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What is quantum superposition?
Superposition is the quantum state in which a qubit, before it is measured, holds the states 0 and 1 together with definite amplitudes.
Superposition is often summed up as “the qubit is 0 and 1 at the same time”; more precisely, the qubit’s state is a sum of |0⟩ and |1⟩ weighted by amplitudes. On measurement, the square of each amplitude gives the probability of that result.
The best-known way to put a qubit into an equal superposition is the Hadamard (H) gate: apply H to a qubit starting in |0⟩ and measurement gives 0 or 1 with 50% probability each. This is known as a quantum coin flip.
Superposition alone gives no speed-up. The power of quantum algorithms comes from amplitudes in superposition reinforcing or cancelling each other through interference.
Try it yourself
The lesson Superposition: a mixture of 0 and 1 lets you try this concept in the browser, with no installation.
Related packages
Related concepts
- Qubit · A qubit is the basic unit of information in a quantum computer: measured, it gives 0 or 1, but before measurement it can be in a superposition of both.
- Hadamard gate (H) · The Hadamard gate is a single-qubit quantum gate that turns a qubit in |0⟩ into an equal superposition of 0 and 1.
- Quantum interference · Quantum interference is the way amplitudes add up like waves, raising the probability of some results and cancelling others to zero.