Scientists have described the process of creating a substance achieved at a temperature “a hair’s breadth” from absolute zero.
What is a Bose-Einstein Condensate?
Bose-Einstein condensate - state of aggregationa substance based on bosons cooled to temperatures close to absolute zero. It is sometimes called the fifth state of matter, along with solids, liquids, gases and plasma. Theoretically predicted at the beginning of the 20th century, the Bose-Einstein condensate, or BEC, was created in the laboratory only in 1995. It is also perhaps the strangest state of matter, and much about it remains unknown to science.
Absolute zero is the temperature atin which molecules stop any movement. Equals –273.15 °C, or zero on the Kelvin scale. When the temperature approaches absolute zero, some rather strange things begin to happen.
Photo: NIST/Wikimedia Commons
BEC occurs when a group of atoms coolswith accuracy to billionths of a degree above absolute zero. Typically, physicists use lasers and magnetic traps to continually lower the temperature of a gas made up of rubidium atoms. At such an ultra-low temperature, the atoms hardly move and begin to behave very strangely.
They are in the samequantum state—almost like coherent photons in a laser—and begin to stick together, occupying the same volume as one indistinguishable superatom. A collection of atoms essentially behaves like one particle.
Bose-Einstein condensate and quantum computing
At the moment, BEC is important for fundamentalresearch and modeling of condensed matter systems. However, it is also useful in quantum information processing. Quantum computing, which is still in its early stages of development, uses a variety of systems. But they all depend on quantum bits, or qubits, being in the same quantum state.
Most BECs are made from dilute gases of ordinary atoms. But until now it has not been possible to create a condensate from exotic atoms.
What are exotic atoms?
Exotic atoms are those in whichone subatomic particle, such as an electron or proton, is replaced by another subatomic particle with the same charge. Positronium, for example, is an exotic atom that consists of an electron and its positively charged antiparticle, the positron.
Exciton is another example of atomic “exoticism”.When light hits a semiconductor, it has enough energy to excite electrons and move from the valence level of the atom to its conduction level. These excited electrons then flow freely in an electrical current, essentially converting light energy into electrical energy. When a negatively charged electron makes this "jump", the remaining space can be thought of as a positively charged particle. The negative electron and the positive empty space are attracted and thus bond.
Together this electron-spatial pairis an electrically neutral quasiparticle known as an exciton. A quasiparticle is a particle-like “entity” that is not considered one of the 17 elementary particles in the Standard Model of particle physics.
The Standard Model is a theoretical construct inelementary particle physics, describing the electromagnetic, weak and strong interaction of all elementary particles. The modern formulation was completed in the 2000s after experimental confirmation of the existence of quarks.
However, she may still haveproperties of an elementary particle - such as charge and rotation. An excitonic quasiparticle can also be described as an exotic atom. That's because it's actually a hydrogen atom, with its single positive proton replaced by a single void with a positive charge.
The researchers applied a non-uniform voltage using a lens mounted under the sample (red cube).
Image Credit & Copyright: Yusuke Morita, Kosuke Yoshioka, and Makoto Kuwata-Gonokami, University of Tokyo
There are two types of excitons:orthoexcitons, in which the electron spin is parallel to the spin of its hole, and paraexcitons, in which the electron spin is antiparallel (parallel, but in the opposite direction) to the spin of its void (hole).
How were electron-void systems used in the past?
Electron-hole systems have been used forcreating other phases of matter, such as electron-hole plasma and even excitonic liquid droplets. Now the scientists wanted to see if they could create a BEC from excitons.
The point is that direct observation of the excitoncondensate in a three-dimensional semiconductor has been in high demand since theorists proposed it in 1962. No one knew whether quasiparticles could undergo Bose-Einstein condensation in the same way as real particles.” As the authors of the new study explain, “this is something of a Holy Grail of low-temperature physics.”
Attempts in the past
Scientists believed that hydrogen-likeParaexcitons created in cuprous oxide (Cu₂O), a compound of copper and oxygen, are best suited for fabricating excitonic BECs in bulk semiconductors. All because of their long lifespan. Attempts to create a paraexciton BEC at liquid helium temperatures of about 2 Kelvin (-271.15 °C) were made back in the 1990s, but were not successful. The problem is that creating a BEC from excitons requires temperatures much lower than this.
Orthoexcitons cannot reach such a lowtemperatures, as they are too short-lived. However, it is experimentally well known that paraexcitons have extremely long lifetimes, exceeding several hundred nanoseconds, which is long enough to cool them to the desired BEC temperature.
What have the scientists done?
As part of the experiment, physicists caughtparaexcitons in a mass of Cu₂O with a temperature below 400 mK (millikelvin). To do this, they used a dissolution refrigerator, a specifically cryogenic device. Scientists are using it in an attempt to realize quantum computers.
The dilution refrigerator is a cryogenic device,first proposed by Heinz London. The cooling process uses a mixture of two helium isotopes: ³He and ⁴He. When cooled below 700 mK, the mixture experiences spontaneous phase separation, forming phases rich in ³He and rich in ⁴He.
Close-up of the apparatus in a non-cryogenic refrigeratorto dissolve. The dark red cubic crystal in the center of the image is cuprous oxide. Credit: Yusuke Morita, Kosuke Yoshioka, and Makoto Kuwata-Gonokami, University of Tokyo
They then directly imaged the exciton BECin real space. They were helped by imaging with induced absorption in the mid-infrared range. This is a type of microscopy that uses light in the mid-infrared range. This way, scientists were able to make precise measurements, including the density and temperature of excitons. In turn, this allowed them to note the differences and similarities between exciton BEC and conventional atomic BEC.
What's next?
Scientists are not going to stop thereachieved. Their next step is to study the dynamics of the formation of an excitonic BEC in a bulk semiconductor and to study the collective excitations of an excitonic BEC.
As a result, physicists hope to build a platformbased on a system of excitonic BECs. This will help elucidate its quantum properties and better understand the quantum mechanics of qubits, which are strongly coupled to their environment.
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On the cover: press.princeton.edu