A qubit (quantum bit) is the basic unit of information in a quantum computer. Unlike a classical bit which can only be 0 or 1, a qubit can exist in a superposition of both states simultaneously until it is measured. Physically, qubits can be implemented as the spin of an electron, the polarization of a photon, the energy levels of a superconducting circuit (transmon), or the electronic states of trapped ions. When a qubit is measured, it collapses to a definite 0 or 1 with probabilities determined by the superposition amplitude. A system of n qubits can represent 2ⁿ states simultaneously, which gives quantum computers their potential for exponential parallelism.
Related Terms
Superposition
FundamentalsThe ability of a quantum system to exist in multiple states at the same time.
Bloch Sphere
FundamentalsA geometric representation of all possible states of a single qubit as a point on a unit sphere.
Measurement
FundamentalsThe act of observing a qubit's state, which collapses the superposition to a definite 0 or 1.
T1 / T2 Time
HardwareT1 is the qubit energy relaxation time; T2 is the coherence (dephasing) time. Both limit circuit duration.