Short answer: A quantized zero-bias conductance peak of 2e^2/h in a superconductor-semiconductor nanowire is proposed as a signature of Majorana zero modes, but papers from 2012 to 2021 show that topologically trivial mechanisms can also produce zero-bias peaks. These works are about interpreting measurements, and none of the abstracts reports a working topological qubit.
What is the signature being looked for?
The 2021 Physical Review B abstract explains the standard picture. Majorana zero modes can appear at the ends of a one-dimensional topological superconductor and show up as a quantized zero-bias conductance peak in tunnelling spectroscopy of normal-superconductor junctions. The 2012 Physical Review Letters paper by Jie Liu, Andrew Potter, K. T. Law and Patrick Lee describes the quantized value of 2e^2/h as one of the simplest proposed experimental probes of a Majorana bound state.
Why is the peak not proof?
The 2012 authors note that when temperature is somewhat larger than the intrinsic width of the Majorana peak, conductance is no longer quantized, though a zero-bias peak can remain. They simulate tunnelling conductance for multiband wires with realistic amounts of disorder, and show that such systems generically exhibit a nonquantized zero-bias peak even when the wire is topologically trivial and has no Majorana end states. They compare their results with experiments reported at the time and discuss the requirements for confirming a Majorana state.
The 2021 paper investigates this systematically. It compares topological Majorana bound states, trivial quasi-Majorana bound states and low-energy Andreev bound states arising from smooth potential variations and disorder-induced subgap states. The authors focus on the peak value relative to 2e^2/h and on how robust the peak is, plateau-like or spike-like, against tuning parameters such as magnetic field and tunnelling gate voltage. They find that for Majoranas and quasi-Majoranas the peak values are no more than 2e^2/h and a quantized plateau forms generically, whereas for Andreev bound states or disorder-induced states the peak can exceed 2e^2/h and a plateau is rarely seen without careful postselection and fine-tuning. They say these findings should help in interpreting experiments.
What did a 2017 experiment report?
A 2017 Physical Review Letters paper from a group in Copenhagen with collaborators reports an experimental study of zero-bias conductance peaks compatible with Majorana zero modes in one-dimensional structures made from an epitaxial semiconductor-superconductor heterostructure. They studied scaling with magnetic field, tunnel coupling and temperature. The abstract says the results are consistent with theory, including a peak conductance proportional to tunnel coupling that saturates at 2e^2/h, decreases as expected with the field-dependent gap, and collapses onto a simple scaling function of the ratio of temperature to tunnel coupling. The abstract says "compatible with" and "consistent with" Majorana zero modes, and does not claim a confirmed detection.
How to read this
The theory papers show that a peak alone is ambiguous, and the experiment shows consistency with a theoretical model. Neither establishes a working topological qubit, and none of the abstracts claims one. This post is based on abstracts and visible summary text and full texts were not checked. It makes no claim about products or about any company's results.
FAQ
What is 2e^2/h?
The quantized value of the zero-bias tunnelling conductance peak that the 2012 and 2021 abstracts associate with an ideal Majorana bound state.
Can a trivial state mimic a Majorana peak?
Yes. The 2012 and 2021 papers show that disorder, Andreev bound states and quasi-Majorana states can produce zero-bias peaks.
How can the cases be told apart?
The 2021 authors point to the peak value and to whether a quantized plateau forms robustly across parameters, without postselection.
Has a Majorana qubit been demonstrated here?
No. These abstracts concern conductance peaks and their interpretation.
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