Contents

# Contents

## Idea

In solid state physics, the term “Majorana zero mode” (often abbreviated “MZM” or just “Majorana”) has come to refer to (hypothetical and so far eluve) ground states of certain effectively 1-dimensional quantum materials (quantum/nano-wires) which are acted on by a “Majorana operator” (namely the Hermitian combination $c + c^\dagger$ of fermion annihilation/creation operators, only vaguely related to relativistic Majorana spinors) and which have been argued to potentially behave like Majorana anyons, in some sense (beware that these modes, being stuck to wires, would not be mobile and hence would not admit adiabatic braiding operations in the usual sense, see here).

These Majorana zero modes were theoretically introduced in a spin chain model by Kitaev 2001 (“Kitaev spin chain”), originally as a theoretical toy example for gapped and degenerate ground states vaguely as expected for topological order, but then argued to be realizable on interfaces of superconductors with certain topological insulators by Fu & Kane 2008; and their experimental realization in super-/semi-conducting nano-wires has been proposed in Lutchyn, Say & Das Sarma 2010, Oreg, Refael, von Oppen 2010.

Following these proposals and especially after Microsoft Quantum (with QuTech at TU Delft) declared (Nov 2016) the concrete aim of realizing topological quantum computation based on topological qbits given by such “Majorana zero modes” (following the plan laid out in Das Sarma, Freedman & Nayak 15), the topic attracted enormous attention in solid state physics.

But prominent claims of experimental detection of (these kinds of) Majorana zero modes had to be retracted:

While some (probably most) researchers now dismiss the whole approach (e.g. BSSA21, p. 3) there is a new claim of detection by Nayak 22 & MicrosoftQuantum 22 – but see Frolov & Mourik 22a, 22b and Frolov 22.

## References

### General

The general strategy of realizing Majorana zero modes in supercondocuting/semiconducting nanowires is due to

reviewed in:

• Pasquale Marra: Majorana nanowires for topological quantum computation: A tutorial [arXiv:2206.14828]

Discussion in the context of topological quantum computation:

General review and experimental status:

### Non-Detection

On the general problem of distinguishing the expected effect from noise:

we believe that similar confirmation bias applies to many other topological discovery claims in the literature during 2000–2020 where a precise knowledge of what one is looking for has been the key factor in the discovery claim, with the experimental quantization results themselves not being sufficiently compelling. […] Our results certainly apply to most of the Majorana experiments during 2012–2021 in the literature.

p. 3: The quantum physics community is sufficiently aware that when certain qubit technologies do not produce any reasonable result after several years of effort, they should be gently removed from the list of quantum candidates. After working with the physics colleagues in Delft, we saw that happening with the Majorana qubits that could not be confirmed in any follow-up experiment.

Non-retracted claims of experimental realization of something in the direction of Majorana zero modes:

• Gerbold C. Ménard, Andrej Mesaros, Christophe Brun, François Debontridder, Dimitri Roditchev, Pascal Simon, Tristan Cren, Isolated pairs of Majorana zero modes in a disordered superconducting lead monolayer, Nat Commun 10 2587 (2019) $[$doi:10.1038/s41467-019-10397-5$]$

• Chetan Nayak, Microsoft has demonstrated the underlying physics required to create a new kind of qubit, Microsoft Research Blog (March 2022)

• Microsoft Quantum, InAs-Al Hybrid Devices Passing the Topological Gap Protocol [arXiv:2207.02472, video presentation]

but see commentary in:

• Sergey M. Frolov, Vincent Mourik: Majorana Fireside Podcast, Episode 1: The Microsoft TGP paper live review [video, conclusion at: 1:01:31]

1:01:52 The signal is fully consistent, from what we see, with not having discovered any Majorana or topological superconductivity here. At the same time, the amount of data is extremely narrow.

• Sergey M. Frolov, Superconductors and semiconductors, nanowires and majorana, research and integrity [video, general caution: 55:34, concrete criticism: 1:01:41]

1:01:50: The claims of the discovery of Majorana have been overblown and are false. Majorana has not been discovered in nanowires. I don’t believe in any other system it has been discovered either.

On how this could happen:

• Elizabeth Gibney, Inside Microsoft’s quest for a topological quantum computer (Interview with Alex Bocharov), Nature (2016) [doi:10.1038/nature.2016.20774]

[Bocharov:] We’re people-centric, rather than problem-centric.