In Qiskit, apply Pauli X or Z to a chosen qubit with qc.x(q) or qc.z(q). X flips computational-basis values; Z changes the phase of the |1⟩ component. Both are single-qubit instructions that can be added in sequence or alongside gates on other qubits.
Apply X or Z in Qiskit
Create a QuantumCircuit with enough qubits, then call the method for the qubit you want to affect:
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from qiskit import QuantumCircuit
qc = QuantumCircuit(1)
qc.x(0) # apply Pauli X to qubit 0
qc.z(0) # then apply Pauli Z to qubit 0
print(qc.draw())The official IBM Quantum lesson on bits, gates, and circuits demonstrates adding X to a single-qubit circuit; the XGate and ZGate API references document the corresponding gate methods. The call order is circuit order: in this example, X is applied first, followed by Z.
Use a specific qubit
For a two-qubit circuit, pass the index of the target qubit to each method:
qc = QuantumCircuit(2)
qc.x(0) # X acts on qubit 0
qc.z(1) # Z acts on qubit 1Each instruction acts on the selected qubit and leaves the other subsystem unchanged. When reading a drawn circuit or displayed bitstring, pay attention to the framework’s qubit-indexing and display conventions.
What X and Z do to a qubit
A qubit in a general pure state can be written as α|0⟩ + β|1⟩, where α and β are amplitudes. Applying a gate transforms those amplitudes as follows:
- Pauli X:
α|0⟩ + β|1⟩ → α|1⟩ + β|0⟩. - Pauli Z:
α|0⟩ + β|1⟩ → α|0⟩ − β|1⟩.
The matrix forms and computational-basis effects make the distinction clear:
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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minute| Gate | Matrix | Effect on basis states | Common description | Qiskit method |
|---|---|---|---|---|
| Pauli X | [[0, 1], [1, 0]] |
|0⟩ → |1⟩; |1⟩ → |0⟩ |
Bit flip | qc.x(q) |
| Pauli Z | [[1, 0], [0, −1]] |
|0⟩ → |0⟩; |1⟩ → −|1⟩ |
Phase flip | qc.z(q) |
On the all-zero starting state, X on a single qubit produces |1⟩. Z leaves |0⟩ unchanged, so its effect is not visible on that state alone; its sign change matters when the |0⟩ and |1⟩ components are in superposition.
Check the circuit’s state
In the IBM Quantum learning workflow, the state can be inspected with Qiskit’s Statevector:
from qiskit.quantum_info import Statevector
state = Statevector(qc)
print(state)This gives the state implied by the circuit instructions. To see measurement outcomes, a measurement operation and an execution or simulator workflow are needed; a statevector inspection by itself is not a measurement.
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Does the order of X and Z matter?
Yes. On the same qubit, the operators anticommute: XZ = −ZX. Their resulting statevectors therefore differ by a minus sign if their order is reversed. For an isolated state, that overall sign is a global phase and does not change measurement probabilities. But operator order must not be treated as interchangeable in controlled operations or larger constructions, where phase relationships can affect the result.
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Pauli gates versus rotation gates
Qiskit’s Pauli X and Z instructions are not exactly the same unitary matrices as π rotations around the corresponding axes: RX(π) = −iX and RZ(π) = −iZ. The difference is a global phase for an isolated state, so it does not change its measurement probabilities. Keep the factor when comparing exact unitary operators or composing gates in a larger construction.
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