Entangling polar molecules in optical tweezers

Short answer: Polar molecules such as calcium monofluoride and sodium-cesium can be held one at a time in optical tweezers and made to interact through their electric dipoles. Three papers from 2023 to 2024 report Bell-state entanglement of pairs of molecules, with reported fidelities of 0.89 for calcium monofluoride and 94% for sodium-cesium. These are two-molecule laboratory experiments, and the fidelities are conditioned on both molecules being present.

Why molecules

The Science abstract on calcium monofluoride (CaF) says ultracold polar molecules are promising candidate qubits for quantum computing and quantum simulation. Long-lived rotational states form robust qubits, and the long-range dipolar interaction between molecules provides entanglement. The Nature abstract adds that trapped polar molecules have been proposed as a platform with scalability and single-particle addressability, while keeping the complexity and strong couplings of molecules.

CaF: spin exchange and a Bell state

In the first Science paper, the authors demonstrate dipolar spin-exchange interactions between single CaF molecules trapped in an optical tweezer array. They realize the spin-1/2 quantum XY model by encoding an effective spin-1/2 system into rotational states, and use it to generate a Bell state through an iSWAP operation. Conditioned on verified existence of molecules in both tweezers at the end of the measurement, they obtain a Bell state fidelity of 0.89 with an uncertainty of 0.06. They also demonstrate single-site molecular addressability using interleaved tweezer arrays.

A second Science paper: on-demand entanglement

A separate Science paper on on-demand entanglement of molecules notes that deterministic entanglement of individually controlled molecules had been a long-standing experimental challenge. Its authors use the electric dipolar interaction between pairs of molecules prepared in a reconfigurable optical tweezer array and deterministically create Bell pairs. They say the results demonstrate key building blocks for quantum applications and may advance quantum-enhanced tests of fundamental physics. The abstract does not name the molecular species, so this post does not either.

NaCs: a two-qubit gate

The Nature abstract says universal two-qubit gates had not been demonstrated with molecules before this work. The authors implement a two-qubit iSWAP gate using individually trapped sodium-cesium (NaCs) molecules. By letting the molecules interact for 664 microseconds at a distance of 1.9 micrometers, they create a maximally entangled Bell state with a fidelity of 94% (uncertainty 3%) in trials in which both molecules are present.

They report that residual excitation of the lowest few motional states along the axial trapping direction is the primary source of decoherence. They encode a qubit in two non-interacting hyperfine states within the ground rotational level, and toggle the interaction by transferring between interacting and non-interacting states. They verify the gate by measuring its logical truth table.

Limits

Both fidelity figures exclude runs where a molecule was missing, so they describe successful loading and are not unconditional. Each experiment involves two molecules. The papers use different species and methods, so the numbers are not a head-to-head ranking.

FAQ

What is an iSWAP gate?

A two-qubit operation based on exchanging spin states, used to create Bell states in these papers.

How good were the Bell states?

0.89 for CaF and 94% for NaCs, each conditioned on both molecules being present.

What limited the NaCs result?

Residual motional excitation, per the authors.

Why use molecules and not atoms?

The authors cite long-lived rotational states and strong dipolar interactions.

Is this a molecular quantum computer?

No. These are two-molecule experiments.