- Prepare 0
- H: equal superposition
- H again: return to 0
background
Amplitudes can reinforce or cancel
Probabilities are never negative, but amplitudes can have signs and phases. Quantum operations can combine amplitudes so that contributions reinforce for some outcomes and cancel for others. The squared magnitude of the resulting amplitude determines its probability.
Sources: IBM Quantum Learning
background
Two ideal Hadamard gates
Start a qubit in 0. One Hadamard gate creates equal measurement probabilities for 0 and 1. Apply the same gate again without measuring in between: the ideal final result is 0 with certainty. A coin-toss analogy misses this cancellation. Real hardware introduces errors.
Sources: IBM Quantum Learning
Check your understanding
What changes if you measure between the two gates?
Reveal the answer
The intermediate measurement removes the superposition in that basis. The second gate then gives equal outcome probabilities rather than the original guaranteed 0 in this ideal example.
Sources and review scope
Source-linked. Editorial review: .