Optical tweezer manipulation of single atoms
Atoms can be captured and guided by light through optical dipole interactions. However, applying this method to guide many atoms simultaneously and independently has remained difficult due to the low inertia of atoms. Recently, a KAIST research team has reported their successful demonstration of moving 42 rubidium atoms along arbitrary paths, using dynamic holographic optical tweezers. This enables for many single-atom-level machinery examples previously recognized as impossible, including (i) bottom-up design of arbitrary atom lattices, (ii) deterministic atom loading, (iii) atom sorting, and even (iv) single-atom-level machinery, to list a few.
Optical tweezer, or the single-beam gradient force trap, is a scientific instrument used to hold or move microscopic particles. After the first demonstration by Ashkin in 1970, this instrument led not only to the invention of laser cooling and trapping of neutral atoms, but also to the huge impact on the study of biological systems at the level of single molecules. One important example characterized thus are molecular motors. Among the many applications arising from the implementation of optical tweezers are (i) cell sorting and, more recently, (ii) particle sorting, both of which are performed with dynamic rearrangements of optical tweezers to achieve otherwise unnatural spatial configurations of these species. However, it has been known that such arrangements are not easily achievable for microscopic objects like atoms due to their small inertia. When optical tweezers are dynamically changed, their interference causes a certain amount of intensity flickering between frames; as a result, atoms even with a little kinetic energy as low as Doppler energy easily escape the optical trap.
In a recent issue of Nature Communications published in October 31, 2016, a team led by Prof. Jaewook Ahn of the Department of Physics at KAIST, reported optical rearrangements of single atoms are also possible as freely as those of macroscopic particles. By using a two-dimensional liquid-crystal array, updated with intensity-flicker-free optical array solutions, single-atom-level dynamic holographic optical tweezers (atom-DHOTs) demonstrated a full range of motion, useful in simultaneous and individual control of constituent atoms. In the experiment, cold rubidium atoms cooled in a magneto-optical trap were captured by a set of optical tweezers (far-off resonant optical dipole-traps); the positions of the optical tweezers, captured atoms were adjusted by the liquid-crystal array to simultaneously drag the atoms to the positions of a target configuration. The experimental demonstrations include rotation, translation, and path-following of a set of atoms, simultaneously and independently. Words such as “KAIST” and “ATOM” were able to be formed by the rearrangement of single atoms from their initial random locations.
The science of physics behind this simultaneous and independent transport of individual atoms is the capability of holographic optical tweezers to sustain the trapped atoms while the hologram is being actively updated, by minimizing the intensity flickering of individual optical dipole traps. Solutions for flicker-reduced hologram updates are devised either using an analytic design, superposition, or adaptive algorithm. Considering the fact that the temperature of a single atom is as low as about 120 K in a deep 1.4 mK trap, an intensity flickering that is smaller than 5% allows the atom to remain in the trap over 99.9% in each hologram frame update. Therefore, the single-atom rearrangements with a high success rate over 99% are accomplished with distances of up to a 10 m translation.
The primary advantage of using atom-DHOTs is the bottom-up design of atom lattices. Not only does this method enable for previously impossible lattice configurations, but also provides an example in atom lattice formation replaces a probabilistically-loaded high-entropy configuration, initially formed as such due to the pairwise collisional loss of atoms in an optical tweezer, to a completely-filled zero-entropy configuration. By combining these two features, deterministic loading of atoms in an arbitrary array configuration with unit occupancy has become possible. Since creating scalable and highly controllable quantum systems is an essential cornerstone for quantum computing, quantum simulation, and quantum many-body physics, this bottom-up approach to form single-atom networks is hoped to be useful in those quantum technologies.
This study was published online in Nature Communications (October, 2016).Hyosub Kim, Woojun Lee, Han-gyeol Lee, Hanlae Jo, Yunheung Song, and Jaewook Ahn, “In situ single-atom array synthesis using dynamic holographic optical tweezers,” Nature Communications 7, 13317 (2016).
DOI: 10.1038/ncomms13317paper link: http://www.nature.com/articles/ncomms13317
* Lab information
Jaewook Ahn, Ultrafast Quantum Optics Laboratory, Department of Physics, KAIST