Low-entropy arrays of microwave-shielded molecules prepared by interaction blockade

Abstract

Ultracold molecules are becoming an increasingly important technology for quantum simulation, computation, and sensing, but their state preparation in large, low-entropy arrays remains a key challenge. We propose to deterministically load single molecules into optical tweezer arrays or lattices from either thermal or degenerate gases, with a high probability of occupying the tweezer's motional ground state. Strong repulsion between microwave-shielded molecules prevents multiparticle occupancy. Our proposal represents a robust scheme for deterministic single molecule preparation directly in the motional ground state with expected fidelities exceeding 99 percent for small trap volumes and highly polar species. This method can be scaled to thousands of traps limited by the reservoir molecule number, opening the door to large, low-entropy polar molecule arrays for quantum computation, quantum simulation, and precision measurement.

Figures (5)

• Figure 1: Illustration of deterministic loading of single molecules via interaction blockade. (a) A red optical tweezer with waist $w_0$ is overlapped with thermal reservoir gas (blue) with many polar molecules, which experience repulsion from microwave shielding, (b) with intermolecular energy shown versus distance. The length scale for the $c_6$ potential, $R_6$, can approach or exceed the tweezer waist, blocking a second molecule from remaining inside the tweezer. (c) Energy scales relevant to our model include the reservoir thermal energy $k_BT$ and the single-particle binding energy $\epsilon$ of the tweezer. The deterministic single-particle loading pictured in (a) will occur approximately in the conditions that $U\gg\epsilon\gg k_BT$. As molecules collide inside a small tweezer volume, a sufficiently strong two-particle repulsive energy $U$ guarantees that multiparticle occupancy is not supported in thermal equilibrium. (d) Compared to stochastic loading from a thermal gas, which produces disordered arrays (right), our scheme can deterministically load single molecules in tweezer traps (middle) with high probability of ground state occupancy (left), leading to low-entropy arrays.
• Figure 2: Blocking double occupancy by the interaction blockade. Panel ( a) illustrates interacting (non-interacting) two-molecule energy levels as dashed (solid) levels in tweezers of varying depth $D$. In deep tweezers [red shading], the repulsion $U$ is small compared to the binding energy, and double occupancy is not prevented. At intermediate depths [white shading], two-molecule bound states exist but are energetically unfavored for $U>\epsilon$, and only at the shallowest depths [blue shading], two-molecule bound states are fully expelled. Panel ( b) illustrates the single particle binding energy $\epsilon$ and energetic cost to loading a second molecule, $U-\epsilon$, for KAg in a 300 nm tweezer. Results of DVR calculations for other species and tweezer parameters are shown in panel ( c).
• Figure 3: Deterministic loading of shielded molecules for fixed densities $10^{10}$ and $10^{12}$ cm$^{-3}$ in panel ( a) and ( b), respectively. Dashed lines show the infidelity $1-\mathcal{F}$ for single molecule preparation; solid lines show $1-\mathcal{F}$ for preparing that single molecule in its motional ground state. Near perfect loading is possible for highly dipolar molecules and for small-waist tweezers. Full dependence on temperature and phase-space density is shown in the Appendix.
• Figure 4: Infidelity of deterministic single molecule preparation as a function of phase space density and temperature. Lines indicate the region of constant densities of $10^{10}$ and $10^{12}$ cm$^{-3}$, respectively, used in the main text Fig. 3.
• Figure 5: Infidelity of single molecule preparation in the motional ground state as a function of phase space density and temperature. Lines indicate the region of constant densities of $10^{10}$ and $10^{12}$ cm$^{-3}$, respectively, used in the main text Fig. 3.