Topological materials have emerged as a promising platform for quantum sensing due to their unique electronic properties, particularly spin-momentum locking and inherent noise resilience. These materials host protected surface or edge states that are robust against perturbations, making them ideal for high-precision sensing applications. Unlike conventional semiconductors, topological insulators and semimetals exhibit a bulk bandgap while supporting conducting states at their boundaries, a feature that arises from strong spin-orbit coupling and time-reversal symmetry. This article explores the mechanisms behind spin-momentum locking and noise resilience in topological materials, with a focus on their implications for quantum sensing.

Spin-momentum locking is a hallmark of topological surface states, where the spin of an electron is intrinsically tied to its momentum. In materials like HgTe, the Dirac-like surface states exhibit a helical spin texture, meaning that electrons moving in opposite directions possess opposite spin polarizations. This property is a direct consequence of spin-orbit interaction and time-reversal symmetry, which enforce a one-to-one correspondence between momentum and spin orientation. For quantum sensing, spin-momentum locking offers a way to manipulate and probe spin states without external magnetic fields, reducing noise from stray magnetic fluctuations. The robustness of these states against non-magnetic impurities further enhances their suitability for sensing applications, as scattering events that would typically degrade signal fidelity are suppressed.

Noise resilience in topological materials stems from the topological protection of their edge or surface states. Backscattering is strongly suppressed because flipping the momentum of an electron would require flipping its spin, a process forbidden by time-reversal symmetry in the absence of magnetic perturbations. This property is particularly advantageous in environments where decoherence and noise are significant challenges, such as in biological systems or high-temperature regimes. For instance, HgTe-based quantum wells have demonstrated exceptional stability in their conductive channels, even in the presence of disorder. The quantized conductance observed in these systems is a direct manifestation of their topological protection, ensuring that the sensing signal remains intact under varying external conditions.

HgTe is a prototypical example of a topological material exploited for quantum sensing. When strained or grown as quantum wells, HgTe transitions from a conventional semiconductor to a topological insulator, hosting edge states with spin-momentum locking. Experimental studies have shown that these edge states exhibit quantized conductance, a feature that is highly desirable for metrology and precision sensing. The ability to tune the bandgap via quantum confinement or external strain further allows optimization for specific sensing applications. Unlike traditional semiconductors, where defects and impurities introduce significant noise, the topological nature of HgTe ensures that the edge states remain coherent over micron-scale distances, even at elevated temperatures.

Another class of materials with potential for quantum sensing is topological semimetals, such as Weyl and Dirac semimetals. These materials possess bulk band touchings with linearly dispersing states, leading to high carrier mobility and low dissipation. The surface states of Weyl semimetals, known as Fermi arcs, also exhibit spin-momentum locking, though their connectivity and dispersion differ from those of topological insulators. The unique Fermi arc surface states can be probed for sensing applications, leveraging their sensitivity to external perturbations like strain or electric fields. Cd3As2 and Na3Bi are examples of Dirac semimetals where the bulk Dirac cones and surface states have been explored for their potential in high-sensitivity detectors.

The noise resilience of topological materials extends beyond immunity to non-magnetic impurities. Their surface states are also less susceptible to certain types of phase noise, which often plagues conventional quantum sensors. For example, in superconducting quantum interference devices (SQUIDs), flux noise can limit sensitivity, but topological materials could offer alternative readout mechanisms that circumvent these limitations. The spin-polarized currents in topological insulators can be harnessed to detect minute magnetic fields without the need for superconducting components, potentially enabling room-temperature operation. This advantage is critical for applications where cryogenic cooling is impractical.

In addition to HgTe, other two-dimensional topological insulators like bismuth bilayers and WTe2 have shown promise for quantum sensing. These materials exhibit quantum spin Hall effects, where the edge states are spin-polarized and counter-propagating. The absence of backscattering in these channels makes them ideal for low-noise sensing applications. WTe2, in particular, has demonstrated non-saturating magnetoresistance and high carrier mobility, properties that are beneficial for detecting weak magnetic or electric fields. The flexibility of these materials in thin-film form also allows integration into heterostructures, where proximity effects can further enhance their sensing capabilities.

The interplay between topology and superconductivity in certain materials opens additional avenues for quantum sensing. Proximity-induced superconductivity in topological insulators can lead to the formation of Majorana bound states, which are theoretically predicted to exhibit non-Abelian statistics. While the primary focus of Majorana research is on quantum computing, their topological protection could also be exploited for sensing applications. The zero-bias conductance peaks associated with Majorana states are highly sensitive to local perturbations, suggesting potential use in scanning probe microscopy or nanoscale magnetic field detection.

Despite the progress, challenges remain in harnessing topological materials for practical quantum sensing. Material quality and interfacial defects can still degrade performance, particularly in heterostructures where lattice mismatch is an issue. Furthermore, the integration of topological sensors with existing readout electronics requires careful engineering to preserve their unique properties. Advances in epitaxial growth and defect passivation will be crucial to realizing the full potential of these materials. For instance, optimizing the growth conditions of HgTe quantum wells has already led to improved mobility and coherence lengths, directly enhancing sensing performance.

The future of topological materials in quantum sensing lies in the exploration of new compounds and heterostructures that combine multiple topological phases. For example, coupling a topological insulator with a ferromagnet can break time-reversal symmetry, leading to anomalous quantum Hall effects that may be leveraged for novel sensing modalities. Similarly, stacking two-dimensional topological materials with different properties could create moiré superlattices with tunable electronic states, offering unprecedented control over sensor response. The ongoing discovery of new topological phases, such as higher-order topological insulators, further expands the toolkit available for quantum sensing.

In summary, topological materials offer a unique combination of spin-momentum locking and noise resilience that is highly advantageous for quantum sensing. Materials like HgTe and WTe2 exemplify the potential of these systems, with their protected edge states and robust electronic properties. While challenges in material synthesis and integration persist, the inherent advantages of topological materials make them a compelling choice for next-generation sensors. As research progresses, the development of tailored topological heterostructures and improved fabrication techniques will likely unlock new possibilities in high-precision sensing technologies.