# Quantum Circuits

**Quantum circuits** are the fundamental way to describe quantum computations in Qiskit—an ordered sequence of operations applied to qubits. A circuit specifies which gates to apply, in what order, on which qubits, and where to measure. Think of it like assembly code: abstract and portable, executable on any backend that supports the required gate set.

In Qiskit, a `QuantumCircuit` object holds qubits (quantum registers) and classical bits (classical registers). Qubits hold quantum state; classical bits hold measurement results. Gates modify qubit state; measurements collapse qubits and write results to classical bits.

```python
from qiskit import QuantumCircuit

# Create a 3-qubit, 3-bit circuit
qc = QuantumCircuit(3, 3)

# Apply gates
qc.h(0)           # Hadamard on qubit 0
qc.cx(0, 1)       # CNOT: qubit 0 controls qubit 1
qc.rz(0.5, 2)     # RZ rotation on qubit 2

# Measure all qubits
qc.measure([0, 1, 2], [0, 1, 2])

# Draw the circuit
print(qc.draw())
```

Circuits are abstract—they don't execute until you send them to a backend (simulator or hardware). This abstraction lets you design once and run on different backends. Before execution, [[qiskit-transpilation|transpilation]] adapts your circuit to the backend's native gates and qubit topology.

## Circuit Construction

Gates can be chained, and Qiskit provides hundreds: single-qubit gates (H, X, Y, Z, S, T, RX, RY, RZ), two-qubit gates (CNOT, CZ, SWAP), and multi-qubit gates (Toffoli, controlled operations). Use the circuit's methods to add gates: `qc.h(0)`, `qc.cx(0, 1)`, etc.

You can also inspect and manipulate circuits: get the number of qubits/bits, decompose gates, remove or insert operations, and compose smaller circuits together.

```python
# Get circuit info
print(qc.num_qubits)      # 3
print(qc.num_clbits)      # 3
print(qc.depth())         # Circuit depth (longest qubit path)

# Compose two circuits
qc2 = QuantumCircuit(3, 3)
qc2.h([0, 1, 2])
combined = qc.compose(qc2)
```

## Parameterized Circuits

[[qiskit-parameterized-circuits|Parameterized circuits]] replace fixed rotation angles with parameters, useful for [[qiskit-variational-algorithms|variational algorithms]]. Define parameters once, then bind them to different values for each evaluation without rebuilding the circuit.