Quantum computing is a model of computation that stores and manipulates information in quantum states rather than classical bits. A quantum bit, or qubit, can exist in a superposition of the states 0 and 1, and multiple qubits can become entangled so that their combined state cannot be described independently of one another.
Certain problems — such as factoring large integers or simulating quantum systems — admit quantum algorithms believed to require far fewer computational steps than the best known classical algorithms, though for most everyday tasks classical computers remain more practical. Building a quantum computer that outperforms classical machines on a useful task, while controlling the errors that quantum states are prone to, is the central engineering challenge of the field.