Glossary

• AmplitudeThe number a qubit holds for each possible result. Squaring it gives the chance of that result.
• BitThe smallest unit of ordinary information: a 0 or a 1.
• DecoherenceThe loss of a qubit's blend through contact with its surroundings, which turns it into ordinary randomness.
• Elliptic-curve cryptographyPublic-key cryptography based on a hard problem about points on a curve. It protects most web traffic and Bitcoin.
• EntanglementA link between qubits in which their measurement results are correlated in a way no prearranged plan could produce.
• Grover's algorithmA quantum algorithm, published by Lov Grover in 1996, that searches unsorted possibilities in about the square root of the usual number of tries.
• Harvest now, decrypt laterRecording encrypted data today in order to decrypt it once a capable quantum computer exists.
• InterferenceAmplitudes adding up or cancelling when they combine. Quantum algorithms use it to make wrong answers unlikely.
• Logical qubitA reliable qubit built from many physical qubits using error correction.
• MeasurementReading a qubit. It returns 0 or 1 at random, with chances set by the squared amplitudes, and ends the blend.
• No-cloning theoremThe rule that an unknown quantum state can't be copied.
• PhaseThe part of an amplitude beyond its size. It decides how amplitudes add up or cancel.
• Physical qubitOne actual qubit on a chip or in a trap. Physical qubits make errors, roughly once every few hundred to few thousand operations today.
• Post-quantum cryptographyEncryption that runs on ordinary computers and is designed to resist attacks by quantum computers.
• Q-DayThe day a quantum computer can break the public-key encryption in wide use today, such as RSA-2048 or 256-bit elliptic curves.
• Quantum advantageA quantum computer doing a specific task far faster than any ordinary computer could.
• Quantum error correctionMethods that protect quantum information by spreading it over many qubits and repeatedly checking for errors.
• Quantum gateAn operation that changes the amplitudes of one or more qubits, the quantum version of a logic gate.
• QubitThe basic unit of a quantum computer: it can be 0, 1, or a blend of both until it is measured.
• RSAA public-key encryption method from 1977 whose security relies on how hard it is to factor the product of two large primes.
• Shor's algorithmA quantum algorithm, published by Peter Shor in 1994, that factors large numbers and breaks the encryption most of the internet uses for key exchange and signatures.
• SuperpositionA qubit state that is a blend of 0 and 1, with an amplitude for each.

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