# Which other physical quantities, other than the polarization of classical light be used as a qubit?

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

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