Photonic computing seeing the light of day

Read three articles this past week or so that shed some light on Photonic computing, that is doing computing with light rather than electronic signals. We discuss the first two directly below and save the last for the end.

The first was a TechCrunch article, Bill Gates, Neo, [&] Gigafund backing Luminous in photonic computing, was about a new startup that raised $9M in seed funding to focus on AI computing using photonics rather than electronics. Luminous is working on a AI accelerator chip.

The second article was from MIT News, Chip design drastically reduces energy needed to compute with light, which discusses a paper in Physics Review (Large scale optical neural networks based on photo-electric multiplication) again focused on AI DL acceleration using photonics.

AI accelerators are cropping up everywhere (see my posts on Intel DL Boost, New Graphcore GC2 chips, and AI processing at the edge). So having a photonic, AI accelerator should be easy to integrate with.

Luminous from Princeton

Neuromorphic architecture with level-tuned neurons. The internal state of a primary neuron is used to enable a set of level-tuned neurons. Credit: Pantazi et al. ©2016 IOP Publishing

Luminous Computing is based on research that’s been done over the last decade at Princeton University, and seems to have as it’s goal to ship a single chip to replace 3000TPUs. We’ve talked about Google TPUs before (see Google releases new Cloud TPU, and TPU and hardware vs. software innovation – round 3 posts). We’ve also discussed photonics before (see our Photonic or Optical FPGAs on the horizon post).

Luminous Computings, CTO Mitchell Nahmias has helped author a number of papers, articles, & books on photonics with others from Princeton University, perhaps the most impressive being Principals of neuromorphic photonics. If they are following this approach, it would seem likely they are creating a new photonic neuromorphic chip.

There is absolutely minimal information on their web site so, I am assuming they’re targeting a neuromorphic chip from the CTO’s history of research publications. I could be completely incorrect on this assumption but will continue with it for now.

Neuromorphic AI is in contrast to standard neural network deep learning approaches to AI. We have discussed Neuromorphic computing in a number of posts (e.g. see IBM introduces the SYNAPSE chip from 2011 or University of Manchester fires up the largest neuromorphic chip from 2018, more if interested, just search for “Neuromorphic”). Neuromorphic approaches to AI although had early promise, have been losing the technological arms race, as standard AI DL using GPUs, implementing neural networks, seems to have vastly more adherents both in academia as well as industry.

On the other hand, simulating actual neurons, as in neuromorphic computing, has the potential to be a significant breakthrough. But only if it can be orders of magnitude faster, cheaper or somehow better than standard AI DL neural network processing.

In one of the news reports, it mentioned that Luminous Computing has working silicon. Assuming their approach has significant AI training advantages, will it also require a neuromorphic chip to perform AI inferencing?

Photonics from MIT

The MIT group states, that based on their simulation of the photonics chip, they can perform AI DL using neural network training with much less energy. In their paper, they mention that their matrix multiplication is performed passively by optical interference alone. In this case, both the input and the multiply and accumulate function (MAC) are done using photonics.

AI DL neural network training takes multiple iterations of matrix multiplications and accumulate (MAC) functions. A standard CPU takes about 20 pj/MAC (10**-12 pico-Joules/MAC) and a GPU takes about 1 pj/MAC. Whereas in simulation they were able to show their photonic chip should be able to perform 10 fJ/MAC (10**-15 femto-Joules/MAC) This would put the MIT photonic chip design power consumption per MAC ~1000X better than CPUs and ~100X better than GPUs.

The paper goes on to say that theoretically, they could perform at 50 to 100 zJ/MAC (10**-21 zepto-Joules/MAC), which would add another factor of 1000 to their power advantages.

In addition, MIT’s photonic chip design is being constructed using “free [space] optical components” and should be able to scale much better than “nano photonic implementations”. Nano photonics requires waveguides and optical couplers whereas free-space photonics uses lasers beams traveling through space.

Their paper claims that nano-photonics can only support ~1000 node neural networks but with their free-space photonic solution, they believe they can support a 1,000,000 node neural network. Not sure how this would work in a chip design with lasers visible, being routed via micro-mirrors and other optical components, around a chip.

Nothing I could find indicates that the MIT team has working silicon nor have spun out a startup with seed funding.

Energy use for AI DL modeling

There have been many news reports noting that training a AI DL neural network is 5X more carbon polluting than a driving a car. This statement is based on the third article, a single report, Energy and policy considerations for deep learning in NLP .

One can argue with the numbers. Also, the fact that the car has historically consumed gas, a non-renewable energy source and the GPU could theoretically use solar, wind or hydro renewable power for its energy needs also needs to be stated. Moreover, one trains a AI DL (NLP) model once with all the data and then may subsequently train it on new data that comes in, but it gets used a gazillion times. Most adults (at least in the US) have a car and drive it often, multiplying automobile energy impacts by 100s of millions in the US alone.

But the fact is that AI DL training, that iterates multiple times over 1000s to 1,000,000s of data items, takes energy and lots of it. How that energy is produced matter to global warming. And anything that has the potential to reduce this energy consumption should be welcomed.

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Photonics may be ready for prime time but only if they can significantly improve AI training .

Photo Credit(s): Logo from Luminous Computing website.

Figure from Neuromorphic computing mimics important brain feature Phys.org article

Screen grab of Figure 1 from the Large-Scale Optical Neural Networks Based on Photoelectric Multiplication paper.

Screen grab of Figure 1 from the Energy and Policy Considerations for Deep Learning in NLP paper.

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