Qubit

The basic unit of a quantum computer: a two-state system that can hold a superposition of 0 and 1.

• A qubit's state is two numbers called amplitudes; measuring it yields 0 or 1 with probabilities set by them, and the superposition is lost.
• n entangled qubits need 2ⁿ amplitudes to describe, so 300 qubits exceed the atoms in the observable universe; algorithms exploit interference among them.
• That doesn't make them faster at everything: the known big wins are narrow, such as simulating molecules and breaking some encryption.
• Any stray interaction with the environment destroys the state ('decoherence'), so error rates matter more than qubit counts.