Gray-Molasses Optical-Tweezer Loading: Controlling Collisions for Scaling Atom-Array Assembly

Abstract

To isolate individual neutral atoms in microtraps, experimenters have long harnessed molecular photoassociation to make atom distributions sub-Poissonian. While a variety of approaches have used a combination of attractive (red-detuned) and repulsive (blue-detuned) molecular states, to date all experiments have been predicated on red-detuned cooling. In our work, we present a shifted perspective—namely, the efficient way to capture single atoms is to eliminate red-detuned light in the loading stage and use blue-detuned light that both cools the atoms and precisely controls trap loss through the amount of energy released during atom-atom collisions in the photoassociation process. Subsequent application of red-detuned light then assures the preparation of maximally one atom in the trap. UsingΛ-enhanced gray-molasses for loading, we study and model the molecular processes and find we can trap single atoms with 90% probability even in a very shallow optical tweezer. Using 100 traps loaded with 80% probability, we demonstrate one example of the power of enhanced loading by assembling a grid of 36 atoms using only a single move of rows and columns in 2D. Our insight is key in scaling the number of particles in a bottom-up quantum simulation and computation with atoms, or even molecules.

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Plain Language Summary

One of the big challenges in quantum simulation and computing is the ability to assemble and individually control large arrays of atoms. While researchers have long used ultracold gases of neutral atoms for simulating quantum physics on a macroscopic scale, the community needs new ways to engineer large ensembles with single-atom control to make the next leap into quantum computing and simulation. Here, we enable preparation of such systems by experimentally demonstrating a new technique for isolating single atoms in a trap with much greater efficiency than standard approaches.

In our work, we densely load atoms into a large optical tweezer array using a laser-based cooling technique known as gray molasses, which relies on light whose frequency is tuned slightly higher than an electronic transition in the atoms. We find we can catch and hold onto single atoms with nearly 90% efficiency in a trap much shallower than required for standard loading—which peaks at about 60% efficiency—as we avoid formation of molecules that cause two atoms to be lost from the trap. Given that available laser power typically limits the array size, we can thus create more traps and load them more efficiently. With the help of single-atom fluorescence imaging, we then rearrange the atoms to an ordered, defect-free array in a single move by switching rows and columns with defects and contracting the remaining ensemble.

We predict that our technique will scale up neutral-atom array assembly by improving existing rearrangement algorithms while enabling newer and faster ones. This technique realistically allows for the creation of systems containing hundreds of individually addressable qubits, which provides a pathway to the most powerful quantum machines to date.