What is a Qubit?
A qubit (quantum bit) is the quantum equivalent of the classical bits we've just explored, but with a crucial difference: while a classical bit can only be in one of two states (0 or 1), a qubit can exist in a superposition of both states simultaneously!
When a qubit is measured (similar to the manner in which we "measured" the state of the cat earlier by opening the box), its superposition collapses to either state |0⟩ or |1⟩ (rather than simply 0 and 1). This measurement process is irreversible, which is another key difference from classical systems.
Another important property of qubits is entanglement, where the state of one qubit becomes correlated with the state of another, regardless of the distance separating them. Einstein famously called this "spooky action at a distance."
Visualizing Qubits: The Bloch Sphere
A qubit's state can be visualized as a point on a sphere called the Bloch sphere. The north pole represents |0⟩, the south pole represents |1⟩, and points on the equator represent equal superpositions.
Interactive Bloch Sphere
State: |0⟩
Probability of measuring |0⟩: 100%
Probability of measuring |1⟩: 0%