Physicists at the University of Tokyo have developed the first Bose-Einstein condensate from quasiparticles, a landmark breakthrough that could significantly advance quantum computing.
The Bose-Einstein condensate is known as the enigmatic “fifth state” of matter, alongside solids, liquids, gases, and plasmas. Now experts have created the first Bose-Einstein condensate from quasiparticles – entities that aren’t elementary particles but exhibit similar properties, such as charge and spin.
For decades, scientists wondered if quasiparticles could undergo Bose-Einstein condensation in the same way as real particles, with their findings potentially having important ramifications for advancing quantum technologies.
The research paper, “Observation of Bose-Einstein condensates of excitons in a bulk semiconductor”, is published in Communication Nature.
What is a Bose-Einstein condensate?
The Bose-Einstein condensate was first predicted in the early 20th century and was not created in a laboratory until 1995, and remains the strangest and most mysterious state of matter. Bose-Einstein condensates occur when a group of atoms is cooled to a few billionths of a degree above absolute zero. To achieve this, scientists traditionally use lasers and magnetic traps to gradually reduce the temperature of a gas, usually made up of rubidium atoms.
The atoms barely move at this temperature and begin to show unusual behavior. They experience the same quantum state and begin to merge, occupying the same volume as an indistinguishable “super atom” that essentially behaves like a single particle.
Bose-Einstein condensates are the subject of much fundamental research, including in the simulation of condensed matter systems, and have a range of applications in quantum information processing. Quantum computing is still in its infancy and uses various systems, all of which depend on quantum bits (qubits) in the same quantum state. Predominantly, a Bose-Einstein condensate is created from dilute gases of ordinary atoms, a Bose-Einstein condensate made from exotic atoms having never been made until now.
Understanding quasiparticles
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, positronium 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, transforming light energy into electrical energy. When 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 the positive hole being attracted and bound together.
This electron-hole pair is an electrically neutral quasi-particle called an exciton. Quasiparticles do not count as one of the 17 elementary particles in 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 liquid droplets of excitons, leading research to determine if they could produce a condensate of Bose-Einstein from excitons.
Makoto Kuwata-Gonokami, a physicist at the University of Tokyo and co-author of the paper, commented: “Direct observation of an exciton condensate in a three-dimensional semiconductor has been much sought after since its first theoretical proposal. in 1962. No one knew if quasiparticles could undergo Bose-Einstein condensation in the same way as real particles. It is the holy grail of low temperature physics.
Pioneer of an exciton condensate
The team believed that the most promising candidate for making Bose-Einstein exciton condensates in a bulk semiconductor were the hydrogen-like paraexcitons created in cuprous oxide (Cu2O), a compound of copper and oxygen, due to their long lifespan.
In the 1990s, researchers attempted to create a Bose-Einstein paraexciton condensate at liquid helium temperatures of around 2 K, but failed due to the need for much colder temperatures. Orthoexcitons cannot reach such a low temperature because they have too short a lifetime, while paraexcitons have an extremely long lifetime of several hundred nanoseconds – long enough to cool them to the desired temperature of a condensate of Bose Einstein.
Physicists were able to trap paraexcitons in the bulk of Cu2O below 400 millikelvins using a dilution refrigerator that cools 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 obtain precision measurements, such as exciton density and temperature, allowing them to observe the differences and similarities between the exciton and regular Bose-Einstein atomic condensates.
The researchers now aim to study the dynamics of the formation of the Bose-Einstein exciton condensate in the bulk semiconductor and its collective excitations. Their primary objective is to build a platform based on a system of these Bose-Einstein exciton condensates to better understand the quantum mechanics of qubits.
#BoseEinstein #condensate #created #quasiparticles #time