Measure qubits

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Package versions

The code on this page was developed using the following requirements. We recommend using these versions or newer.

qiskit[all]~=2.4.0

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 $|1\rangle$:

$$|q\rangle \xrightarrow{measure}\begin{cases} 0 (\text{outcome}+1), \text{with probability } p_0=|\langle q|0\rangle|^{2}\text{,} \\ 1 (\text{outcome}-1), \text{with probability } p_1=|\langle q|1\rangle|^{2}\text{.} \end{cases}$$

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:

# Added by doQumentation — required packages for this notebook
!pip install -q qiskit

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.

<qiskit.circuit.instructionset.InstructionSet at 0x7eff8c636c20>

from qiskit import QuantumCircuit

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

<qiskit.circuit.instructionset.InstructionSet at 0x7eff8c636830>

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

<qiskit.circuit.instructionset.InstructionSet at 0x7eff8c6369e0>

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, that is, qubits 0 and 2.

Next steps

Recommendations

• Measure class
• measure_all method
• measure_active method
• random_circuit method
• Mid-circuit measurements (Available only when using Qiskit Runtime.)