Princeton team achieves millisecond-scale atom reloading while preserving qubit coherence

Neutral-atom quantum processors have made progress in scale and operation precision in recent years, and researchers have demonstrated quantum error correction and algorithms based on logical qubits. But if one can only use a batch of atoms prepared before computation begins, then as atom loss and leakage errors accumulate, the depth of executable quantum circuits becomes limited. Background gas collisions, atom transport, gate operation errors, and some readout methods can all cause atom loss.

Replenishing atoms itself is not easy: it often requires magnetic field gradients and cooling, and imaging light, which may disrupt the states of existing qubits. A previous two-species reloading experiment that preserved quantum coherence had a characteristic loading time of about 90 milliseconds. How to simultaneously achieve reloading speed and coherence is precisely the problem this research aims to solve.

The team set up a continuously replenished high-density cold-atom reservoir near the computation region, used moving optical tweezers to extract atoms from it, and then sent them to subsequent operation regions. Optical tweezers are tiny optical traps formed by focused lasers that can capture and move atoms; an array composed of multiple optical tweezers can process many sites simultaneously.

This speed depends on a continuously operating atom supply system: cold atoms start from a three-dimensional magneto-optical trap in an adjacent vacuum chamber, are guided about 35 centimeters along a cavity-enhanced optical dipole trap to the reservoir, and are then decelerated and cooled by one-dimensional optical molasses. The time constant for establishing the reservoir is about 5 seconds, but once steady state is reached it can supply continuously, without needing to refill from scratch after each extraction.

The optical tweezer array used for extraction is generated by crossed acousto-optic deflectors and operates at a wavelength of 488 nanometers. During loading, the array overlaps with the reservoir; after loading, a galvanometer mirror moves it to an imaging region about 300 micrometers away to reduce interference from atomic fluorescence in the reservoir. During imaging, the optical molasses is briefly turned off; atoms in the reservoir can still be retained, but their average replenishment rate is affected by the duty cycle of the molasses being on.