{
  "_about": "Words, each defined in one or two plain sentences. 'see' points at the chapter that uses it.",
  "terms": [
    {"term": "Amplitude", "plain": "The number underneath a quantum probability. It can be positive, negative or point in any direction; the chance of an outcome is its size squared. Amplitudes can cancel each other; chances cannot.", "see": "bit"},
    {"term": "Annealer", "plain": "A quantum machine that finds a low-energy arrangement instead of running a program step by step. D-Wave builds them. It cannot run Shor's or Grover's methods.", "see": "myths"},
    {"term": "Bell test", "plain": "An experiment that tells entangled particles apart from particles that secretly decided in advance. Every one run since 1972 has come out on the quantum side. 2022 Nobel Prize.", "see": "entangle"},
    {"term": "Bit", "plain": "One yes or no. The unit of ordinary computing. A coin on the table.", "see": "bit"},
    {"term": "Bloch sphere", "plain": "The picture of one qubit as an arrow from the centre of a ball: up is 0, down is 1, anywhere else is a mix.", "see": "sphere"},
    {"term": "Circuit", "plain": "A quantum program drawn as wires (one per qubit) and boxes (gates), read left to right.", "see": "gates"},
    {"term": "CNOT", "plain": "The two-qubit gate that flips the second qubit if the first is 1. With an H gate it makes an entangled pair.", "see": "gates"},
    {"term": "Coherence time", "plain": "How long a qubit stays in superposition before the outside world wears it down. Written T1 and T2. Millionths of a second for chips, seconds for ions.", "see": "noise"},
    {"term": "Decoherence", "plain": "The wearing-down. Anything that touches a qubit, heat, stray light, a vibration, counts as a look, and the superposition leaks away.", "see": "noise"},
    {"term": "Entanglement", "plain": "Two or more qubits sharing one state, so measuring one fixes what the others will say, at any distance, with neither having decided in advance.", "see": "entangle"},
    {"term": "Error correction", "plain": "Storing one qubit's worth of information across many qubits and checking the relationships between them, without reading the information, so errors can be found and undone.", "see": "correct"},
    {"term": "Fault tolerance", "plain": "Error correction that works even though the correcting itself makes errors. The goal; a machine that has it can run as long as you like.", "see": "correct"},
    {"term": "Fidelity", "plain": "How close a gate or a state is to the intended one, written as a percentage. 99.9% means one error in a thousand. Error-correction plans want 99.99% or better.", "see": "noise"},
    {"term": "Gate", "plain": "One step of a quantum program: a turn applied to one or two qubits.", "see": "gates"},
    {"term": "Grover's algorithm", "plain": "Finding a marked item in an unsorted list in about the square root of the number of looks a plain computer needs.", "see": "grover"},
    {"term": "Hadamard gate (H)", "plain": "The gate that tips a 0 or 1 into an even mix. It puts the coin in the air.", "see": "sphere"},
    {"term": "Interference", "plain": "Waves adding where they agree and cancelling where they disagree. The engine of every quantum speed-up.", "see": "waves"},
    {"term": "Logical qubit", "plain": "A reliable qubit made out of many unreliable physical ones by error correction. The number that counts.", "see": "correct"},
    {"term": "Measurement", "plain": "Reading a qubit. It lands on 0 or 1 with the odds its state gives, and the superposition is gone.", "see": "bit"},
    {"term": "NISQ", "plain": "'Noisy intermediate-scale quantum': the current era of machines with tens to hundreds of qubits and no error correction. John Preskill's term, 2018.", "see": "noise"},
    {"term": "No-cloning", "plain": "An unknown quantum state cannot be copied. Proved in 1982. It rules out backups.", "see": "correct"},
    {"term": "Phase", "plain": "Which way round the equator a qubit's arrow points. Measurement cannot see it; interference is made of it.", "see": "sphere"},
    {"term": "Physical qubit", "plain": "One actual qubit in the hardware, noisy. Hundreds to thousands of them make one logical qubit.", "see": "correct"},
    {"term": "Post-quantum cryptography", "plain": "New locks for the internet that do not rest on factoring, so Shor's algorithm does not open them. NIST published the first standards in August 2024.", "see": "shor"},
    {"term": "qLDPC code", "plain": "A newer family of error-correcting codes that needs far fewer physical qubits per logical one than the surface code, in exchange for connections between qubits that are not next to each other. IBM's 2029 plan rests on one.", "see": "correct"},
    {"term": "Quantum advantage", "plain": "A quantum computer doing a task faster or better than the best ordinary method. 'Supremacy' was the earlier word. Each claim has been followed by a better ordinary method, so the bar moves.", "see": "myths"},
    {"term": "Qubit", "plain": "A quantum bit: a two-level system that can be in a mix of 0 and 1 until measured. A coin in the air.", "see": "bit"},
    {"term": "Rydberg state", "plain": "An atom with one electron kicked far out, so the atom swells and shoves its neighbours. Neutral-atom machines use it to entangle.", "see": "machines"},
    {"term": "Shor's algorithm", "plain": "Finding the prime factors of a big number by finding a rhythm with a quantum Fourier transform. It would break RSA. No real key has been broken.", "see": "shor"},
    {"term": "Superconducting qubit", "plain": "A qubit printed on a chip as a resistance-free circuit, cooled to a hundredth of a degree above absolute zero. Google's and IBM's kind.", "see": "machines"},
    {"term": "Superposition", "plain": "Being in a mix of 0 and 1 at once, with a lean. Not 'both' and not 'unknown'; a third thing.", "see": "bit"},
    {"term": "Surface code", "plain": "The error-correcting code most chip-based machines plan on: a checkerboard where checker qubits keep asking their four neighbours whether they still agree.", "see": "correct"},
    {"term": "Threshold", "plain": "The error rate, about one in a hundred, below which a bigger error-correcting code gets better instead of worse. Crossed in hardware in December 2024.", "see": "correct"},
    {"term": "Transmon", "plain": "The 2007 design of superconducting qubit that most chips use.", "see": "machines"},
    {"term": "Trapped ion", "plain": "A single charged atom held in mid-air by electric fields and worked with lasers. The cleanest kind of qubit. Quantinuum's and IonQ's kind.", "see": "machines"}
  ]
}