Quantum light (i.e. photons), linear optics, adaptive measurements and feed forward can be used to build an efficient universal quantum computer. ref: Knill et. al. 2001.

Clearly the above resources can be also be used to build an efficient classical computer (since a QC can efficiently simulate a CC).

Can an efficient classical computer be built using classical light (instead of photons), linear optics and some detection model? I expect the answer is yes.

I am looking for a constructive proof. Particularly

  • bits: degree of freedom of classical light
  • universal gate set: what operations and measurements need to be performed to implement one such set.
  • bonus: do a complexity analysis to show efficiency in time and resources.

Failing a constructive proof, a non-constructive proof will be welcome.


1 Answer 1


It is possible to build a classical computer using just NAND gates, so proof of concept would be an all optical NAND gate. Here is one of many examples

  • $\begingroup$ For another example of how just about anything can be used to construct a NAND gate (and hence a computer), try fluidics: a computer using fluids (water, air, etc). See en.wikipedia.org/wiki/Fluidics $\endgroup$
    – hdhondt
    Oct 30, 2014 at 9:20
  • 2
    $\begingroup$ One caveat about optical computers. They have been discussed for decades as "the next big thing" but never make it. Part of the reason is that modern chips now use features smaller than the wavelength of any light that might be used in a practical machine modeled on existing architectures. In other words, they cannot achieve the same degree of integration and circuit density. $\endgroup$
    – user56903
    Oct 30, 2014 at 9:29
  • $\begingroup$ @DirkBruere I am not particularly interested in the edge-of-tech-implementation sort of answer. I work in the field of optical quantum computing and am trying to understand this the resources needed for optical classical computing vs those needed for optical quantum computing. $\endgroup$ Oct 30, 2014 at 23:57
  • $\begingroup$ Well, the only resource needed is that NAND gate. A google search should bring up almost all current implementations of a classical optical NAND gate. $\endgroup$
    – user56903
    Oct 31, 2014 at 8:32

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