Why Optical Tweezers Are Unlikely to Lead To Atomically Precise Manufacturing
You should still read about them, however.
To start, optical tweezers are an incredible piece of technology that more people should read about.
Optical tweezers can catch and throw atoms, enable optical arrays that contain thousands of atoms, and even combine two atoms into a new molecule.
In terms of being a scalable quantum computing technology, ultracold optically-trapped neutral atoms are among the best options in the world.
However, when it comes to atomically precise manufacturing, optical tweezers have a number of technical problems which makes them poorly suited for the job.
First, optical tweezers do not apply that much force to atoms (typically pico-newtons, or 10^-12 N). This is much less than even the weak bonds of atoms on substrates, so it would be difficult to break those bonds for atomic manipulation.
Second, I have not found any literature where someone actually assembles an object atom by atom with optical tweezers. All of the papers with “atomic assembly” in them referred to optical tweezer arrays, which user laser light to make what’s almost like an egg carton-shaped potential energy surface for cooled atoms would rest in to perform quantum computation.
Third, optical tweezers have poor coherence times. The 6100 atoms Caltech paper had a coherence time of 12.6 seconds for its qubits, and the 2018 Harvard two atoms build a molecule paper only had the NaCs in the Na + Cs → NaCs reaction last for 30 nanoseconds. I would prefer my molecular machines to last much, much longer. (The coherence of quantum computers is it’s own subfield and something I’d love to cover more in-depth, but is out of scope for this piece.)
Fourth, there were a few interesting nano-manufacturing via optical tweezer papers, but they were all in micro-fluidics and the particles were much bigger than atoms. While Drexler does mention solution-phase bootstrapping as one idea of getting to APM, that is more analogous to the ribosome rather than using optical tweezers.