# Why isn’t light polarization used as a physical realization of a quantum computer?

Constructing a qubit requires something that can be represented as a linear combination of two states. The physical realizations are numerous

https://en.wikipedia.org/wiki/Quantum_computing

But I do not see the use of light polarization in this list. If we let |0> be vertically polarized light and |1> be horizontally polarized light, then a qubit can assume the polarization of light as a sum of these two components. It is easy to read the state of a qubit by measuring the residual brightness after passing the photon through a linear Polaroid filter. The polarization of a qubit can be changed by applying a magnetic field to the photon. The polarization can be stored in a hologram.

https://www.researchgate.net/publication/258369620_Polarization_Holography

Obviously there is a reason no one has done this so perhaps someone can enlighten me.

• To make a quantum computer, you need to be able to implement a set of 2-qubit gates on any pair of qubits. This might be difficult with light. I think you'd need some kind of nonlinear multi-photon effect to implement such gates, and these phenomena might have very low probabilities of occurring. Apr 10 '20 at 16:33
• why 2 qubit gates? photons can be entangled. and this reference (which I just found) en.wikipedia.org/wiki/Qubit lists light polarization in its first entry as a physical implementation. And this paper iopscience.iop.org/article/10.1088/1367-2630/15/5/053007 shows the use of photons with coherence times of ~500ns. Apr 10 '20 at 22:09
• There is no issue with polarization representing a qubit. My point was about quantum gates acting on multiple qubits. Phrased alternatively, I'm making the argument that it seems difficult (although probably not impossible) to implement quantum gates that generate arbitrary types of entanglement. This is different from coherence also. To perform arbitrary types of quantum computation, you need arbitrary unitary operators on your composite space of all qubits. The standard way to do this is to choose a universal set of 2-qubit gates which you can compose into arbitrary unitaries. Apr 11 '20 at 1:33
• Searching for "photon polarization qubits" gives loads of hits. I can't understand how you come to claim that this is not been done. Apr 11 '20 at 11:47
• @NorbertSchuch (sigh) If you read my comment again, you will see I am NOT claiming that photon polarization has not been used to build qubits. I am claiming that coupling photon polarization qubits via 2-qubit gates is very difficult due to the need for non-linear optics effects which have low probabilities. Apr 15 '20 at 14:44