QuantumCircuit Basics

qc/programming qc/qiskit

QuantumCircuit is Qiskit’s abstract representation of a quantum program — a sequence of gates (and measurements) acting on registers. Every code snippet elsewhere in this vault assumes you can read this layer fluently.

from qiskit import QuantumCircuit
 
qc = QuantumCircuit(2, 2)   # 2 qubits, 2 classical bits
qc.h(0)                     # gate on qubit 0
qc.cx(0, 1)                 # two-qubit gate: control=0, target=1
qc.measure([0, 1], [0, 1])  # qubit i -> classical bit i

Registers

QuantumCircuit(n) creates n qubits with no classical register (fine for statevector work — see Simulators — Statevector vs Shot-Based). QuantumCircuit(n, m) adds m classical bits for storing measurement outcomes. Explicit QuantumRegister/ClassicalRegister objects exist for naming/grouping qubits, but the shorthand above covers most circuits in this vault.

Composing circuits

.compose() stitches one circuit into another — this is how 1D Ising Chain and the Mirror Trick builds the mirror benchmark: mirror.compose(ising, inplace=True) followed by mirror.compose(ising.inverse(), inplace=True). It’s the general tool for building circuits out of reusable sub-circuit pieces rather than writing every gate inline.

Barriers

qc.barrier() is not a gate — it’s an instruction to the transpiler: “don’t reorder or cancel anything across this line.” See 1D Ising Chain and the Mirror Trick for why this matters in practice (without a barrier, the transpiler can silently optimize a mirror circuit’s $U^{†}U$ down to nothing).

Inspecting and drawing

qc.draw()          # text diagram
qc.draw('mpl')      # matplotlib diagram
qc.depth()          # see [[Circuit Depth]]
qc.data             # list of CircuitInstruction — see [[Circuit Introspection Cheat Sheet]]

Official reference: Qiskit documentation.

Related

• The Primitives Family — what actually runs a built circuit
• Parameterized Circuits
• Bell States — the canonical h(0); cx(0,1) pattern in practice
• Circuit Introspection Cheat Sheet
• 1D Ising Chain and the Mirror Trick — .compose() and .barrier() used together at scale

Self-Check

• What’s the difference between QuantumCircuit(3) and QuantumCircuit(3, 3), and when would you need the second form?
• What does qc.barrier() actually do — is it a gate?
• Why does 1D Ising Chain and the Mirror Trick need both .compose() and .barrier() to build its benchmark correctly?