Advances in Neutral Atom Quantum Computing A New Approach to Capturing Atoms
Atoms are notoriously difficult to control. They pass through the strongest of containers and jitter even at temperatures close to absolute zero. Nevertheless, scientists need to capture and manipulate individual atoms in order for quantum devices such as atomic clocks/quantum computers to function properly. If individual atoms can be captured and controlled in large arrays, they can be used as quantum bits whose states or orientations could eventually be used to perform calculations far faster than the fastest supercomputers available today.
The research results were published on August 1 in the journal PRX QUANTUM under the title "Single-atom capture in super-surface lens optical tweezers" [1].
The experimental results are illustrated using a graphical representation of light focusing on a flat glass surface studded with millions of "metalens" (super lenses) forming an optical tweezer. a) The cross-section of the device depicts planar light waves, focused by secondary waves generated by nanopillars of different sizes; b) The same nanopillars are used to capture and image individual rubidium atoms.
The optical tweezers that won the 2018 Nobel Prize in Physics feature bulky centimeter lenses, or microscope objectives, holding individual atoms outside of a vacuum.NIST and JILA had previously used this technique with great success: the creation of an atomic clock.
In the new design, instead of using a typical lens, the NIST team used an unconventional optical device - a square sheet of glass about 4 mm long with millions of "pillars" a few hundred nanometers (billionths of a meter) in height printed on it. Together, they act as tiny lenses. These imprinted surfaces, called "supersurfaces," focus laser light to capture, manipulate, and image individual atoms in the vapor. Unlike ordinary optical tweezers, the hyperlenses can operate in the vacuum where the trapped atomic clouds are located.
Supersurface optics trapped with optical tweezers. (a) Amorphous silicon (a-Si) nanopillar (height 660 nm) containing a periodic array (lattice constant = 280 nm) on a 500 µm thick fused silica substrate (light blue); (b) conceptual diagram of the super-surface lens operation showing light propagation (pink), wavefront surface (dashed line), and secondary sub-waves (black semicircle) re-emitted by the nanopillar that inter interfering to form a focused wavefront surface; (c) optical setup for trapping (pink) and fluorescence imaging (green) of single atoms in the array created with multiple input beams generated by a biaxial acousto-optical deflector; (d) an image of the trapped 87Rb array created by averaging the results over multiple experimental iterations (100) with a probability of approximately 52% of single atoms in each image.
Supersurface mounting in vacuum. (a) Photograph of a supersurface sample optically contacted on a wedge-shaped fused silica sample holder; (b) metal sample with NA = 0.55 designed for the 852 nm tweezer beam; (c) schematic of how the tweezer and charge-coupled device (CCD) camera are aligned to the supersurface sample by substrate back reflection; (d) schematic of the optical tweezer imaging path, which shows that the lenses L1 and L2 introduce no additional aberration ; (e) end view of the vacuum chamber, showing the orientation of the probe beam (also the resonant heating beam) in relation to the metal sample and the tweezer beam.
The process involves several steps. First, an incident fiber with a particularly simple form, called a "plane wave" (a plane wave is like a moving parallel sheet of light with a uniform, homogeneous wavefront or phase, whose vibrations remain synchronized with each other and neither diverge nor converge as it moves), hits groups of tiny nanopillars. The grouping of nanopillars converts the plane wave into a series of wavelets (wavelets), each of which is slightly out of synchronization with its neighboring wavelets. As a result, they can peak at different times.
These wavelets analytically combine or "interfere" with each other so that they concentrate all the energy at a specific location - the location of the atom to be captured.
Depending on the angle at which the incident plane light wave hits the nanopillar, the wavelet analysis is focused at slightly different locations, allowing the optical system to capture a series of individual atoms at slightly different locations.
NIST researcher Amit Agrawal said [2], "Because the miniature planar lens can be operated in a vacuum chamber and does not require any moving parts, it is possible to capture atoms without having to build and manipulate a complex optical system." In this new study, Agrawal and two other NIST scientists, Scott Papp and Wenqi Zhu, as well as collaborators from Cindy Regal's group at JILA, designed, fabricated and tested the hypersurface and performed single-atom capture experiments.
The researchers report that they captured nine rubidium atoms each, according to the paper.Agrawal said, "The same technique, scaled up by using multiple hypersurfaces or one with a large field of view, should be able to confine hundreds of single atoms and could lead the way in routinely capturing arrays of atoms using chip-scale optics. " The system holds the atoms in place for about 10 seconds, which is enough to study the quantum mechanical properties of the particles and use them to store quantum information (quantum experiments are performed on time scales of ten millionths of a second to thousandths of a second).
To demonstrate their success in capturing rubidium atoms, the researchers illuminated them with a separate light source, causing them to fluoresce. Then, the supersurface played a second key role. Initially, they shaped and focused the incident light that captured the rubidium atoms; now, the hypersurface captures and focuses the fluorescence emitted by those same atoms, redirecting the fluorescent radiation into a camera that allows imaging of the atoms.
Supersurfaces can do more than just capture individual atoms: by precisely focusing light, supersurfaces can direct individual atoms into special quantum states tailored to specific atom capture experiments. For example, polarized light guided by a tiny lens can cause an atom's spin (a quantum property similar to the Earth's rotation around its axis) to point in a specific direction. These interactions between focused light and individual atoms are useful for many types of atomic-level experiments and devices, including future quantum computers.
Reference link：
[1]https://journals.aps.org/prxquantum/abstract/10.1103/PRXQuantum.3.030316
[2]https://www.nist.gov/news-events/news/2022/08/nist-researchers-develop-miniature-lens-trapping-atoms
