Endohedral fullerenes, particularly nitrogen atom encapsulated in a C60 cage (N@C60), have emerged as promising candidates for qubit implementation in quantum computing. The unique electronic structure of these systems, where an atom or molecule is trapped inside a carbon cage, provides isolated spin states with long coherence times, a critical requirement for quantum information processing. The nitrogen atom in N@C60 retains its atomic-like electronic configuration due to weak interaction with the carbon cage, making it an ideal system for spin-based qubits.

The spin coherence time (T2) is a key metric for qubit performance, determining how long quantum information can be stored and manipulated. For N@C60, experimental studies have demonstrated spin coherence times exceeding 100 microseconds at room temperature, with further extension possible at cryogenic temperatures. The carbon cage serves as a protective shield, minimizing interactions with the external environment that could lead to decoherence. This is a significant advantage over other solid-state qubits, where decoherence due to environmental noise is often a limiting factor. The spin coherence time can be further enhanced by using purified samples and isotopic engineering, such as employing carbon-13 depleted C60 cages to reduce nuclear spin-induced decoherence.

Spin manipulation in endohedral fullerenes is achieved through microwave pulses, leveraging the electron paramagnetic resonance (EPR) transitions of the encapsulated atom. The nitrogen atom in N@C60 has a spin S=3/2 ground state, providing multiple addressable transitions that can be used for qubit operations. Precise control over these transitions allows for the implementation of single-qubit gates, such as rotations around the X, Y, and Z axes. The ability to perform these operations with high fidelity is essential for building a functional quantum computer. Experiments have shown that Rabi oscillations, indicative of coherent spin manipulation, can be observed in N@C60 with high contrast, demonstrating the feasibility of this approach.

The scalability of endohedral fullerene-based qubits is another area of interest. One proposed method involves embedding these molecules into a solid-state matrix, such as a polymer or crystal lattice, where they can be individually addressed using electric or magnetic fields. The spatial separation between molecules reduces cross-talk, a common issue in dense qubit arrays. Additionally, the chemical stability of fullerenes allows for integration into various host materials without significant degradation of spin properties. This flexibility in assembly is a notable advantage over other qubit platforms that require stringent fabrication conditions.

Challenges remain in the practical implementation of endohedral fullerene qubits. While the spin coherence times are promising, they are still shorter than those achieved in some other systems, such as superconducting qubits or trapped ions. Improving T2 times through material engineering and optimized pulse sequences is an active area of research. Another challenge is the readout of spin states, which currently relies on EPR techniques that may not be easily scalable. Developing efficient and scalable readout methods is crucial for the advancement of this technology.

The potential of endohedral fullerenes extends beyond nitrogen-based systems. Other atoms or molecules, such as phosphorus or metal ions, can also be encapsulated, offering a range of spin states and properties for qubit design. For example, P@C60 has been studied for its potential as a multi-level qubit system, where the nuclear spin of phosphorus could provide additional degrees of freedom for quantum information storage. The versatility of endohedral fullerenes in hosting different spin-active species makes them a rich platform for exploring new qubit architectures.

In summary, endohedral fullerenes represent a promising avenue for spin-based quantum computing. Their long spin coherence times, ability to be manipulated with microwave pulses, and potential for scalable integration make them attractive candidates for qubit implementation. While challenges such as improving coherence times and developing scalable readout methods remain, ongoing research continues to advance the feasibility of these systems. The unique properties of endohedral fullerenes position them as a viable alternative to other qubit platforms, with the added benefit of chemical tunability and robustness. As the field progresses, further exploration of these molecules and their derivatives will likely yield new insights and innovations for quantum computing.