A Sum That Would Not Stay Light

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A Sum That Would Not Stay Light

Ruth wanted one matrix-vector product done in light. That product is the heavy step when a layer in a neural net multiplies its inputs by its weights, and the board beside her was already warm from doing it with electrons. If the light path worked, the same product could show up as interference instead of as a long chain of transistors.

The guides on the chip were silicon, the platform people call silicon photonics, and they can sit with electronic circuits. A beam splitter divided the beam. A phase shifter nudged one path. She had set the split and the phase to the weights she cared about. Where the paths met, interference built a weighted sum, the step a matrix-vector product is made of. A second wavelength traveled the same guide as the first, two streams in one pipe.