Superposition
Superposition
Overview
- In quantum mechanics, superposition is the ability of a quantum system to be in a combination of several states at once. A qubit can be partly 0 and partly 1; an electron can be spread over several positions.
Key Principles
- The state is a weighted sum of possible outcomes. The weights, called amplitudes, are complex numbers, and the square of each amplitude's magnitude gives the probability of that outcome when the system is measured.
- Measurement returns one outcome. After it, the system is found in the state that was measured.
- Amplitudes can add or cancel, which is how superposed states interfere. The interference pattern in the double-slit experiment comes from this.
Examples
- Quantum algorithms put qubits in superposition and then use interference so that the amplitudes of wrong answers cancel and the amplitudes of right answers add up.
- Person/Erwin Schrödinger/Cat — Person/Erwin Schrödinger's thought experiment carrying an atom's superposition up to a cat that is both dead and alive.
Misconceptions
- A superposition is one definite quantum state. It differs from a system that is secretly in one state that we just don't know yet, and interference experiments can tell the two apart.
Sources