# Qubit A **qubit** is the smallest unit of quantum information: a physical system with two distinguishable states, $|0\rangle$ and $|1\rangle$, that obeys the rules of quantum mechanics rather than classical logic. ## First principles A classical bit is forced to be either 0 or 1. A qubit instead lives in a **superposition** — a weighted combination of both possibilities at once: $ |\psi\rangle = \alpha|0\rangle + \beta|1\rangle, \qquad |\alpha|^2 + |\beta|^2 = 1 $ The numbers $\alpha$ and $\beta$ are complex *amplitudes*. You never observe them directly. When you measure the qubit, it collapses to $|0\rangle$ with probability $|\alpha|^2$ or $|1\rangle$ with probability $|\beta|^2$. The act of looking destroys the superposition. Two ideas give qubits their power: - **Superposition** lets $n$ qubits represent a combination of all $2^n$ basis states simultaneously. - **[[Quantum Entanglement|Entanglement]]** lets qubits share correlations with no classical analogue, so the joint state cannot be described as separate independent parts. > [!intuition] Why this is not just "parallel computing" > A superposition is not a list of answers you can read out. Measurement gives you only one outcome. The art of a quantum algorithm is arranging interference so that *wrong* answers cancel and *right* answers reinforce before you measure. ## What makes a good physical qubit Any two-level system can in principle be a qubit — an atom's energy levels, a photon's polarization, the direction of a spin, a current in a superconducting loop. To be *useful* it must satisfy competing demands (often summarized as the DiVincenzo criteria): it must be well isolated from noise yet controllable on demand, measurable, and connectable to other qubits. Two metrics capture the core tension: how long the fragile quantum state survives ([[Coherence Time]]) and how accurately operations can be applied ([[Gate Fidelity]]). ## Why one qubit is never enough Physical qubits are noisy. To compute reliably, many physical qubits are bundled together to encode one robust [[Physical vs Logical Qubits|logical qubit]] protected by [[Quantum Error Correction]]. ## Related - [[Quantum Entanglement|Entanglement]] - [[Coherence Time]] - [[Gate Fidelity]] - [[Physical vs Logical Qubits]] - [[Quantum Error Correction]]