Apr 2 2019
A certain amount of effort would be required to experimentally achieve the combination of various phases of water—liquid water, solid ice, and water vapor. For example, if the aim is to place ice next to vapor, the water would have to be continuously chilled to maintain the solid phase while also heating it to maintain the gaseous phase.
Theoretical calculations performed by (left to right) Neil Robinson, Robert Konik, Alexei Tsvelik, and Andreas Weichselbaum of Brookhaven Lab’s Condensed Matter Physics and Materials Science Department suggest that Majorana fermions exist in the boundaries of magnetic materials with different magnetic phases. Majorana fermions are particle-like excitations that emerge when single electrons fractionalize into two halves, and their unique properties are of interest for quantum applications. (Image credit: Brookhaven National Laboratory)
Condensed matter physicists vie for this potential to form different conditions in the same system since intriguing phenomena and properties usually materialize at the interfaces between two phases. Their interest at present is toward the conditions under which Majorana fermions might show up close to these boundaries.
Majorana fermions, which are particle-like excitations known as quasiparticles, are formed following the fractionalization, or splitting, of individual electrons into two halves. Simply put, an electron turns into an entangled, or linked, pair of two Majorana quasiparticles, where the link persists irrespective of the distance between them.
Researchers hope to use Majorana fermions that are physically isolated in a material to securely store information in the form of qubits, which are the building blocks of quantum computers. The exotic properties of Majoranas, such as their high insensitivity to electromagnetic fields and other environmental “noise,” render them perfect for carrying information over long distances without any loss.
However, until now, Majorana fermions have only been achieved in materials at extreme conditions, such as at frigid temperatures near absolute zero (−459 °F) and under high magnetic fields. Moreover, despite being “topologically” protected from local atomic impurities, defects, and disorder that exist in all materials (i.e. their spatial properties stay unchanged even when the material is twisted, bent, distorted, or stretched), they cannot survive under the impact of strong perturbations. Furthermore, the temperature range over which they can function is extremely narrow. Therefore, Majorana fermions cannot be used readily for practical technological application.
Currently, a group of physicists headed by the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory and involving collaborators from Germany, China, and the Netherlands has come up with an innovative theoretical method for creating more powerful Majorana fermions. Based on their calculations, reported in a paper published in Physical Review Letters on January 15th, 2018, these Majoranas appear at higher temperatures (by several orders of magnitude) and are not affected much by noise and disorder. Despite being not topologically protected, they can withstand if the perturbations gradually vary from one point to another in space.
Our numerical and analytical calculations provide evidence that Majorana fermions exist in the boundaries of magnetic materials with different magnetic phases, or directions of electron spins, positioned next to one other. We also determined the number of Majorana fermions you should expect to get if you combine certain magnetic phases.
Alexei Tsvelik, Study Co-Author, Senior Scientist, Brookhaven National Laboratory
Tsvelik is the leader of the Condensed Matter Theory Group in Brookhaven Lab’s Condensed Matter Physics and Materials Science (CMPMS) Department.
For the theoretical study, the researchers focused on magnetic materials known as spin ladders—crystals made of atoms with a three-dimensional (3D) structure that is subdivided into pairs of chains resembling ladders. For many years, the researchers have been analyzing the properties of spin ladder systems and anticipated that they would give rise Majorana fermions, but they were not sure about how many. They performed their calculations by applying the mathematical framework of quantum field theory to describe the fundamental physics behind elementary particles, as well as a numerical technique (density-matrix renormalization group) to simulate quantum systems in which electrons behave in a strongly correlated way.
We were surprised to learn that for certain configurations of magnetic phases we can generate more than one Majorana fermion at each boundary.
Robert Konik, Study Co-Author, Chair, CMPMS Department, Brookhaven National Laboratory
If Majorana fermions are to be practically useful in quantum computing, they must be produced in huge numbers. The belief of computing experts is that the minimum threshold at which quantum computers will have the ability to solve problems that cannot be solved by classical computers is 100 qubits. It is also necessary for the Majorana fermions to be movable in such a manner that they can turn entangled.
The researchers intend to follow up their theoretical analyses with experiments with the help of engineered systems like quantum dots (nanosized semiconducting particles) or trapped (confined) ions. In comparison to the properties of real materials, it is possible to tune and manipulate the properties of engineered materials more easily to introduce the different phase boundaries in which Majorana fermions may emerge.
What the next generation of quantum computers will be made of is unclear right now. We’re trying to find better alternatives to the low-temperature superconductors of the current generation, similar to how silicon replaced germanium in transistors. We’re in such early stages that we need to explore every possibility available.
Robert Konik, Study Co-Author, Chair, CMPMS Department, Brookhaven National Laboratory