Words

35 words, each in a sentence or two, with the chapter that uses it.

Amplitude: 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. — A coin on the table, a coin in the air
Annealer: 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. — What it is not
Bell test: 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. — Two coins that always match
Bit: One yes or no. The unit of ordinary computing. A coin on the table. — A coin on the table, a coin in the air
Bloch sphere: 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. — Where the arrow points
Circuit: A quantum program drawn as wires (one per qubit) and boxes (gates), read left to right. — The switches
CNOT: The two-qubit gate that flips the second qubit if the first is 1. With an H gate it makes an entangled pair. — The switches
Coherence time: 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. — Why it falls apart
Decoherence: The wearing-down. Anything that touches a qubit, heat, stray light, a vibration, counts as a look, and the superposition leaks away. — Why it falls apart
Entanglement: 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. — Two coins that always match
Error correction: 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. — Fixing errors without looking
Fault tolerance: 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. — Fixing errors without looking
Fidelity: 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. — Why it falls apart
Gate: One step of a quantum program: a turn applied to one or two qubits. — The switches
Grover's algorithm: Finding a marked item in an unsorted list in about the square root of the number of looks a plain computer needs. — Finding a needle in fewer tries
Hadamard gate (H): The gate that tips a 0 or 1 into an even mix. It puts the coin in the air. — Where the arrow points
Interference: Waves adding where they agree and cancelling where they disagree. The engine of every quantum speed-up. — Ripples that cancel
Logical qubit: A reliable qubit made out of many unreliable physical ones by error correction. The number that counts. — Fixing errors without looking
Measurement: Reading a qubit. It lands on 0 or 1 with the odds its state gives, and the superposition is gone. — A coin on the table, a coin in the air
NISQ: 'Noisy intermediate-scale quantum': the current era of machines with tens to hundreds of qubits and no error correction. John Preskill's term, 2018. — Why it falls apart
No-cloning: An unknown quantum state cannot be copied. Proved in 1982. It rules out backups. — Fixing errors without looking
Phase: Which way round the equator a qubit's arrow points. Measurement cannot see it; interference is made of it. — Where the arrow points
Physical qubit: One actual qubit in the hardware, noisy. Hundreds to thousands of them make one logical qubit. — Fixing errors without looking
Post-quantum cryptography: 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. — Breaking the locks
qLDPC code: 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. — Fixing errors without looking
Quantum advantage: 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. — What it is not
Qubit: A quantum bit: a two-level system that can be in a mix of 0 and 1 until measured. A coin in the air. — A coin on the table, a coin in the air
Rydberg state: An atom with one electron kicked far out, so the atom swells and shoves its neighbours. Neutral-atom machines use it to entangle. — Machines
Shor's algorithm: 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. — Breaking the locks
Superconducting qubit: 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. — Machines
Superposition: Being in a mix of 0 and 1 at once, with a lean. Not 'both' and not 'unknown'; a third thing. — A coin on the table, a coin in the air
Surface code: The error-correcting code most chip-based machines plan on: a checkerboard where checker qubits keep asking their four neighbours whether they still agree. — Fixing errors without looking
Threshold: 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. — Fixing errors without looking
Transmon: The 2007 design of superconducting qubit that most chips use. — Machines
Trapped ion: 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. — Machines