Squeezed state protection of fine structure in "Poor Man's Majorana" via quantum spin coupling

TL;DR

This paper demonstrates that the 'Poor Man's Majorana' zero mode can be protected against quantum spin perturbations through squeezing effects, challenging the notion that it lacks topological protection.

Contribution

It introduces a model showing protection of the 'Poor Man's Majorana' zero mode via quantum spin coupling, revisiting its supposed lack of topological protection.

Findings

Half of the fine structure remains unchanged under perturbation.

The zero mode is squeezed and pinned at zero frequency.

Protection persists despite exchange coupling perturbations.

Abstract

The "Poor Man's Majorana" [Phys. Rev. B 86, 134528 (2012)] devoid of topological protection has been theoretically predicted to rely on the minimal Kitaev chain. Afterward, a pair of superconducting and spinless quantum dots turned the proposal practicable and differential conductance pinpointed consistent fingerprints with such a scenario [Nature 614, 445 (2023) and Nature 630, 329 (2024)]. In this work, we propose a model wherein the "Poor Man's Majorana" presents protection when one of the dots is exchange coupled to a quantum spin. If this quantum dot is perturbed by tuning the exchange coupling, the well-known spill over-like behavior of this Majorana surprisingly remains unchanged, and solely half of the fine structure is unexpectedly viewed. As a matter of fact, the "Poor Man's Majorana" zero mode consists in squeezing of the other half at zero frequency, which imposes its pinning there and prevents the mixing of the mode with the explicit fine structure. We claim that if the supposed unavoidable split of the zero mode by the fine structure is unexpectedly absent, then the "Poor Man's Majorana" can be considered robust against the quantum spin. In this way, it becomes protected and the lack of topological protection paradigm of the "Poor Man's Majorana" has been revisited, pushing this seemingly well-established issue into a new direction.