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What is the no-cloning theorem?
The no-cloning theorem states that it is impossible to make an exact copy of an unknown quantum state; it is a fundamental constraint for quantum cryptography and error correction.
Shown in 1982 by Wootters, Zurek and Dieks, the theorem follows from the linearity of quantum mechanics: an operation that copies |0⟩ and |1⟩ does not copy a state in superposition but produces an entangled state instead. So a qubit’s content cannot be duplicated like a classical bit.
Its consequences cut both ways. Quantum key distribution protocols such as BB84 rely on an eavesdropper being unable to copy and keep the qubits. On the other hand, error correction cannot work by copying; information is spread over many qubits through entanglement. Quantum teleportation does not copy a state either, it moves it: the state of the original qubit is destroyed.
Related packages
Related concepts
- Quantum key distribution (QKD, BB84) · Quantum key distribution lets two parties share a secret encryption key while detecting eavesdropping attempts thanks to the nature of quantum measurement.
- Quantum teleportation · Quantum teleportation is a protocol that transfers the unknown state of a qubit to another qubit using a shared entangled pair and two classical bits.
- Quantum error correction · Quantum error correction spreads one logical qubit over many physical qubits so that errors caused by noise can be found and fixed without measuring the qubits directly.