Superconductivity in two-dimensional (2D) materials has emerged as a rich field of study, offering insights into unconventional pairing mechanisms and potential applications in quantum technologies. Among these materials, twisted bilayer graphene (TBG) and transition metal dichalcogenides (NbSe2) have demonstrated remarkable superconducting properties, distinct from their bulk counterparts. The reduced dimensionality, strong electron correlations, and tunable electronic structures in these systems enable unique superconducting phases, including Ising superconductivity, and provide platforms for developing quantum sensors based on Josephson junctions.

Twisted bilayer graphene exhibits superconductivity when two graphene layers are stacked at a specific "magic angle," typically around 1.1 degrees. At this angle, the moiré superlattice formed between the layers creates flat electronic bands, leading to enhanced electron-electron interactions and correlated insulating states. Upon doping, these insulating states transition into superconducting phases with critical temperatures (Tc) reaching up to 3 K. The pairing mechanism in TBG remains debated, with proposals including unconventional Cooper pairing mediated by electronic correlations rather than conventional phonon coupling. The absence of a strong isotope effect suggests a non-phononic origin, further supporting unconventional superconductivity in this system.

In contrast, NbSe2, a layered transition metal dichalcogenide, displays Ising superconductivity, where the electron spin is locked perpendicular to the 2D plane due to strong spin-orbit coupling. This spin-momentum locking protects the superconducting state from in-plane magnetic fields, allowing critical fields far exceeding the Pauli limit. Monolayer NbSe2 exhibits a Tc of approximately 3 K, slightly lower than bulk NbSe2 (7.2 K), but maintains robust superconductivity even at the 2D limit. The pairing mechanism in NbSe2 is primarily phonon-mediated, though the Ising protection mechanism introduces additional stability against magnetic perturbations, making it attractive for spin-triplet superconductivity studies.

Other 2D superconductors, such as twisted trilayer graphene and intercalated MoS2, also exhibit tunable superconducting phases. Twisted trilayer graphene shows higher Tc values (up to 4 K) compared to TBG, attributed to additional electronic bands enhancing density of states. Intercalated MoS2, where alkali metals are inserted between layers, achieves superconductivity with Tc values around 6 K, demonstrating the role of carrier doping in inducing superconducting transitions.

Josephson junctions fabricated from these 2D superconductors are critical for quantum sensing applications. In TBG, Josephson junctions exhibit gate-tunable supercurrents, enabling the design of programmable quantum circuits. The Fraunhofer patterns observed in these junctions confirm the existence of spatially uniform superconducting order parameters. NbSe2-based Josephson junctions, on the other hand, leverage Ising protection to maintain coherence under high magnetic fields, making them suitable for ultrasensitive magnetometers. The critical current oscillations in these junctions provide evidence of phase-coherent transport, essential for superconducting quantum interference devices (SQUIDs).

The superconducting gap symmetry in these materials varies significantly. TBG shows evidence of a nodal gap structure, consistent with unconventional pairing, while NbSe2 exhibits a fully gapped s-wave symmetry. Measurements using tunneling spectroscopy and Andreev reflection reveal these gap structures, providing insights into the underlying pairing interactions. For instance, scanning tunneling microscopy (STM) studies on TBG show a V-shaped density of states near the Fermi level, indicative of nodal superconductivity, whereas NbSe2 displays a U-shaped gap characteristic of conventional s-wave pairing.

The role of disorder and substrate interactions in 2D superconductors cannot be overlooked. Substrate-induced strain and charge inhomogeneities can suppress Tc or introduce localized superconducting puddles. Encapsulation with hexagonal boron nitride (hBN) has proven effective in preserving intrinsic superconducting properties by reducing environmental disorder. For example, hBN-encapsulated TBG devices show more homogeneous superconducting transitions compared to those on SiO2 substrates.

Proximity effects in heterostructures further expand the possibilities for engineering superconducting states. When a 2D superconductor is coupled to a topological insulator or a ferromagnet, exotic phenomena such as topological superconductivity or spin-polarized supercurrents can emerge. For instance, NbSe2 coupled to a topological insulator exhibits signatures of Majorana bound states, which are of interest for topological quantum computing.

The development of high-temperature 2D superconductors remains a key challenge. While most 2D systems exhibit Tc values below 10 K, recent work on electron-doped ZrNCl has reported Tc up to 15 K in the monolayer limit. This suggests that exploring new material systems or heterostructures could yield higher Tc values, though the mechanisms driving enhanced superconductivity in these cases require further investigation.

In summary, 2D superconductors like twisted bilayer graphene and NbSe2 offer a versatile platform for studying unconventional superconductivity and developing quantum devices. Their unique properties, such as Ising protection and gate-tunable superconductivity, pave the way for advanced applications in quantum sensing and computing. Future research will likely focus on increasing Tc, understanding pairing mechanisms, and integrating these materials into scalable quantum circuits. The interplay between dimensionality, electronic correlations, and spin-orbit coupling continues to drive discoveries in this rapidly evolving field.