QUBIT

Qubit is the basic unit of quantum information. It is a two-level quantum system that can exist in a superposition of the classical states 0 and 1. A qubit state is described by a normalized vector a|0> + b|1>, where a and b are complex amplitudes. The probabilities of measuring 0 or 1 are |a|^2 and |b|^2, and the state can be represented on the Bloch sphere, with pure states corresponding to unit vectors.

Qubits are initialized to a known state, commonly |0>, and then manipulated with quantum gates. Quantum gates implement unitary transformations, changing the amplitudes and relative phase. A universal set of gates typically includes arbitrary single-qubit rotations and a two-qubit entangling gate such as the controlled-NOT (CNOT).

Measurement collapses a qubit to one of the basis states, with probabilities determined by the state amplitudes. Because of superposition and entanglement, measurements on multiple qubits reveal correlations that have no classical counterpart. Entanglement is a key resource in many quantum algorithms.

Physical realizations vary. Common platforms include superconducting circuits, trapped ions, semiconductor quantum dots, and photonic qubits. Each platform faces challenges such as decoherence, gate errors, and readout imperfections. Coherence time and gate speed are key figures of merit; fault-tolerant operation requires error rates below threshold and scalable error correction.

Qubits enable quantum computing, quantum simulation, and certain communication protocols. While quantum computers can efficiently solve some problems, building large, reliable systems remains an active area of research, with ongoing work in qubit quality, interconnects, error correction, and architectures.