Comment on "InAs-Al hybrid devices passing the topological gap protocol", Microsoft Quantum, Phys. Rev. B 107, 245423 (2023) (2502.19560v1)

Abstract: The topological gap protocol (TGP) is presented as "a series of stringent experimental tests" for the presence of topological superconductivity and associated Majorana bound states. Here, we show that the TGP, 'passed' by Microsoft Quantum [PRB 107, 245423 (2023)], lacks a consistent definition of 'gap' or 'topological', and even utilises different parameters when applied to theoretical simulations compared to experimental data. Furthermore, the TGP's outcome is sensitive to the choice of magnetic field range, bias voltage range, data resolution, and number of cutter voltage pairs - data parameters that, in PRB 107, 245423 (2023), vary significantly, even for measurements of the same device. As a result, the core claims of PRB 107, 245423 (2023) are primarily based on unexplained measurement choices and inconsistent definitions, rather than on intrinsic properties of the studied devices. As such, Microsoft Quantum's claim in PRB 107, 245423 (2023) that their devices have a "high probability of being in the topological phase" is not reliable and must be revisited. Our findings also suggest that subsequent studies, e.g. Nature 638, 651-655 (2025), that are based on tuning up devices via the TGP are built on a flawed protocol and should also be revisited.

Critical Examination of the Topological Gap Protocol in InAs-Al Hybrid Devices

In the pursuit of unambiguous topological superconductors, the identification of Majorana bound states (MBSs) stands as a pivotal goal due to their potential applications in quantum computation. However, discerning truly topological from trivial states in semiconductor-superconductor nanowires has been an intricate issue. In this vein, Henry F. Legg critiques the validity and implementation of the Topological Gap Protocol (TGP) proposed by Aghaee et al. (2023) in their work on InAs-Al hybrid devices.

The TGP, heralded by Microsoft Quantum as a stringent test for topological superconductivity, was observed to lack a consistent definition of 'topological' or 'gap.' A comprehensive analysis of the TGP reveals several critical inconsistencies and methodological weaknesses that cast doubt on the claims of topological phase presence.

Key Issues with the TGP

1. Gap Identification Discrepancies: The TGP's reliance on nonlocal conductance for gap identification raises concerns as the method of determining the conductance threshold ($G_{\rm th}$) is inconsistently reported and applied. The protocol, in practice, uses the maximum conductance across all bias voltages rather than a high-bias maximum, leading to sensitivity to the parameters of measurement such as bias range.
2. Variability in Experimental Parameters: The paper highlights the substantial variability in magnetic field range, bias voltage, data resolution, and cutter voltage pairs across different experimental setups. This lack of standardization indicates that outcomes of the TGP could be a consequence of selective experimental parameter choice rather than an intrinsic property of the devices. This variability inherently challenges the reliability of detecting true topological phases.
3. Inconsistencies between Theory and Experiment: The TGP applied to simulations and experiments is not uniform, with distinct versions (analyze_2 and analyze_two) being used for different parts of the paper. This discrepancy undermines the reliability of the TGP in ensuring no false positives come from simulations, as false positives were identified with the experimental TGP (analyze_two).
4. Weakness in 'Topological' Definition: The paper raises significant concerns about the definition of 'topological' within the paper, noting that a single pixel with $\det(r) < 0$ at either end satisfies the topological criterion in their simulations. This diluted definition results in a large expanse of phase space being incorrectly labeled as topological, further undermining the claims of device topology.

Implications and Speculation

The implications of these findings reach beyond the specific paper by Aghaee et al., impacting subsequent research reliant on the TGP framework. Additionally, this raises pertinent questions regarding bias in experimental implementations aimed at detecting topological phases, possibly requiring a reevaluation or refinement of the TGP itself.

A coherent and universally applicable protocol is crucial for reliably advancing this field. Without these modifications, findings risk being seen as artifacts of experimental setup rather than validated results. Future work must address these identified inconsistencies to establish a more robust testing mechanism for topological phases.

Conclusion

Legg’s examination suggests that the results presented by Aghaee et al. are largely shaped by procedural inconsistencies and selective definitions rather than fundamental device properties. Given the potential ramifications on related academic and technological pursuits, the concerns raised here serve as a crucial reminder of the need for standardized definitions and methodological rigor in topological phase identification efforts within quantum materials research. These insights necessitate careful reexamination and adaptation of testing protocols, promoting scientific integrity in ongoing explorations of topological superconductivity.