Researchers from Tsinghua University and Institute of Physics, Chinese Academy of Sciences in Beijing have shown that the states of matter within multilayered magnetically doped semiconductors can be controlled, in turn controlling their internal resistance, with the help of the Quantum Anomalous Hall (QAH) effect.
The QAH effect is found in certain uniquely designed materials where electrons have the ability to move a millimeter-scale distance without losing their energy. The capability of applying this effect to devices would enable a new revolution in computation speed and energy efficiency.
In a research published in the Chinese Physics Letters journal, scientists have reported that they have developed an artificial material that can be used for creating Topological Quantum Computer with the help of Molecular Beam Epitaxy, an innovative technique that enables the synthesis of single-molecule-thick layers of crystal to be stacked, and by employing the QAH effect.
A quantum computer exploits the potential of subatomic particles to be in multiple states at the same time (rather than the binary 0 or 1 observed in existing computers), thereby enabling them to solve specific types of problems in a highly efficient manner. The Topological Quantum Computer would be a prospective evolution on this. Rather than physical particles, they use a particular type of quasiparticle, known as the anyon, for encoding the information. It has been found that anyons are highly resistant to errors in both storage and processing of information.
We can indeed realise QAH multilayers, or a stack of multiple layers of crystal lattices that are experiencing the QAH effect, with several magnetically doped films spaced by insulating Cadmium Selenide layers. Since we do it by molecular beam epitaxy, it is easy to control the properties of each layer to drive the sample into different states.
Ke He, Professor at Tsinghua University.
Cadmium selenide is a molecule that is formed of one Cadmium atom and one Selenium atom used as a semiconductor. The conductive properties of this material can be modified by the addition of impurities.
The ability to synthesize multilayers of thin crystals facilitates the sandwiching of an insulating film between the layers that conduct electric current. This prevents the undesirable interaction of the electrons between the sheets, the same way wires are prevented from crossing in electronics. Structures such as these are highly fascinating to investigate since they force some of the electrons into a so-called “edge state” that, to date, was very challenging to fabricate. This “edge state” functions as a path for a portion of the electrons to flow through without any resistance; since several layers are stacked on top of one another, the effect gets amplified by pushing a greater fraction of the electrons into this state.
“By tuning the thicknesses of the QAH layers and Cadmium Selenide insulating layers; we can drive the system into a magnetic Weyl semimetal ... a state of matter that so far has never been convincingly demonstrated in naturally occurring materials.”
First observed in July 2015, a Weyl semimetal is an exotic state of matter categorized as a solid state crystal with the ability to conduct electricity using the massless “Weyl Fermions,” instead of electrons. This considerable difference in mass between the Weyl Fermions and the electrons enables the electric current to flow through circuits in a highly effective way, thereby enabling faster devices.
“Now what interests me most is to construct QAH bilayers with the two layers able to be independently controlled. If we could get a pair of counter-propagating edge states, while putting a superconducting contact on the edge of the sample, the two edge states might bind together due to the superconducting contact, leading to Majorana modes which can be used to build a Topological Quantum Computer.”
It is considered that Majorana modes can be used in Quantum Error Correcting code, a property specific to topological quantum computers, and an essential part of information theory used to help reduce errors that normally occur in data transmission and to help neutralize the interference effects. It is also considered that this prospectively gives them the ability to not only process quantum information but also store it more effectively in the future.