Quantum Bit (Qubit)

A quantum bit, or qubit, is the basic unit of information in a quantum computer. An ordinary bit is always either 0 or 1, but a qubit can exist in a superposition, a weighted combination of both, until it is measured and gives a single answer. Qubits can also be entangled, so that the state of one is linked to others. Together these effects let a quantum computer explore certain problems in ways ordinary machines cannot. Qubits are built from superconducting circuits, trapped ions, photons or neutral atoms, and they are fragile: heat, vibration and stray signals cause errors. Engineers therefore combine many physical qubits into a smaller number of error-corrected "logical" qubits. The number of reliable qubits a machine can run is the figure most often used to judge how close it is to threatening today's public-key cryptography, a moment called Q-Day.