Quantum glossary

Expectation value

Also: observable, ⟨O⟩

The average result you would get measuring an observable on many copies of the same state.

Variational algorithms minimise an expectation value — usually an energy. On hardware it is estimated from finitely many shots, so it always comes with statistical error bars.

⟨O⟩ = ⟨ψ|O|ψ⟩