Molecular MOC

Curated map of Zoo entries in the Molecular family.

Entries

Entry	Type	Status
molecular-qubit	qubit	demonstrated
nuclear-magnetic-resonance-qubit	qubit	demonstrated
polar-molecule-qubit	qubit	demonstrated

Composition

• qubit: 3

Curated synthesis

“Molecular” names the physical carrier, not a common processor architecture. The useful comparison is therefore not molecule versus atom, but where each branch places addressability, interaction, and scaling cost. molecular-qubit puts design freedom into synthesis and now spans ensemble coherence, coupled dimers, optical spin control, and electrically driven single molecules, but still lacks a standard scalable initialization/readout stack; polar-molecule-qubit gains individual addressing and a native dipolar entangler by paying an assembly, cooling, and survival-detection cost; nuclear-magnetic-resonance-qubit has exceptionally mature coherent control but pays an exponential pseudo-pure-state signal penalty because it addresses ensembles rather than individual processors.

This makes experimental age a poor proxy for architectural maturity. NMR reached algorithm demonstrations first yet closed off as a scalable route; molecular spin qubits show that chemistry can engineer coherence without by itself supplying an integration stack; trapped polar molecules are the newest branch but already expose a credible two-particle gate primitive. Read the family as three different answers to the same question: which part of the quantum computer is delegated to molecular structure, and which unsolved burden remains outside the molecule?

Conceptual anchors

• coherence-time-hierarchy is the right comparison frame here, because molecular platforms compete less on raw gate speed and more on how chemical design reshapes dephasing channels and extends usable spin coherence.
• spin-orbit-coupling-for-qubit-control captures one of the central molecular design tensions: stronger ligand-field and spin-orbit structure can unlock richer control, but usually at the price of opening additional relaxation and dephasing pathways.
• divincenzo-criteria explains why this family remains strategically interesting but still immature, because synthetic tunability is strong while scalable entangling gates and qubit-specific readout remain the limiting criteria.

Family structure

• molecular-qubit is the chemistry-designed solid-state spin branch: ligand fields and molecular synthesis shape the spin Hamiltonian.
• polar-molecule-qubit is the trapped-particle branch: rotational states carry the qubit and electrically induced dipoles provide entangling interactions.
• nuclear-magnetic-resonance-qubit is the ensemble-computing branch: nuclear spins inside molecules are controlled spectroscopically, but room-temperature demonstrations use pseudo-pure ensembles rather than individually addressable pure-state qubits.
• These are genuinely different hardware models. Their shared label is molecular degrees of freedom, not a shared readout stack or scaling architecture.

Molecular platform routing table

Branch	Logical degree of freedom	Interaction / control route	Readout model	Decisive scaling bottleneck
molecular-qubit	Electronic spin states engineered by ligand field, anisotropy, and hyperfine structure	Microwave, optical, or exchange-mediated electrical control; chemically coupled dimers	Ensemble EPR, optical detection, or local ESR-STM; no standard scalable readout stack	Convert chemically engineered coupling and local control into calibrated gates with repeatable initialization/readout
polar-molecule-qubit	Rotational and hyperfine states of individually trapped molecules	Switchable electric dipole exchange, naturally producing iSWAP-family gates; see sqrt-swap-as-universal-gate	State-selective survival detection in optical tweezers	Molecule assembly, cooling, loss, and parallel control
nuclear-magnetic-resonance-qubit	Nuclear spins within an ensemble of molecules	RF pulses plus intramolecular scalar $J$-coupling	Bulk ensemble magnetization	Pseudo-pure-state signal falls exponentially and cannot support single-shot error correction

Use this table as a routing test: chemical synthesis alone points to the first branch, individually trapped dipolar rotors to the second, and bulk spectroscopic ensembles to the third. A new molecular entry should not be merged into an existing branch merely because its carrier is a molecule.

Molecular-spin evidence ladder

The molecular-qubit branch now contains several experimental regimes that should not be collapsed into one maturity claim:

1. Ensemble coherence shows that isotope, ligand, and nuclear-spin engineering can preserve a molecular electronic spin for useful times, but does not establish individual addressability.
2. Coupled molecular dimers show that chemically designed interactions can outrun decoherence, but an interaction timescale is not a calibrated two-qubit-gate fidelity.
3. Optically detected room-temperature control supplies a spin–optical interface and ambient operation, but currently serves sensing-style operation more directly than a scalable register.
4. Single-molecule electrical control demonstrates local tuning and coherent drive on surfaces, while initialization, nondestructive readout, and repeatable multi-qubit integration remain open.

Read these as complementary rungs rather than a performance leaderboard: ensemble $T_2$, single-molecule $T_1$, optical contrast, and exchange times measure different parts of the processor stack. The next decisive evidence is not another isolated record but a repeatable experiment combining local initialization, coherent interaction, and qubit-specific readout in one architecture.

The three branches should remain separate in comparisons: chemistry-designed spin registers, trapped polar rotors, and ensemble NMR solve different problems and have radically different scaling limits.