A quantum circuit represents a computation as an ordered sequence of gates and measurements applied to a fixed set of qubits. In Qiskit, QuantumCircuit records those operations and can include classical bits for measurement results. section 2.1.2
Constructing a useful circuit requires:
- Declaring the required qubits and classical bits before appending operations. section 2.1.2
- Adding gates by method shortcuts or append, while preserving qubit order and identifying each target explicitly. section 2.1.2 section 6.5.2
- Preparing complete executable circuits when measurements, simulation, or state analysis are required. section 4.5.2
- Testing behavior against classical calculations or expected basis-state results. section 4.5.2 section 6.4.2
Preparing and composing circuits
A single-qubit state can be prepared with an Ry rotation whose angle is derived from the target coefficients. Larger circuits compose reusable operations such as controlled-X, XOR, AND, and OR, assigning input, auxiliary, and output qubits in a documented order. section 2.1.2 section 6.5.2
Listing 2.1 Qiskit code to prepare a single-qubit state
from math import atan2 from qiskit import QuantumCircuit alpha, beta = 0.6, 0.8 circ = QuantumCircuit(1) #1 theta = 2 * atan2(beta, alpha) #2 circ.ry(theta, 0) #3
Inspect a minimal Qiskit circuit that derives a rotation angle and prepares a one-qubit state. section 2.1.2
