June 10, 2023
Observation of Bose-Einstein condensates of excitons

Scientists have created the first Bose-Einstein condensate quasi-particle

Bose-Einstein condensates are sometimes described as the fifth state of matter. They were only created in a lab as recently as 1995. They experience the same quantum state—almost like coherent photons in a laser—and begin to aggregate, occupying the same volume as an indistinguishable superatom.

Currently, BECs remain the subject of much basic research for the simulation of condensed matter systems, but in principle, they have applications in quantum information processing. Most BECs are made from thin gases of ordinary atoms. But until now, a BEC made of exotic atoms has never been achieved.

Scientists from the University of Tokyo wanted to see if they could make a BEC out of excitons. Using quasi-particles, they created the first Bose-Einstein condensate – the mysterious “fifth state” of matter. The finding is set to significantly influence the development of quantum technologies, including quantum computing.

The combined electron-hole pair is an electrically neutral “quasiparticle” called an exciton. The exciton quasiparticle can also be described as an exotic atom because it is, in fact, a hydrogen atom that has had its single positive proton replaced by a single positive hole.

Experimental setup inside the cryogen-free dilution cooler The copper oxide crystal (red cube) was placed on a sample stage in the center of the dilution cooler. The researchers attached windows to the cooler shields that allowed visual access to the sample stage in four directions. Windows in two directions allowed transmission of excitation light (orange solid line) and luminescence from paraexcitons (yellow solid line) into the visible region. Windows in the other two directions allowed transmission of detector light (blue solid line) for induced absorption imaging. To reduce incoming heat, the researchers carefully designed the windows by minimizing the numerical opening and using a specific window material. This specialized window design and the high cooling capacity of the cryogen-free dilution cooler made it easy to achieve a minimum core temperature of 64 millikelvin. ©2022 Yusuke Morita, Kosuke Yoshioka and Makoto Kuwata-Gonokami, The University of Tokyo

Makoto Kuwata-Gonokami, a physicist at the University of Tokyo and co-author of the paper, said: “The direct observation of an exciton condensate in a three-dimensional semiconductor has been highly sought after since it was first proposed theoretically in 1962. No one knew whether quasiparticles could undergo Bose-Einstein condensation in the same way as real particles. It’s kind of the holy grail of low-temperature physics.”

Due to their extended lifetime, paraexcitons generated in copper oxide (Cu2O), a mixture of copper and oxygen, were considered as one of the most promising possibilities for creating exciton BECs in bulk semiconductor. In the 1990s, attempts were made to produce paraexciton BECs at liquid helium temperatures of about 2 K. However, they had failed because much lower temperatures are required to produce a BEC from excitons. Because they are very transient, orthoexcitons cannot achieve such a low temperature. However, it is known from experiments that paraexcitons have a very long lifetime of over a few hundred nanoseconds, which is sufficient to cool them to the necessary temperature of a BEC.

The team used a dilution refrigerator, a cryogenic device cooled by fusing two isotopes of helium and often used by scientists trying to develop quantum computers, to trap paraexcitons in the bulk of Cu2O below 400 millikelvins. Next, they used mid-infrared induced absorption imaging, a type of microscopy that uses light in the mid-infrared region, to directly see the excitonic BEC in real space.

As a result, the team was able to obtain precise measurements of the density and temperature of the excitons, which allowed them to identify differences and similarities between the exciton BEC and the conventional atomic BEC.

Schematic illustration of the physical processes involved for excitons in the sample
Schematic illustration of the physical processes involved for excitons in the sample The researchers applied an inhomogeneous voltage using a set of lenses below the sample (red cube). The inhomogeneous voltage results in an inhomogeneous strain field that acts as a trap potential for excitons. The excitation beam (orange solid line) was focused at the bottom of the trapping potential in the sample. An exciton (yellow sphere) consists of an electron (blue sphere) and a hole (red sphere). The team detected excitons either by luminescence (shaded yellow) or differential transmission of probe light (shaded blue). An objective lens placed behind the sample collected luminescence from excitons. The detector beam was also propagated through the objective lens. ©2022 Yusuke Morita, Kosuke Yoshioka and Makoto Kuwata-Gonokami, The University of Tokyo

The scientists further want to investigate the dynamics of how the exciton BEC is formed in the bulk semiconductor and investigate the collective excitations of the 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 tightly coupled to their environment.

Journal Reference:

  1. Yusuke Morita, Kosuke Yoshioka, and Makoto Kuwata-Gonokami, “Observation of Bose-Einstein exciton condensates in a bulk semiconductor,” Nature Communications: September 14, 2022. DOI: 10.1038/s423-430-

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