Fermions are fundamental particles, or elementary particles, which means that they have no constituent particles. They have half-integer intrinsic spins, such as +1/2 and -1/2, and are classified as either quarks or leptons. They can contribute to composite fermions, such as protons, neutrons, atoms, and molecules, and with assistance from force-carrying bosons they comprise all matter in the universe. The stability of matter is owed to fermions never occupying identical quantum states simultaneously, which is known as the Pauli exclusion principle.
All known fermions, with the possible exception of neutrinos, are Dirac fermions. These particles all have distinct antiparticles. A Majorana fermion, or Majorana particle, is a hypothesized fermion that is both a particle and its own antiparticle; it has the same properties either way. It is also inherently resilient to noise, which makes the theoretical Majorana qubit highly sought after for fault-tolerant quantum computing (FTQC). In regard to neutrinos, it remains unconfirmed whether they are Dirac fermions or Majorana fermions.
For more information, “Topology And Physics” by Chen Ning Yang, Mo-lin Ge, and Yang-hui He is a 232-page book published by World Scientific. It includes a chapter titled “Majorana Fermions and representations of the braid group.” For a relatively-quick visual explainer, QuTech Academy offers a free 6:27-minute video titled “Majorana fermions and where to find them.” The explanation includes some mathematics, but it also includes illustrations and some simplified descriptions.
Dirac fermions are like a traditional pair of gloves, with one glove comfortably fitting the left hand and one glove comfortably fitting the right hand, or a traditional pair of slippers, with one slipper comfortably fitting the left foot and one slipper comfortably fitting the right foot. The particles and antiparticles can be thought of as mirror images of each other, similar but not identical. In contrast, Majorana fermions would be the type of wool gloves or hotel slippers that can fit either the left or the right comfortably. In a mirror, the particles and antiparticles are identical.
The Italian theoretical physicist Ettore Majorana laid the theoretical foundations for Majorana fermions back in 1937. Some of the key points today include:
Research continues into understanding and discovering Majorana fermions. Some of this research has already extended beyond theory and into attempts to experimentally confirm their existence.
Experimentation has not yet confirmed the existence of Majorana fermions. However, the following are some of the leading experiments and initiatives toward achieving that aim:
The controversy over Majorana particles is eroding confidence in the field. More accountability and openness are needed — from authors, reviewers and journal editors.
A paper titled “Quantum computing’s reproducibility crisis: Majorana fermions,” published in Nature, notes that there is considerable controversy in the field. Claims of Majorana particle detection have been heralded as breakthroughs upon their announcements, only to end up not being reproducible experimentally. Their existence, unfortunately, remains unconfirmed.
In condensed matter physics, quasiparticles condense and some can exhibit the behaviors of Majorana particles. There are several areas from which Majorana fermions are predicted to emerge:
Although each of these areas are not without challenges, the confirmation of Majorana particles could lead to breakthroughs in:
In other words, the confirmation of Majorana particles would be a breakthrough both technologically and scientifically. The resultant applications and advancements could be far-reaching. Just thinking about quantum technologies, potential benefits include:
The most exciting prospects of all might just be the ones that remain unforeseen.