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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 . That sharing is wavelength division multiplexing . At the detector she had a number . The number would not stay light . The next layer needed it the way a program needs a value still in memory . Electronic RAM hands a stored charge back when asked . Nothing on her bench would hand a light - held value back in that way . A detector turned the photons into an electrical signal , and the electronic side wrote the value down . The crossing is a toll . Each time the result leaves the light , part of the reason for using light is spent . Loss had already taken a share . A guide absorbs a little , scatters a little , and a poor joint leaks a little . Raise the beam to replace what was lost , and the boost brings noise with it . She had a linear step , the kind passive optics can do , and then a return into electronics . She left the larger stories on the shelf . A nonlinear material can let one beam alter another , because strong light changes the stuff it passes through , and that is a sketch of an optical switch . An Ising machine can use optical pulses to search for a low - energy arrangement of a hard problem . Quantum optics can send single photons and use entanglement . She had not just finished any of those , and she would not paste them onto today's number . She labeled the file as one product , stored on the electronic side , and shut the laser off .
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