The creation of the first Bose-Einstein condensate could have a significant impact on the future of quantum computing
Physicists have created the first Bose-Einstein condensate — the mysterious ‘fifth state’ of matter — made from quasiparticles, entities that do not count as elementary particles but that can still have elementary-particle properties like charge and spin.
For decades, it was unknown whether they could undergo Bose-Einstein condensation in the same way as real particles, and it now appears that they can.
The finding is set to have a significant impact on the development of quantum technologies including quantum computing.
Bose-Einstein condensates are sometimes described as the fifth state of matter, alongside solids, liquids, gases and plasmas.
BECs occur when a group of atoms is cooled to within billionths of a degree above absolute zero.
Researchers commonly use lasers and ‘magnet traps’ to steadily reduce the temperature of a gas, typically composed of rubidium atoms.
At this ultracool temperature, the atoms barely move and begin to exhibit very strange behaviour.
They experience the same quantum state — almost like coherent photons in a laser — and start to clump together, occupying the same volume as one indistinguishable ‘super atom’.
The collection of atoms essentially behaves as a single particle.
same quantum state
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, still in early stages of development, makes use of a number of different systems.
But they all depend upon quantum bits, or qubits, that are in the same quantum state.
Most BECs are fabricated from dilute gases of ordinary atoms. But until now, a BEC made out of exotic atoms has never been achieved.
Exotic atoms are atoms in which one subatomic particle, such as an electron or a proton, is replaced by another subatomic particle that has the same charge.
Positronium, for example, is an exotic atom made of an electron and its positively charged anti-particle, a positron.
An ‘exciton’ is another such example. When light hits a semiconductor, the energy is sufficient to ‘excite’ electrons to jump up from the valence level of an atom to its conduction level.
These excited electrons then flow freely in an electric current — in essence transforming light energy into electrical energy.
When the negatively charged electron performs this jump, the space left behind, or ‘hole’, can be treated as if it were a positively charged particle.
The negative electron and positive hole are attracted and thus bound together. Combined, this electron-hole pair is an electrically neutral ‘quasiparticle’ called an exciton.
A quasiparticle is a particle-like entity that does not count as one of the 17 elementary particles of the standard model of particle physics, but that can still have elementary-particle properties like charge and spin.
The exciton quasiparticle can also be described as an exotic atom because it is in effect a hydrogen atom that has had its single positive proton replaced by a single positive hole.
Excitons come in two flavours: orthoexcitons, in which the spin of the electron is parallel to the spin of its hole; and paraexcitons, in which the electron spin is anti-parallel (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 out of excitons.
Makoto Kuwata-Gonokami, a physicist at the University of Tokyo and co-author of the paper, said: “Direct observation of an exciton condensate in a three-dimensional semiconductor has been highly sought after since it was first theoretically proposed in 1962.
“Nobody 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.”
The team managed to trap paraexcitons in the bulk of Cu2O below 400 millikelvins using a dilution refrigerator, a cryogenic device that cools by mixing two isotopes of helium together and which is commonly used by scientists attempting to realize quantum computers.
They then directly visualized the exciton BEC in real space by the use of mid-infrared induced absorption imaging, a type of microscopy making use of light in the middle of the infrared range.
This allowed the team to take precision measurements, including the density and temperature of the excitons, that in turn enabled them to mark out the differences and similarities between exciton BEC and regular atomic BEC.
The research is published in Nature Communications.
Image: A close-up picture of the apparatus in a cryogen-free dilution refrigerator. A dark red-coloured cubic crystal in the centre of the picture is cuprous oxide. A zinc selenide meniscus lens placed behind the crystal is an objective lens. A rod and a stage below the crystal is used for generation of an inhomogeneous strain field in the crystal that acts as a trap potential for excitons.
Credit: Yusuke Morita, Kosuke Yoshioka and Makoto Kuwata-Gonokami, The University of Tokyo.