This question is inspired by "What is the difference between a qudit system with d=4 and a two-qubit system?", as an experimental follow-up.
Consider for illustration these two particular cases:
- Molecular Spin Qudits for Quantum Algorithms, where in one important example a single d=4 qudit arises from a nuclear spin quadruplet I=3/2
- A modular design of molecular qubits to implement universal quantum gates, where in one important example a two-qubit system arises from an electronic spin quadruplet S=3/2 (which in turn arises from a ferromagnetic coupling between two ideal electronic spin doublets)
In general I'm referring to experimental cases where in practice there is an always-on-but-sometimes-weak coupling between two two-state systems, producing a ground quadruplet.
My question is: in experiments such as these, are 2·qubit and d=4 qudit (a) strictly distinguishable beasts, or (b) theoretical idealizations which are more or less adequate depending on practical considerations?