The model describes a way to start with a small group of polar molecules and isolate a single molecule in each optical tweezer, a tightly focused laser trap, with a predicted single-site fidelity above 99%. The proposed sequence combines static-electric-field and microwave shielding with an electric-field gradient used for controlled spilling. For an array whose sites begin with two molecules, the same model estimates 95% one-molecule loading per tweezer.
The work is a preprint dated 21 September 2026, according to its front matter. Its proposed starting point is a small, stochastic occupancy of roughly five or fewer molecules per tweezer. The numerical reference system is NaCs, with a static electric field of 2.366 kilovolts per centimeter.
A gap between resonances
The first part of the protocol seeks a gap between field-linked bound states, molecular states associated with the applied fields. In one ac-dc(X) shielding calculation, with the microwave ellipticity angle set to 45 degrees, the model found a range without those states. It placed resonances near 0.23 and 0.5 times the microwave detuning on either side of the range.
The same comparison found that the resonance-free region grew as the ellipticity angle decreased. For angles at or above roughly 10 degrees, the scattering length, a low-energy measure of molecular interactions, remained negative even without a field-linked state.
To check the approximation used to map the interactions, the authors compared an effective-potential calculation with a fuller close-coupling calculation of molecular scattering. The two approaches agreed on the magnitude of the scattering length and on the positions of the field-linked resonances.
At a 100-nanokelvin collision energy, with the microwave coupling set to 0.4 times its detuning and the detuning matched to the Förster energy defect, the model reported an elastic-to-loss rate ratio above a million for Rabi frequencies below 10 megahertz when divided by 2π. The ratio compares elastic collisions with loss-producing collisions. The predicted loss-rate coefficient was below about 10⁻¹⁴ cubic centimeters per second.
Turning two molecules into one
The proposed spilling step uses an interaction-induced energy shift to make the first and second molecules tunnel, or leak through the trapping barrier, on different timescales. In the geometry modeled for this step, the maximum computed shift was 0.6372 kilohertz, and the authors used 0.55 kilohertz as a representative value. The calculation treated first-molecule tunneling, second-molecule tunneling and two-body loss as three competing exponential processes.
Those timescales separated by nearly four orders of magnitude in the model. At the optimal spill time, it predicted more than 99% single-molecule isolation when the interaction shift was above 0.5 kilohertz. The number is an output of the spilling model, not a measured fidelity.
For the array extension, the modeled NaCs shielding window spanned plus or minus 2.8 volts per centimeter. In the case of initially doubly occupied sites, the statistical estimate reached more than 99% one-molecule loading at one site and 95% per tweezer across the array.
The numbers are targets, not measurements
The main caveat is that the treatment becomes less complete as occupancy rises. For more than two molecules in a tweezer, the study uses two-body scattering as a proxy rather than a full multiband many-body calculation. In that proxy, with the Rabi frequency set to 0.4 times the detuning and equal to 2π times 5 megahertz, the elastic-to-loss ratio was at least roughly 10,000 up to collision energies of 10 microkelvin.
Taken together, the results define a numerical protocol and parameter targets for NaCs tweezers, not an experimental demonstration. No experimental sample or measured fidelity is reported, and the headline percentages apply to the stated modeled conditions, including small initial occupancies and, for the array estimate, initially doubly occupied sites. Whether those figures are reached in a physical array remains an open test.
Paper data and sources
Original title: A Protocol for Shielding-Enhanced Loading of Single Polar Molecules into Optical Tweezers
Authors: Reuben R. W. Wang, Christian H. Nunez, Conner Williams et al.
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-20
DOI: Not available
Original paper · Full text