Polar-Molecule Qubit

Figure

Description

A polar-molecule qubit encodes information in rotational and hyperfine states of an individually trapped ultracold molecule. Long-lived hyperfine states in the rotational ground manifold provide storage, while a rotationally excited state turns on an electric dipole-dipole exchange interaction between nearby molecules. This separates memory from interaction without changing particles.

The platform is distinct from chemically synthesized molecular spin qubits: here intact diatomic molecules such as NaCs are assembled and trapped in optical tweezers, individually addressed, and coupled through their permanent electric dipoles.

Hamiltonian

For one molecule,

$H_{\mathrm{mol}}=B_{\mathrm{rot}}{\mathbf{N}}^2+H_{\mathrm{hf}}-\mathbf{d}\cdot \mathbf{E},$

where $\mathbf{N}$ is rotational angular momentum, $H_{\mathrm{hf}}$ resolves nuclear-spin sublevels, and the Stark term controls dipole character. Within an interacting two-state manifold, resonant dipolar exchange reduces to

$H_{\mathrm{ex}}=\frac{J_{\perp }}{2}({\sigma }_x^{(1)}{\sigma }_x^{(2)}+{\sigma }_y^{(1)}{\sigma }_y^{(2)}),$

which generates an iSWAP at the appropriate interaction time. Microwave transfer between storage and interacting states toggles $J_{\perp }$.

Motivation

• Combine atomic-style coherence and single-particle trapping with intrinsically strong, switchable electric dipoles.
• Simulate long-range spin models and implement exchange gates without Rydberg excitation.
• Use rich rotational/hyperfine structure as a controllable qubit or qudit resource.

Experimental Status

DeMille proposed dipolar quantum computation with trapped polar molecules in 2002. Picard et al. later implemented a two-qubit iSWAP gate between individual NaCs molecules 1.9 μm apart, using a 664 μs interaction and producing a post-selected Bell state with 94(3)% fidelity.

Key Metrics

Metric	Value	Notes	Fidelity reference
Bell-state fidelity	94(3)%	Post-selected on both NaCs molecules being detected	Picard et al. 2025
iSWAP interaction time	664 μs	Molecules separated by 1.9 μm	Picard et al. 2025
Storage encoding	Hyperfine states	Non-interacting states in the rotational ground manifold	Picard et al. 2025

Scaling Considerations

• Molecule assembly, motional ground-state cooling, and survival detection reduce usable duty cycle.
• Motion-rotation coupling and differential trapping shifts currently limit gate fidelity.
• The dense internal spectrum is a resource for control but creates leakage and calibration burden.
• Larger arrays require repeatable molecule formation and parallel microwave/optical addressing.

References

• D. DeMille, “Quantum Computation with Trapped Polar Molecules,” Phys. Rev. Lett. 88, 067901 (2002).
• L. R. B. Picard et al., “Entanglement and iSWAP gate between molecular qubits,” Nature 637, 821–826 (2025).

Linked Papers

• demille-2002-polar-molecule-quantum-computation
• picard-2025-molecular-iswap

Evergreen context

• coherence-time-hierarchy — separates long-lived storage states from the shorter interaction and motional timescales.
• sqrt-swap-as-universal-gate — the dipolar exchange interaction naturally generates iSWAP-family entanglers.
• divincenzo-criteria — useful for separating compelling two-molecule physics from processor-scale readiness.

Related Entries

• molecular-qubit — chemically synthesized molecular spin qubits rather than ultracold tweezer-trapped molecules.
• neutral-atom-qubit — shares optical trapping and single-particle assembly but uses atomic rather than molecular internal structure.