Simulating Physics with Computers

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   Feynman argues that nature is quantum, not classical, so any machine that wants to process it faithfully must itself use quantum phenomena, not merely classical bits.

   Simulating Physics with Computers · 1982

   Nature computes quantum-mechanically, so only a quantum machine can simulate it faithfully.

   Feynman starts from a simple observation. Simulating a physical system on a classical computer grows exponentially with the system's size, because quantum states cannot be represented efficiently with ordinary bits. So he proposes a computer that is itself quantum, one that processes amplitudes and interference rather than mere digits. Light is exactly such a system: photons propagate, interfere and combine according to quantum equations. An optical processor lets light perform the operation directly, through interference, instead of simulating it step by step in silicon. A neural network is, at its core, a chain of matrix multiplications, and an optical matrix performs that multiplication as the beam travels through it.

   “Nature isn't classical, dammit, and if you want to make a simulation of nature, you'd better make it quantum mechanical, and by golly it's a wonderful problem, because it doesn't look so easy.”

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