Caveat: the paper mainly focuses on the "standard quantum limit" which is the fundamental photon energy needed for the operations. If other things are taken into account (for example, modulation energy for the weights in this homodyne scheme, which scales with N² and not N, or the limits of the ADC) then the energy they are proposing is nowhere near possible. Furthermore, substantial alignment and packaging problems exist for free space optical systems, which prevents them from beating integrated approaches in the near term. In fact, it seems that Fathom Computing has potentially pivoted away from free space, based on the latest verbiage on their website, and they've been trying to get it to work for 3 years now.
However, it still presents an interesting case for the fact that the fundamental floor on optical scaling is absolutely tiny. It'll be interesting to see who wins in this space :)
Does the paper propose building these devices in free space? I got the sense this was all intended to be produced lithographically with waveguides, so alignment wouldn't be a problem.
The title is wrong about it being an integrated design. Here's an excerpt from the paper abstract:
"This paper presents a new type of photonic accelerator based on coherent detection that is scalable to large (N≳106) networks and can be operated at high (gigahertz) speeds and very low (subattojoule) energies per multiply and accumulate (MAC), using the massive spatial multiplexing enabled by standard free-space optical components"
However, it still presents an interesting case for the fact that the fundamental floor on optical scaling is absolutely tiny. It'll be interesting to see who wins in this space :)