of formation of the material, which was achieved at hair amplitude temperatures of absolute zero. Nature Communications.

Bose-Einstein condensates are sometimes described as the fifth state of matter, along with solids, liquids, gases, and plasmas. Theoretically predicted in the early 1900s, Bose-Einstein condensates, or BECs, were only created in the lab as recently as 1995. They may also be the strangest state of matter, with many remaining unknown to science.

BECs occur when a group of atoms is cooled to a billionth of a degree above absolute zero. Researchers typically use lasers and magnet traps to steadily reduce the temperature of a gas, which is typically composed of rubidium atoms. At this super-cold temperature, the atoms barely move and start to show very strange behavior.

They experience the same quantum state – roughly like coherent photons in a laser – and begin to clump together, occupying the same volume as one indistinguishable super atom. The group of atoms basically behaves as a single particle.

Currently, BECs remain the subject of much basic research, and for simulations of condensed matter systems, however, in principle, they have applications in quantum information processing. Quantum computing, still in its early stages of development, uses a number of different systems. But they all rely on quantum bits, or qubits, that are in the same quantum state.

Researchers have created the first Bose-Einstein quasi-condensate

A copper oxide crystal (red cube) was placed on the sample stage in the center of the dilution refrigerator. The researchers installed windows on the refrigerator shields allowing visual access to the sample stage in four directions. Two-way windows allow transmission of excitation light (orange solid line) and fluorescence from parasketons (solid yellow line) into the visible region. Windows in the other two directions allow transmission of probe light (solid blue line) for induced absorption imaging. To reduce incoming heat, the researchers carefully designed the windows by reducing the numerical aperture and using a specific window material. This specialized window design and high cooling power of the Refrigerant-Free Thin Refrigerator make it easy to achieve a core temperature of at least 64 mK. Credit: Yosuke Morita, Kosuke Yoshioka, and Makoto Kawata-Junokami, University of Tokyo

Most BECs are made from dilute gases of ordinary atoms. But so far, BEC composed of exotic atoms has not been achieved.

Exotic atoms are atoms in which a subatomic particle, such as an electron or a proton, is replaced by another subatomic particle of the same charge. Positronium, for example, is a strange atom made up of an electron and its positively charged antiparticle, the positron.

The exciton is another such example. When light strikes a semiconductor, the energy is enough to excite the electrons to jump from the atom’s valence level to its conduction level. These excited electrons then flow freely in an electric current – in essence converting light energy into electrical energy. When a 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 bind together.

Taken together, this electron-hole pair is an electrically neutral quasiparticle called an exciton. A quasiparticle is a particle-like entity that is not one of 17 elementary particles of the Standard Model of particle physics, but can still have elementary particle properties such as charge and spin. The exciton particle can also be described as an exotic atom because it is actually a hydrogen atom whose only positive proton has been replaced by a single positive hole.

Excitons come in two flavors: orthoexcitons, where the electron’s spin is parallel to its hole spin, and parexcitons, where the electron’s spin is antiparallel (parallel but opposite) to its hole.

Electron-hole systems have been used to create other phases of matter such as electron-hole plasmas and even droplets of exciton liquid. The researchers wanted to see if they could produce BEC from excitons.

Researchers have created the first Bose-Einstein quasi-condensate

The researchers applied an inhomogeneous strain using a lens array below the sample (the red cube). The heterogeneous stress results in a heterogeneous stress domain that acts as a potential trap for excitons. Focus the excitation beam (orange solid line) on the bottom of the trap potential in the sample. An exciton (yellow ball) consists of one electron (blue ball) and one hole (red ball). The team detected the excitons by either luminescence (yellow shade) or differential transmission of the probe’s light (blue shade). An objective lens placed behind the sample collected luminescence from the excitons. The probe beam is also scattered through the objective lens. Credit: Yosuke Morita, Kosuke Yoshioka, and Makoto Kawata-Junokami, University of Tokyo

“Direct observation of exciton condensation in 3D semiconductors has been researched since it was first theoretically proposed in 1962,” Makoto Kawata said. “No one knew if quasiparticles could undergo Bose-Einstein condensation in the same way as real particles.” . Junokami, a physicist at the University of Tokyo and a co-author of the paper. “It’s kind of the holy grail of low-temperature physics.”

The researchers believe that the hydrogen-like paraxyitions were created in copper oxide (Cu2O), a compound of copper and oxygen, has been one of the most promising candidates for the fabrication of exciton BECs in bulk semiconductors due to its long life. Attempts to create a BEC at liquid helium temperatures around 2 K were made in the 1990s, but failed because in order to create a BEC from excitons, the temperatures are much lower than those required.

Orthoxitones cannot reach a low temperature because they are short-lived. However, it is experimentally known that paraxitones have a very long lifetime in excess of several hundred nanoseconds, which is long enough to cool them to the temperature required for BEC.

The team succeeded in catching parxiton in the bulk of the copper2O below 400 mK using a dilution refrigerator, a cryogenic device that cools by mixing two isotopes of helium together and is commonly used by scientists trying to manufacture quantum computers.

They then imaged the BEC exciton directly in real space using mid-infrared induced absorption imaging, a type of microscopy that uses light in the mid-infrared range. This allowed the team to take precise measurements, including the density and temperature of the excitons, which in turn enabled them to determine the differences and similarities between a BEC exciton and a normal atomic BEC.

The group’s next step will be to investigate the dynamics of how a BEC exciton forms in bulk semiconductors, and to investigate the collective excitation of exciton BECs. Their ultimate goal is to build a platform based on exciton BECs, to further elucidate their quantum properties, and to develop a better understanding of the quantum mechanics of qubits strongly related to their environment.

Predict the temperature of the Bose-Einstein condensation of excitons

more information:
Yosuke Morita et al., Observing Bose-Einstein condensates of excitons in bulk semiconductors, Nature Communications (2022). DOI: 10.1038 / s41467-022-33103-4

Presented by the University of Tokyo

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