Theory

10 chapters in the order to read them. 9 of them have something to press.

1. A coin on the table, a coin in the air
   A bit is a coin lying on the table: heads or tails. A qubit is a coin in the air.
2. Where the arrow points
   One qubit is an arrow that can point anywhere on a ball. Up is 0, down is 1, and the sideways part is what makes it quantum.
3. Ripples that cancel
   Drop two stones in a pond. Where crest meets crest the water leaps; where crest meets trough it goes flat. A quantum computer is arranged so wrong answers meet trough and go flat.
4. Two coins that always match
   Two entangled qubits are two coins in the air that, when you catch them, always land the same way, no matter how far apart, and neither one had picked before you looked.
5. The switches
   A quantum program is a short list of turns applied to qubits, drawn as wires and boxes. Everything the machine does is a few kinds of turn, over and over.
6. Finding a needle in fewer tries
   Grover's method finds one marked item in a million by looking about a thousand times. An ordinary computer looks half a million. The trick is amplifying the right answer's wave a little with every pass.
7. Breaking the locks
   The locks on the internet rest on one fact: multiplying two big primes is easy, and undoing it is not. Shor showed a quantum computer can undo it, by finding a rhythm.
8. Why it falls apart
   A qubit in the air is a coin balanced on its edge in a barn full of drafts. Anything that touches it, heat, a stray radio wave, a cosmic ray, counts as a look, and the coin falls.
9. Fixing errors without looking
   You cannot copy a qubit and you cannot look at it. So you spread it across many qubits and ask the neighbours whether they still agree, without asking what they say.
10. What it is not
    It does not try every answer at once, it is not faster at everything, and it is not about to replace your laptop.

Nothing here needs any maths past squaring a number. Where a technical word appears, it is defined the first time and again at Words.