# Clarification on what we can consider a qubit to be

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## Main Question or Discussion Point

In a 2 level quantum system, should I consider the states

$$|0>$$

and

$$|1|>$$

to be qubits by themselves?

Or is only the SUPERPOSITION of these two states,

$$\alpha |0> + \beta |1>$$

considered to be a qubit?

## Answers and Replies

Related Quantum Physics News on Phys.org
Nevermind, they have to be qubits as well. If we consider our superposition to be a qubit, then we can set $\alpha = 1$ and $\beta = 0$ and that should be an appropriate qubit state.

A. Neumaier
Science Advisor
2019 Award
In a 2 level quantum system, should I consider [...] to be qubits by themselves?
The qubit is the 2-state system, just like a classical bit is a binary variable. The states are not qubits, but the qauntum analogues of the classical values.

Strilanc
Science Advisor
You should consider the pair to form a qubit. On their own they're not very useful.

Another useful way to think about what a qubit is is as an anti-commuting pair of observables, such as the observable $|0\rangle$-vs-$|1\rangle$ paired with the observable $\frac{1}{\sqrt{2}} \left( |0\rangle + |1\rangle \right)$-vs-$\frac{1}{\sqrt{2}} \left( |0\rangle - |1\rangle \right)$.