Scheme for Deterministic Loading of Laser-Cooled Molecules into Optical Tweezers

Etienne F. Walraven, Michael R. Tarbutt, and Tijs Karman

Phys. Rev. Lett. 132, 183401 – Published 3 May 2024

Abstract

We propose to repeatedly load laser-cooled molecules into optical tweezers, and transfer them to storage states that are rotationally excited by two additional quanta. Collisional loss of molecules in these storage states is suppressed, and a dipolar blockade prevents the accumulation of more than one molecule. Applying three cycles loads tweezers with single molecules at an 80% success rate, limited by residual collisional loss. This improved loading efficiency reduces the time needed for rearrangement of tweezer arrays, which would otherwise limit the scalability of neutral molecule quantum computers.

Received 12 January 2024
Accepted 27 March 2024

DOI:https://doi.org/10.1103/PhysRevLett.132.183401

Physics Subject Headings (PhySH)

Cold gases in optical lattices Dipolar molecules Rotational states Ultracold collisions

Cooling & trapping

Atomic, Molecular & Optical

Authors & Affiliations

Etienne F. Walraven ¹, Michael R. Tarbutt ², and Tijs Karman ^1,*

¹Institute for Molecules and Materials, Radboud University, Nijmegen, The Netherlands
²Centre for Cold Matter, Blackett Laboratory, Imperial College London, Prince Consort Road, London SW7 2AZ, United Kingdom

^*tkarman@science.ru.nl

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Vol. 132, Iss. 18 — 3 May 2024

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Images

Figure 1
Light-assisted collisions. Shown schematically are interaction potentials in the electronic ground state $X$ and excited state $A$ . Wavy vertical lines indicate the Condon point where the attractive or repulsive dipole-dipole interaction in the excited state compensates for the red or blue detuning, respectively. Nonadiabatic transitions occur at the Condon point with probability $p$ , leading to a short-range encounter. For blue detuning, there can be a light-assisted collision on the repulsive branch, releasing kinetic energy $h Δ$ which can be controlled to eject only one of the two particles. This requires a process that is adiabatic on the way in, and nonadiabatic on the way out, so has probability $p (1 - p)$ whose maximum value is 0.25. For atoms, short-range encounters are usually elastic, so the atoms have many opportunities for such controlled light-assisted collisions. For molecules, short-range encounters usually lead to loss, and the loss dominates since $p$ is always larger than $p (1 - p)$ .
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Figure 2
Proposed scheme for deterministic loading of laser-cooled molecules into optical tweezers. (a) Illustrates the different processes that can occur in each tweezer in each cycle. Row 1: single $j = 1$ molecules are loaded with 50% probability due to collisional blockade. They are transferred to $j = 3$ using two microwave $π$ pulses. Row 2: subsequent cycles transfer molecules to different hyperfine states of $j = 3$ , indicated by different colors. Row 3: molecules loaded in previous cycles are not addressed by subsequent microwave pulses and remain stored. Row 4: when a tweezer already contains a $j = 3$ molecule, the dipolar blockade prevents accumulating additional $j = 3$ molecules. (b) Illustrates loading of the array after multiple cycles of the scheme.
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Figure 3
Molecular interaction potentials (a) Attractive rotational dispersion interaction for two CaF molecules in the $j = 1$ rotational state of the electronic ground state (green solid). Also shown is a schematic of the dressed potentials where one molecule is excited by the cooling light to the $A$ ${}^{2}Π$ state, resulting in resonant dipole-dipole interactions (purple dashed). Light-assisted collisions occur at the crossing between the curves. (b) Repulsive rotational dispersion interaction for two CaF molecules in the $j = 1$ and $j = 3$ rotational states [33], which suppresses loss in short-range encounters. Excitation of the $j = 1$ molecule by the cooling light does not produce resonant dipole-dipole interactions in this case, so there are no light-assisted collisions. Multiple green lines indicate different possible total angular momentum states as indicated.
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Figure 4
Performance of the scheme in the presence of residual collisional loss. Loading probability as a function of the loss per cycle for 2, 3, and 4 cycles. The vertical line indicates the probability of loss of CaF molecules during a 10 ms loading cycle when the peak density is $1.5 \times 10^{13} {cm}^{- 3}$ .
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