qubit

A classical bit is a tiny switch: it is either 0 or 1, and that is the whole story. A qubit is the quantum version of that switch, and it has a richer kind of in-between. Before you measure it, a qubit can be in a superposition of 0 and 1 at once. It helps to picture not a switch but a dial: the qubit can point in many directions, and the direction encodes how much of '0' and how much of '1' it carries. A qubit is the fundamental unit of quantum information, and any controllable two-level quantum system can serve as one: the spin of an electron, two energy levels of an atom, the polarization of a photon, or a superconducting circuit.

Here is the part that everyone gets wrong, so read it slowly. Superposition does not mean the qubit secretly holds both answers and a quantum computer 'tries all of them at once.' The amounts of 0 and 1 are described by two numbers called amplitudes, written |psi> = alpha|0> + beta|1>. When you measure, you do not see the amplitudes; you get a single plain 0 or 1, with probability equal to the amplitude squared, and the superposition collapses to whatever you saw. So one qubit, read out, gives you exactly one bit, just like its classical cousin. The power of quantum computing comes not from holding many answers but from arranging the amplitudes of many qubits so that wrong answers cancel out and the right ones add up before you measure.

Real qubits are also delicate. Their amplitudes drift and leak into the environment in a process called decoherence, measured by times labeled T1 and T2, which is why today's machines are small and noisy and need error correction to scale. Holding 'a bit of 0 and a bit of 1' is easy to say and genuinely hard to keep.