Quantum Information

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14 domain-specific abstractions whose origin domain is Quantum Information.

• Counterfactual quantum computation — A quantum protocol that infers a computational outcome from an interference branch in which the outcome-producing device did not run.
• Diamond norm — The completely bounded trace norm of a linear map on matrices, maximizing output trace norm after arbitrary ancillary extension and operationally measuring one-use distinguishability of quantum channels.
• Fidelity of quantum states — A bounded similarity measure for two quantum states based on the trace norm of their density-operator square roots, equaling transition probability for pure states under the squared convention.
• Incompatibility of quantum measurements — The absence of a single joint quantum measurement whose marginals reproduce a given set of observables, generalizing noncommutativity to general measurements.
• KLM protocol — A universal linear-optical quantum-computing scheme using single photons, passive optics, photodetection, ancillas, teleportation and error correction.
• No-cloning theorem — The quantum-information theorem that no physical operation can produce a perfect independent copy of every arbitrary unknown quantum state while retaining the original.
• Non-local quantum computation — A distributed quantum-computation model in which separated parties use pre-shared entanglement and a single simultaneous communication round to implement a joint operation on distributed inputs.
• Peres–Horodecki criterion — Test bipartite quantum-state separability by partially transposing one subsystem and checking positivity, a necessary condition in all dimensions and a sufficient one only for 2×2 and 2×3 systems.
• Quantum cloning — The hypothetical transformation that would copy an arbitrary unknown quantum state perfectly while leaving the original unchanged, forbidden universally by linear quantum mechanics.
• Quantum simulator — A controllable quantum system engineered or programmed so its Hamiltonian and observables emulate another quantum system whose behavior is difficult to compute or access directly.
• Reduction criterion — Certify a necessary condition for bipartite separability by requiring both reduced-state operators tensored with identity minus the joint density operator to remain positive semidefinite.
• Reflected entropy — A mixed-state correlation measure obtained by canonically purifying a bipartite density operator and taking entanglement entropy across the reflected subsystem split.
• State-merging — A quantum-information protocol that transfers one share of a joint state to a receiver who already holds correlated side information, at entanglement cost given by conditional quantum entropy.
• Superdense coding — A quantum communication protocol transmitting two classical bits by sending one qubit when sender and receiver share an entangled pair.