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MIT researchers have observed mysterious Majorana fermions on Golden Isles. The discovery could lead to a new family of robust qubits for quantum computing.
Physicists’ discovery could lead to a new family of robust qubits for quantum computing.
Physicists in MIT and elsewhere they have observed evidence of Majorana fermions, particles that are theorized to be their own antiparticles, on the surface of a common metal: gold. This is the first sighting of Majorana fermions on a platform that can potentially be expanded. The results, published in the procedures of the National Academy of Sciences, are an important step towards particle isolation as stable, error-proof qubits for quantum computing.
In particle physics, fermions are a class of elementary particles that includes electrons, protons, neutrons, and quarks, which make up the basic components of matter. For the most part, these particles are considered Dirac fermions, after the English physicist Paul Dirac, who first predicted that all fundamental fermion particles should have a counterpart, somewhere in the universe, in the form of an antiparticle, essentially a identical twin of opposite charge
In 1937, the Italian theoretical physicist Ettore Majorana extended Dirac’s theory, predicting that among fermions, there should be some particles, from Majorana fermions, that cannot be distinguished from their antiparticles. Mysteriously, the physicist disappeared during a ferry trip off the Italian coast just a year after making his prediction. Scientists have been searching for the enigmatic Majorana particle ever since. It has been suggested, but not proven, that the neutrino may be a Majorana particle. On the other hand, theorists have predicted that Majorana fermions can also exist in solids under special conditions.
Now, the MIT-led team has observed evidence of Majorana fermions in a system of materials they designed and manufactured, consisting of gold nanowires grown on a superconducting material, vanadium, and dotted with small ferromagnetic “islands” of europium sulfide. . When the researchers scanned the surface near the islands, they saw characteristic signal peaks near zero energy on the upper surface of the gold that, according to the theory, should only be generated by pairs of Majorana fermions.
“Majorana ferminons are these exotic things, which have long been a dream to see, and now we see them in a very simple material: gold,” says Jagadeesh Moodera, senior research scientist at MIT’s Department of Physics. “We have shown that they are there, stable and easily scalable.”
“The next impulse will be to take these objects and turn them into qubits, which would be a huge step towards practical quantum computing,” adds co-author Patrick Lee, a professor of physics at William and Emma Rogers at MIT.
Lee and Moodera’s co-authors include former MIT postdoc and first author Sujit Manna (currently on the faculty of the Indian Institute of Technology, Delhi), and former MIT postdoc Peng Wei of the University of California at Riverside, along with Yingming Xie and Kam Tuen Law of the Hong Kong University of Science and Technology.
High risk
If they could be harnessed, Majorana’s fermions would be ideal as qubits, or individual computational units for quantum computers. The idea is that a qubit would be made up of combinations of Majorana fermion pairs, each of which would be separate from its partner. If noise errors affect one member of the pair, the other should not be affected, thus preserving the integrity of the qubit and allowing it to perform a calculation correctly.
Scientists have searched for Majorana fermions in semiconductors, the materials used in conventional transistor-based computing. In their experiments, the researchers combined semiconductors with superconductors, materials through which electrons can travel without resistance. This combination imparts superconducting properties to conventional semiconductors, which physicists say should induce particles in the semiconductor to divide, forming the Majorana fermion pair.
“There are several material platforms where people think they have seen Majorana particles,” says Lee. “The evidence is getting stronger, but it’s not 100% proven yet.”
Furthermore, semiconductor-based configurations to date have been difficult to scale to produce the thousands or millions of qubits needed for a practical quantum computer, because they require the growth of very precise crystals of semiconductor material and it is very difficult to convert them into high-superconductors. quality.
About a decade ago, Lee, working with his graduate student Andrew Potter, had an idea: Perhaps physicists could observe Majorana fermions in metal, a material that easily becomes superconductive near a superconductor. Scientists routinely convert metals, including gold, into superconductors. Lee’s idea was to see if the state of gold’s surface, its top layer of atoms, could be made superconducting. If this could be accomplished, then gold could serve as a clean and atomically accurate system in which researchers could observe Majorana fermions.
Lee proposed, building on Moodera’s previous work with ferromagnetic insulators, that if placed on a gold state of superconducting surface, then researchers should have a good chance to clearly see the Majorana fermion signatures.
“When we first proposed this, I couldn’t convince many experimenters to try it because the technology was daunting,” says Lee, who eventually partnered with the Moodera experimental group to obtain crucial funding from the Templeton Foundation to carry out the design. Jagadeesh and Peng really had to reinvent the wheel. It was extremely brave to jump into this, because it is really a high risk, but we think it is a great reward. “
“Find Majorana”
In recent years, researchers have characterized the state of gold’s surface and demonstrated that it could function as a platform for observing Majorana fermions, after which the group began making the configuration that Lee envisioned years ago.
They first grew a superconducting vanadium foil, on top of which they overlaid gold-plated nanowires, which were approximately 4 nanometers thick. They tested the conductivity of the top layer of gold and found that it did indeed become superconductive in proximity to vanadium. They then deposited onto the “islands” of europium sulfide gold nanowires, a ferromagnetic material that is capable of providing the internal magnetic fields necessary to create the Majorana fermions.
The team then applied a small voltage and used scanning tunnel microscopy, a specialized technique that allowed researchers to scan the energy spectrum around each island on the surface of the gold.
Moodera and his colleagues looked for a very specific energy signature that only Majorana fermions should produce, if they exist. In any superconducting material, electrons travel through certain ranges of energy. However, there is a desert or “energy gap” where there should be no electrons. If there is a spike within this gap, it is most likely a Majorana fermion firm.
In reviewing their data, the researchers observed peaks within this energy gap at opposite ends of various islands along the direction of the magnetic field, which were clear signatures of pairs of Majorana fermions.
“We only see this peak on opposite sides of the island, as the theory predicted,” says Moodera. “Elsewhere, you don’t see it.”
“In my talks, I like to say that we are finding Majorana, on an island in a sea of gold,” adds Lee.
Moodera says the kit setup, which requires just three layers (gold sandwiched between a ferromagnet and a superconductor) is a “stable and easy-to-achieve system” that should also be economically scalable compared to conventional semiconductor-based approaches to generate qubits.
“Seeing a pair of Majorana fermions is an important step in making a qubit,” says Wei. “The next step is to make a qubit of these particles, and now we have some ideas on how to do it.”
Reference: “Signature of a pair of Majorana zero modes in golden surface superconducting states” by Sujit Manna, Peng Wei, Yingming Xie, Kam Tuen Law, Patrick A. Lee and Jagadeesh S. Moodera, April 6, 2020, procedures of the National Academy of Sciences.
DOI: 10.1073 / pnas.1919753117
This research was funded, in part, by the John Templeton Foundation, the US Office of Naval Research. USA, The National Science Foundation and the US Department of Energy. USA
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