Single metasurface could generate record numbers of trapped neutral atoms

Physicists in China have demonstrated that a structure called an optical metasurface can individually trap up to 78,400 neutral atoms – a promising development in efforts to build a large-scale quantum computer. The method, which is similar to one demonstrated independently by a team at Columbia University in the US, could help overcome a troublesome bottleneck for computers that use neutral atoms as their quantum bits (qubits).

Arrays of trapped neutral atoms are widely employed in physics research, and they are a promising platform for quantum computing. Their main drawback is scalability, explains physicist Zhongchi Zhang, who co-led the new study together with his Tsinghua University colleague Xue Feng. The components normally used to make such arrays, such as spatial light modulators (SLMs) and acousto-optic deflectors (AODs), can only create around 10,000 atom traps at any one time, and are thus limited to a maximum of 10,000 atomic qubits.

Flat optical surfaces made up of 2D arrays of metasurfaces

In their work, which is detailed in Chinese Physics Letters, Zhang and colleagues replaced SLMs and AODs with two-dimensional arrays of metasurfaces – artificial nanostructures that manipulate light in much the same way as traditional optics, but with far less bulk. To do this, they used a method known as a weighted Gerchberg-Saxton algorithm to design a metasurface made up of nanoscale pillars that can transform a single input laser beam into a 280 x 280 array. They then constructed this metasurface from silicon nitride using electron-beam lithography and reactive ion etching. Both methods are compatible with standard complementary metal–oxide–semiconductor (CMOS) manufacturing techniques and are thus highly reproducible.

The result is a set of nanoscale, light-manipulating, pixel-like structures that act like a superposition of tens of thousands of flat lenses. When a laser beam hits these “lenses”, they produce a unique pattern that contains tens of thousands of focal points. As long as the laser light is intense enough, each of these focal points can be used to trap and manipulate atoms via a well-established technique called optical tweezing.

Zhang explains that the main advantage of trapping atoms this way is that the metasurface generates the array of optical tweezers on its own, without the need for additional bulky and expensive optical components such as microscope objectives to focus the light. Another benefit is that such arrays are very robust to high laser intensities, which are a prerequisite when the goal is to trap hundreds of thousands of atoms. Indeed, Zhang says that arrays of this type can handle powers several orders of magnitude higher than is possible with arrays made using SLMs and AODs. The intensity of the light is also highly uniform (90.6%) across the array, and individual beams feature an Airy disk-like profile with an average first dark radius of around 1.017 µm – parameters that Zhang says are “ideal for trapping single atoms”.

Improving fault-tolerant quantum computing

“Our work addresses the critical need for scalable physical qubit arrays required for improving ‘fault-tolerant’ quantum computing and making it more robust to errors,” Zhang tells Physics World. “Since quantum error-correcting codes may call for hundreds of physical qubits to build a single logical qubit, scalability here becomes paramount.”

Researchers at Columbia University also recently demonstrated an atom-trapping array that replaced SLMs and AODs with flat optical metasurfaces. But whereas the Columbia team managed to create 360,000 tweezers with extreme pixel efficiency (around 300 pixels/tweezer, with over 95% uniformity) the Tsinghua University group prioritized the array’s robustness at higher laser power, achieving around 1354 pixels/tweezer. Both studies have validated the use of metasurfaces as a scalable platform beyond the limitations imposed by AODs and SLMs, says Zhang.

Spurred on by their preliminary results, Zhang and colleagues report that they are now fabricating a 19.5 mm-diameter metasurface designed to generate approximately 18,000 optical trapping sites. Their goal is to place this metasurface outside the vacuum chamber that contains the trapped atoms. “Such an external configuration represents a significant departure from conventional approaches and is expected to enable the trapping of over 10,000 atoms, surpassing current records while substantially simplifying the experimental setup,” Zhang explains.

The team is also developing a next-generation integrated architecture in which metasurfaces will replace the fluorescence imaging microscopes used to characterize trapped atoms, as well as the optical tweezer arrays used to trap them. “This approach aims to create a completely new system paradigm for neutral-atom quantum computing that eliminates the need for traditional bulky optics, enabling unprecedented compactness and scalability for future quantum processors,” Zhang says.

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