Passage

Current obstacles

Physics limits

• Silicon can't make its own lightSilicon has an indirect bandgap, so it can't emit light efficiently. Lasers must be made from other materials such as indium phosphide, which cost more and wear out sooner than silicon chips.
• Light can't be squeezed like electronsDiffraction keeps optical waveguides around half a micrometre wide at telecom wavelengths, hundreds of times wider than the finest wires, so photonic circuits can't be packed as densely as electronics.
• Converting to light and back costs energyEach electrical-to-optical conversion and back costs picojoules per bit. Over the few millimetres inside a package, plain copper can still be cheaper, so optics win only beyond a certain distance.