Multiple qubits
n qubits are described by one amplitude per joint outcome — 2ⁿ amplitudes in total — not by n separate pairs. QPU137 displays bitstrings with q0 as the rightmost (least significant) bit, and while some joint states factor into independent per-qubit states, entangled ones cannot.
One qubit needs two amplitudes. Two qubits need one amplitude per joint outcome, and there are four of those: 00, 01, 10 and 11. Each such labelled outcome is called a basis state, and the full list of amplitudes is called the state vector. Three qubits have 8 basis states, and n qubits have 2ⁿ — the list doubles with every qubit you add.
The measurement rule is unchanged: the probability of reading a particular bitstring is the squared magnitude of its amplitude, and the squares must sum to 1. Worked example: suppose the amplitudes are 0.6 on 00, 0 on 01, 0 on 10, and 0.8 on 11. Squares: 0.36, 0, 0, 0.64 — check 0.36 + 0.64 = 1. So 36% of shots read 00, 64% read 11, and 01 and 10 never appear.
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