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This article mentions that polarization of classical light can be used as a qubit. link

Are there other physical quantities in the classical world which can be act as qubit.

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Any two level quantum system can be used as a qubit, examples include:

  • A system with spin 1/2. Like an electron. The projections of spin $|\uparrow\rangle$ and $|\downarrow\rangle$ can be used as $|0\rangle$ and $|1\rangle$
  • A system with two configurations. For example a quantum dot that can be either empty or filled with 1 electron. If the system is small enough the Coulomb repulsion from that electron is high and no additional electron can enter the dot.
  • A system like an atom, molecule (or any other system like a superconductive transmon) where there is a ground state and a first excited states that can be controlled efficiently. The ground state is the $|0\rangle$ state and the $|1\rangle$ the first excited state. If there are other levels that play a role, hopefully they would be very energetic and the effect of these levels will be negligible.

These are some examples, but any qubit would probably fall on any one of these examples (spin projection/polarization, configuration/occupation, energy states).

Here is also response to a similar question that goes into more specific details https://physics.stackexchange.com/a/298537/168640 and a review of qubit implementations from quantum daily A Detailed Review of Qubit Implementations for Quantum Computing

Additional response:

I might have missed the point of the question. If this is about classical properties used as quantum logic gates, then this would be out of my scope. It is a not so popular idea and I would have to read what that really means.

Looking at the citing articles: there is a clear similar candidate, spin waves. See for example Khitun and Wang "Non-volatile magnonic logic circuits engineering." J.App.Phys. 110.3 (2011). The same way that several photons produce a classical light wave. The magnons (quanta of magnetization) produce a macroscopic magnetization wave due to the precessions of spins (or equivalently the magnetic moments) in the material. I would be an obvious analogue but I do not know what are the drawbacks with respect to light.

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  • $\begingroup$ I agree with your points, but I am specifically looking for classical systems where qubit can be implemented, just like in the shared paper in the question. There they use classical wave polarization as qubit, and show there is no need for single photons or use of quantum light. $\endgroup$ Jul 31 at 9:38
  • $\begingroup$ @ChetanWaghela I edited my answer but probably needs more work. Maybe I would come back to this later $\endgroup$
    – Mauricio
    Jul 31 at 9:53
  • $\begingroup$ Thanks for pointing those papers out. I will also try to search add a few here. This article uses focal lenses to simulate a quantum computer. aapt.scitation.org/doi/10.1119/1.5065506 $\endgroup$ Aug 1 at 6:42
  • $\begingroup$ @ChetanWaghela you cited the last paper twice. Also all of those are implementations using light. $\endgroup$
    – Mauricio
    Aug 1 at 10:41
  • $\begingroup$ arxiv.org/abs/1510.03365 $\endgroup$ Aug 1 at 13:38

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