Fermions in superconducting bismuth palladium could make future quantum computers

When it comes to entirely new, faster, more powerful computers, Majorana fermions may be the answer. These hypothetical particles can do a better job than conventional quantum bits (qubits) of light or matter. Why? Because of the spooky way Majorana fermions interact with each other at a distance. When two fermions interact, they usually dissipate energy, whereas two Majoranas are entangled and preserve the quantum state. But where to find these unique particles? Scientists observed a unique state on the surface of a superconducting material made of equal parts bismuth and palladium. While it didn’t host the long sought-after hypothetical Majorana fermions, it will stimulate further search for materials that do, paving a potential pathway for new computer architectures.

Above – The box marks the spot of the Dirac point of surface states: The surface electronic structure interrogates the relationship between superconductivity and topology. Scientists observed spin-polarized surface states in the noncentrosymmetric superconductor bismuth palladium (BiPd). The observation provides insightful information to guide future searches for topological superconductors, which are promising architectures for quantum information and computation technologies.

The study provides vital insight into the origin of superconductivity and the detection of Majoranas at Dirac points on the surface compared to the bulk. In turn, the results may help, one day, identify Majorana fermions. These particles could change how we design quantum computers.

A Majorana fermion also referred to as a Majorana particle, is a fermion that is its own antiparticle. In superconducting materials, Majorana fermions can emerge as (non-fundamental) quasiparticles (which are more commonly referred as Bogoliubov quasiparticles in condensed matter.). This becomes possible because a quasiparticle in a superconductor is its own antiparticle. A realization of Majoranas would allow them to be used to store and process quantum information within a quantum computation. Though the codes typically have no Hamiltonian to provide suppression of errors, fault-tolerance would be provided by the underlying quantum error correcting code.

Majorana Fermions in superconductors could make Topological Quantum computers.

Arxiv – (2012) Introduction to topological superconductivity and
Majorana fermions (21 pages)

MFs are equal superpositions of electrons and holes, can appear as Bogoliubov quasiparticles in effectively spinless superconductors. Their peculiar non-abelian (non-commutative) exchange statistics was discussed, and the basic concepts of Majorana qubits and topological quantum computation was introduced. Finally, we discussed the possibility to realize topological superconductivity by bringing semiconductors with strong spin-orbit coupling into proximity with standard s-wave
superconductors and exposing them to a magnetic field.

Clearly the types of systems investigated in the hunt for Majoranas contain a lot of new and exciting physics, but which experimental observation are indeed genuine Majorana sightings remains to be seen. We also mention that in a real system, with a finite size and interactions which may not be well described within a mean-field picture, there is not necessarily a perfectly clear and unique definition of a MF.

In any case, once MFs can be reproducibly realized and detected in the lab, the work has only started. Further studies will have to investigate their properties in detail, and fabrication and measurement techniques will have to be perfected. No doubt, there will also be a need for additional theoretical work to understand the experimental findings. On a longer timescale, the goal is of course to be able to control and manipulate quantum information stored in Majorana-based qubit systems. If there will be a useful technological application at the end of this long road is too early to predict, but there
is certainly a lot of interesting and new physics to explore.

Given their considerable application potential, from quantum computing to information technologies, noncentrosymmetric (NCS) superconductors have attracted significant experimental and theoretical interest. In the presence of spin-orbit coupling, these materials are potential candidates for topological superconductivity that host protected Majorana fermion surface states. However, evidence for topological superconducting surface states, and spin-orbit coupling, in NCS materials is scarce. This work has revealed the existence of spin-polarized surface states in the NCS material BiPd, providing unique insight into the electronic structure and identifying a potential pathway to the elusive Marjorana fermion surface states. Scientists conducted a systematic high-resolution angle-resolved photoemission spectroscopy (ARPES) and spin-resolved ARPES study of electronic and spin properties in the normal state of this superconductor. The detailed photon energy, temperature-dependent and spin-resolved ARPES measurements, complemented by first-principles electronic structure calculations, demonstrated the presence of surface states at higher binding energy with the location of the Dirac point at around 700?meV below the Fermi level. While these results negate the existence of topological superconductivity in BiPd, they provide critical information for identifying, and in time controlling through electrical gating, topologically protected surface states in NCS materials that could create a new class of quantum devices based on Majorana fermions.

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