Your First Circuit

The first Qiskit object you need is QuantumCircuit.

from qiskit import QuantumCircuit

qc = QuantumCircuit(1)
qc.h(0)
print(qc)

This says:

• create a one-qubit circuit
• apply a Hadamard gate to qubit 0

Starting from |0>, the Hadamard creates the state

\[ \frac{|0\rangle + |1\rangle}{\sqrt{2}} \]

That state is often called the plus state.

A circuit is a recipe, not a state

This distinction is worth learning immediately.

• the circuit is the recipe
• the statevector is the state produced by the recipe
• measurement turns the state into classical data

Beginners often blend these together and then get confused about what a gate is changing.

Look at the state before measuring

from qiskit import QuantumCircuit
from qiskit.quantum_info import Statevector

qc = QuantumCircuit(1)
qc.h(0)

state = Statevector.from_instruction(qc)
print(state)
print(state.draw("text"))

This is the most important beginner debugging move in Qiskit. Before asking what you will observe, ask what state you built.

Two tiny examples that already matter

Preparing |1>:

qc = QuantumCircuit(1)
qc.x(0)
print(Statevector.from_instruction(qc))

Preparing the plus state:

qc = QuantumCircuit(1)
qc.h(0)
print(Statevector.from_instruction(qc))

The point is not that these circuits are hard. The point is that you are starting to connect gate actions to states.

Learn to predict before you run

Try this:

qc = QuantumCircuit(1)
qc.x(0)
qc.h(0)
print(Statevector.from_instruction(qc))

Before running it, ask:

• what state does x create?
• what does h do to that state?

That habit will scale all the way to Grover, QFT, and QML circuits.

Checkpoint Exercises

1. Prepare |1>.
2. Prepare the plus state.
3. Apply x then h and inspect the resulting state.
4. Build two different circuits that end in the same state.

Try These On QCoder

• QPC001 A1, Generate State |1>
• QPC001 A2, Generate Plus state
• QPC002 B1, Generate State e^(i theta)|0>