What a quantum computer computes
By the end: Describe the circuit model and where a quantum advantage could come from.
Learning outcomes
- Distinguish quantum information from classical information
- Predict measurement outcomes from a state
- Explain what superposition does and does not mean
Information has a physical carrier
A classical computer stores information in physical systems that are engineered to behave like bits. A quantum computer stores and transforms quantum states. In the circuit model, a computation prepares qubits, applies gates, and measures selected outputs.
A quantum processor is not a faster version of every laptop task. Its value depends on matching a problem to an algorithm whose structure can be exploited by quantum operations, while accounting for data loading, circuit depth, error rates, and the cost of reading results.
The computation is a sequence
A circuit is a mathematical recipe as well as a hardware instruction. Before measurement, an ideal closed-system circuit evolves linearly and reversibly. Measurement produces ordinary classical data, so the final result is usually a distribution of answers rather than a complete list of all amplitudes.
This course keeps three layers separate: the mathematical model, the algorithm, and the physical device. A simulator can teach the first two without claiming to be quantum hardware.
A one-qubit computation
- Prepare the qubit in |0⟩.
- Apply a Hadamard gate, H.
- Measure in the computational basis.
ResultAn ideal measurement returns 0 or 1 with equal probability. Repeating the circuit estimates that distribution; one run does not reveal both possibilities.
Run your first circuit
Use the local lab to prepare a balanced state and see the difference between probabilities and finite samples.
- Open the lab preset for a balanced superposition.
- Run 20 shots, then 1,000 shots.
- Compare the observed frequency with the 50% prediction.