Measure qubits

To get information about a qubit's state, you can measure it onto a classical bit. In Qiskit, measurements are performed in the computational basis, that is, the single-qubit Pauli-$Z$ basis. Therefore, a measurement yields 0 or 1, depending on the overlap with the Pauli-$Z$ eigenstates $|0\rangle$ and $|1\rangle$:

Apply a measurement to a circuit

There are several ways to apply measurements to a circuit:

QuantumCircuit.measure method

Use the measure method to measure a QuantumCircuit.

Examples:

from qiskit import QuantumCircuit
 
qc = QuantumCircuit(5, 5)
qc.x(0)
qc.x(1)
qc.x(4)
qc.measure(range(5), range(5)) #  Measures all qubits into the corresponding clbit.

from qiskit import QuantumCircuit
 
qc = QuantumCircuit(3, 1)
qc.x([0, 2])
qc.measure(1, 0)    # Measure qubit 1 into the classical bit 0.

Measure class

The Qiskit Measure class measures the specified qubits.

from qiskit.circuit import Measure
 
qc = QuantumCircuit(3, 1)
qc.x([0,1])
qc.append(Measure(), [0], [0])  # measure qubit 0 into clbit 0

QuantumCircuit.measure_all method

To measure all qubits into the corresponding classical bits, use the measure_all method. By default, this method adds new classical bits in a ClassicalRegister to store these measurements.

from qiskit import QuantumCircuit
 
qc = QuantumCircuit(3, 1)
qc.x([0, 2])
qc.measure_all() # Measure all qubits.

QuantumCircuit.measure_active method

To measure all qubits that are not idle, use the measure_active method. This method creates a new ClassicalRegister with a size equal to the number of non-idle qubits being measured.

from qiskit import QuantumCircuit
 
qc = QuantumCircuit(3, 1)
qc.x([0, 2])
qc.measure_active() # Measure qubits that are not idle, i.e., qubits 0 and 2.

Next steps