Quantum superposition describes a state where a particle combines multiple possible basis states before measurement. An electron, for example, can be described as spinning up and down at the same time. Measurement ends the superposition: the particle collapses into one observed state. section 14.1.1
The idea becomes useful in a few linked steps:
- Prepare a particle in a superposed state using a suitable technique. section 14.1.1
- Keep the state unmeasured while it remains a combination of possible outcomes. section 14.1.1
- Measure the particle, producing one possible result; for the illustrated two-state case, each result has a 50% chance. section 14.1.1
- Use qubits and quantum gates to create, transform, and eventually measure superposed states as classical 0s and 1s. section 14.1.2
From a photon to a random number
A quantum random number generator repeatedly places particles such as photons into superposition and measures them. The unpredictable measurement result supplies truly random numbers. Quantum computers use the same general phenomenon differently: they process qubits before measurement, then interpret the measured 0s and 1s with a classical computer. section 14.1.1 section 14.1.2
