Measure qubits

qiskit[all]~=2.1.1

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 $|1\rangle$:

Mid-circuit measurements

Mid-circuit measurements are a key component of dynamic circuits. Prior to qiskit-ibm-runtime v0.43.0, measure was the only measurement instruction in Qiskit. Mid-circuit measurements, however, have different tuning requirements than terminal measurements (measurements that happen at the end of a circuit). For example, you need to consider the instruction duration when tuning a mid-circuit measurement because longer instructions cause noisier circuits. You don't need to consider instruction duration for terminal measurements because there are no instructions after terminal measurements.

In qiskit-ibm-runtime v0.43.0, the MidCircuitMeasure instruction was introduced. As the name suggests, it is a new measurement instruction that is optimized for mid-circuit on IBM® QPUs.

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.

Output:

<qiskit.circuit.instructionset.InstructionSet at 0x7feaa7cd2290>

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

Output:

<qiskit.circuit.instructionset.InstructionSet at 0x7feaa7c4d8d0>

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

Output:

<qiskit.circuit.instructionset.InstructionSet at 0x7feaa7dc6260>

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.

MidCircuitMeasure method

Use MidCircuitMeasure to apply a mid-circuit measurement (requires qiskit-ibm-runtime v0.43.0 or later). While you can use QuantumCircuit.measure for a mid-circuit measurement, because of its design, MidCircuitMeasure is typically a better choice. For example, it adds less overhead to your circuit than when using QuantumCircuit.measure.

from qiskit import QuantumCircuit
from qiskit_ibm_runtime import QiskitRuntimeService
from qiskit_ibm_runtime.circuit import MidCircuitMeasure
from qiskit.circuit import Measure
 
service = QiskitRuntimeService()
backend = service.least_busy(operational=True, simulator=False)
 
circ = QuantumCircuit(2, 2)
circ.x(0)
circ.append(MidCircuitMeasure(), [0], [0])
# circ.measure([0], [0])
# circ.measure_all()
print(circ.draw(cregbundle=False))

Output:

     ┌───┐┌────────────┐
q_0: ┤ X ├┤0           ├
     └───┘│            │
q_1: ─────┤  Measure_2 ├
          │            │
c_0: ═════╡0           ╞
          └────────────┘
c_1: ═══════════════════
                        

Next steps