June 5, 2023
Bose-Einstein condensate created using quasi-particles for the first time

Bose-Einstein condensate created using quasi-particles for the first time

Physicists at the University of Tokyo have developed the first Bose-Einstein condensate of quasiparticles, a breakthrough that could greatly advance quantum computing.

The Bose-Einstein condensate is known as the enigmatic “fifth state” of matter, along with solids, liquids, gases and plasma. Now, experts have created the first Bose-Einstein condensate of quasiparticles — entities that are not elementary particles but exhibit similar properties, such as charge and spin.

For decades, scientists were unsure whether quasiparticles could undergo Bose-Einstein condensation similar to real particles, with their findings potentially having important implications for advancing quantum technologies.

The research paper, “Observation of Bose-Einstein condensates of excitons in a bulk semiconductor,” is published in Nature communications.

What is a Bose-Einstein condensate?

The Bose-Einstein condensate was first predicted in the early 20th century and only created in a laboratory in 1995 and remains the strangest and most mysterious state of matter. Bose-Einstein condensates occur when a group of atoms is cooled to billionths of a degree above absolute zero. To achieve this, scientists traditionally use lasers and magnetotraps to gradually reduce the temperature of a gas, usually composed of rubidium atoms.

Individuals barely move at this temperature and begin to exhibit unusual behavior. They experience the same quantum state and begin to merge, occupying the same volume as an indistinguishable “superatom” that essentially behaves as a single particle.

Bose-Einstein condensates are the focus of much fundamental research, including the simulation of condensed matter systems, and have a number of applications in quantum information processing. Quantum computing is still in its infancy and uses several systems, all of which depend on quantum bits (qubits) in the same quantum state. Primarily, a Bose-Einstein condensate is created from rare gases of ordinary atoms, while a Bose-Einstein condensate produced from exotic atoms has never been achieved so far.

Understanding quasi-particles

An exotic atom is an atom where a subatomic particle, such as an electron or a proton, is replaced by another subatomic particle with the same charge. For example, the positron is an exotic atom consisting of an electron and its positively charged antiparticle, a positron.

An exciton is another example. When light strikes a semiconductor, the energy is strong enough to excite electrons, causing them to jump from an atom’s valence level to its conduction level. These excited electrons can flow freely in an electric current – converting light energy into electricity. When the negatively charged electrons make this jump, the hole left behind can be treated as if it were a positively charged particle, with the negative electron and positive hole attracted and bound together.

This electron-hole pair is an electrically neutral quasiparticle known as an exciton. Quasiparticles are not counted as one of the 17 elementary particles of the standard model of particle physics, but still exhibit elementary particle properties such as charge and spin. There are two forms of excitons: orthoexcitons, in which the spin of the electron is parallel to the spin of its hole, and paraexcitons, where the spin is antiparallel to its hole. Electron-hole systems have been used to create other phases of matter, such as electron-hole plasma and even exciton liquid droplets, leading research to see if they could create a Bose-Einstein condensate of excitons.

Makoto Kuwata-Gonokami, a physicist at the University of Tokyo and co-author of the paper, commented: “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 the particles could undergo Bose-Einstein condensation in the same way as real particles. It’s the holy grail of low-temperature physics.”

Pioneering exciton concentrator

The team believed that the most promising candidate for making Bose-Einstein exciton condensates in a bulk semiconductor were hydrogen-like paraexions created in copper oxide (Cu2O), a compound of copper and oxygen, due to their long life.

In the 1990s, researchers attempted to create a Bose-Einstein paraexciton condensate at liquid helium temperatures of about 2 K, but failed due to the much lower temperatures required. Ortho-excitons cannot reach such a low temperature as they are very short-lived, while para-excitons have extremely long lifetimes of over several hundred nanoseconds – long enough to cool them to the desired temperature of a Bose-Einstein condensate.

The physicists managed to trap the paraexcitons in most of the Cu2O below 400 millikelvins using a dilution refrigerator cooled by combining two isotopes of helium. They then visualized the Bose-Einstein exciton condensate in real space using mid-infrared induced absorption imaging. This allowed the team to take precise measurements, such as the density and temperature of the excitons, allowing them to observe differences and similarities between excitons and normal Bose-Einstein atomic condensates.

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.

The researchers now aim to investigate the dynamics of how the Bose-Einstein exciton condensate forms in the semiconductor and its collective excitations. Their primary goal is to build a platform based on a system of these Bose-Einstein exciton condensates to better understand the quantum mechanics of qubits.

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