A 2017 report of the discovery of a particular kind of Majorana fermion, the chiral Majorana fermion, was a false alarm, finds a new study, which means construction of a topological quantum computer also remains elusive.
Some particle physicists are using underground observatories to discover if the ghost-like particle known as the neutrino, a subatomic particle that rarely interacts with matter, might be a Majorana fermion.
An exotic quantum state known as a "chiral Majorana fermion" is predicted in devices wherein a superconductor is affixed on top of a quantum anomalous Hall (QAH) insulator (left panel). Experiments performed at Penn State and the University of Würzburg in Germany show that the millimeter-size superconductor strip used in the proposed device geometry creates an electrical short, preventing the detection of chiral Majoranas (right panel). Credit: Cui-zu Chang, Penn State
Meanwhile, condensed matter physicists are seeking to discover manifestations of Majorana physics in solid state devices that combine exotic quantum materials with superconductors. In such devices, electrons are theorized to dress themselves as Majorana fermions by stitching together a fabric constructed from core aspects of quantum mechanics, relativistic physics, and topology. This analogous version of Majorana fermions has particularly captured the attention of condensed matter physicists because it may provide a pathway for constructing a "topological quantum computer" whose qubits (quantum versions of binary 0s and 1s) are inherently protected from environmental decoherence--the loss of information that results when a quantum system is not perfectly isolated and a major hurdle in the development of quantum computers.
Theoretical physicists need to rethink the device geometry
A new paper on the 'Angel Particle' studied over three dozen devices similar to the one used to produce the angel particle in the 2017 report and found that the feature that was claimed to be the manifestation of the angel particle was unlikely to be induced by the existence of the angel particle.
Without experimental evidence for the existence of Majorana fermions in condensed matter, a topological quantum computer remains a future talked about in press releases, the physics equivalent of NASA's ubiquitous 'important for implications about life on other planets' publicity stunts. A 2017 paper said it had the experiment but a new paper casts doubt on it.
The team studied devices fashioned from a quantum material known as a "quantum anomalous Hall insulator" wherein the electrical current flows only at the edge. A recent study predicted that when the edge current is in clean contact with a superconductor, propagating chiral Majorana Fermions are created and the electrical conductance of the device should be "half-quantized" (a value of e2/2h where "e" is the electron charge and "h" is Planck constant), when subject to a precise magnetic field.
The recent team studied over three dozen devices with several different materials configurations and found that devices with a clean superconducting contact always show the half-quantized value regardless of magnetic field conditions. This occurs because the superconductor acts like an electrical short and is thus not indicative of the presence of the Majorana fermion.
With completely consistent results using a wide variety of device configurations there is now serious doubt about the validity of the theoretically proposed experimental geometry and questions the 2017 claim of observing the angel particle.
"I remain optimistic that the combination of quantum anomalous Hall insulators and superconductivity is an attractive scheme for realizing chiral Majoranas," said Morteza Kayyalha, a postdoctoral research associate at Penn State who carried out the device fabrication and measurements. "But our theorist colleagues need to rethink the device geometry."
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