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I've been reading a few articles on photons that are capable of carry bits of information. I was wondering if anyone knew if there are any team of research scientist who have tried to use a photon as a particle to build a quantum computer.

From what I've read on quantum computing most scientist who work on quantum computers all face the problem of quantum decoherence.

Have any quantum computer researchers tried to use a photon in a state of superposition for quantum computing?

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  • $\begingroup$ As random number generation is a type of computation, a quantum random number generator is a simple quantum computer, albeit single purpose. This device has been on the market since 2001: marketing.idquantique.com/acton/attachment/11868/f-0226/1/-/-/-/… $\endgroup$ – Peter Diehr Jun 22 '18 at 13:56
  • $\begingroup$ And yes, it is based on the use of photons. I worked on a similar project, building a quantum random number generator, for a quantum computing theory group; their plan was to test some theorems. Alas, we ran out of money before we completed the project. $\endgroup$ – Peter Diehr Jun 22 '18 at 13:58
  • $\begingroup$ It's reported that a chinese team built up to 18 qubit entanglement using photons, where not only polarization, but also frequency and spatial positions are used to carry information. $\endgroup$ – XXDD Sep 28 '18 at 15:02
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Yes, this is an active research area, generally known as Linear Optical Quantum Computing.

The problem of using photons for quantum computing is that it is very hard to make them interact, which makes it very hard to implement two-qubit gates and to entangle those qubits. To counteract this, one scheme that's received a lot of attention is to pre-generate all the entanglement that the protocol will need, and then use a series of projective measurements and feed-forward schemes (i.e. to use the results of one measurement to decide how to do the next measurement) to implement quantum-computation protocols.

Generally, this is a solid scheme and it has a lot of clear advantages, but it suffers from the fact that it is difficult to produce enough photons, with sufficient initial entanglement, to run large-scale calculations. In this regard, push-button single-photon sources that can reliably and deterministically produce single photons would be close to silver bullets, but we're not there yet, so the technology is currently one among several viable candidates.

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There is a lot of research trying to provide two level systems with photons.

Decoherence is indeed a problem (since you won't have any of the quantum properties that make quantum computing faster, for some operations, than classical computing) and so people try to find systems with a long decoherence time.

One of them is quantum dots: see Wikipedia for instance: https://en.wikipedia.org/wiki/Quantum_dot. Other sources such as trapped ions or atoms are also used.

The challenge is getting a very "pure" single photon source (with only one photon in it), a good decoherence time (long enough) and photons that all have the same energy, and which therefore can be used for qubit like processes.

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