What types of molecules are considered potential candidates for implementing molecular qubits?

Several types of molecules are considered potential candidates for implementing molecular qubits, which are quantum bits at the molecular level. One prominent class is molecules with stable electron spins, such as organic radicals and transition metal complexes. The spin of an electron can represent the two states of a qubit (0 and 1), and these molecules are designed to maintain a stable spin state for a reasonably long time. Specific examples include nitroxide radicals and molecules containing metal ions like vanadium, chromium, or copper. Another class is molecules with nuclear spins. The nucleus of certain atoms possesses a spin that can also be used to represent a qubit. Molecules containing isotopes with non-zero nuclear spin, such as ¹³C or ¹⁵N, are often used. These nuclear spins tend to have longer coherence times compared to electron spins, but they are also more difficult to manipulate. Supramolecular structures, such as self-assembled molecular aggregates or DNA scaffolds, are also explored. These structures can provide a framework for organizing multiple qubits and controlling their interactions. They can also offer protection from environmental noise, potentially enhancing coherence times. Furthermore, molecules exhibiting quantum tunneling or superposition are of interest. For example, molecules that can exist in two different conformational states separated by a potential barrier could be used as qubits, with the two states representing 0 and 1. The ability to tunnel between these states provides a mechanism for manipulating the qubit. Finally, hybrid systems combining different types of molecules are also considered. For instance, combining a molecule with a stable electron spin with a molecule that can undergo a conformational change could create a qubit with both spin and conformational degrees of freedom. The key criteria for a good molecular qubit candidate include long coherence times, controllable interactions with other qubits, and ease of manipulation and readout.