A qubit (quantum bit) is the fundamental building block of quantum computing. While a classical bit can only be 0 or 1, a qubit can exist in a superposition of both states at once, described by complex probability amplitudes. When multiple qubits are entangled, the number of states they can represent grows exponentially — a system of n qubits can encode 2^n states simultaneously, which is the source of quantum computing's potential power advantage over classical machines.
Physical qubits are implemented using a variety of technologies, each with distinct advantages and limitations. Superconducting circuits (used by IBM, Google, and Rigetti) store quantum information in oscillating currents in cooled microchips. Trapped ions (IonQ, Quantinuum) use individual charged atoms held in electromagnetic fields. Photonic approaches (PsiQuantum, Xanadu) encode qubits in properties of light particles. Neutral atoms (QuEra, Pasqal) and topological qubits (Microsoft) represent newer approaches with potentially better scaling properties.
The central challenge in qubit engineering is maintaining quantum coherence — keeping qubits in their delicate quantum states long enough to perform useful computations. Environmental noise causes decoherence, introducing errors that accumulate as computations grow longer. Current state-of-the-art systems from IBM and Google operate with hundreds of physical qubits, but millions of high-quality qubits will likely be needed for the most impactful applications like breaking encryption or simulating molecular chemistry. For deeper coverage, see DeepTechIntel's quantum computing section.