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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]~=2.3.0
qiskit-ibm-runtime~=0.43.1

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-ZZ basis. Therefore, a measurement yields 0 or 1, depending on the overlap with the Pauli-ZZ eigenstates ∣0⟩|0\rangle and ∣1⟩|1\rangle:

∣q⟩→measure{0(outcome+1),with probability p0=∣⟨q∣0⟩∣2,1(outcome−1),with probability p1=∣⟨q∣1⟩∣2.|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}

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.

tala

The MidCircuitMeasure instruction maps to the measure_2 instruction reported in the backend's supported_instructions. However, measure_2 is not supported on all backends. Use service.backends(filters=lambda b: "measure_2" in b.supported_instructions) to find backends that support it. New measurements might be added in the future, but this is not guarenteed.

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.
<qiskit.circuit.instructionset.InstructionSet at 0x7fdc54260490>
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 0x7fdc54260820>

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 0x7fdc54260df0>

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))
┌───┐┌────────────┐
q_0: ┤ X ├┤0           ├
     └───┘│            │
q_1: ─────┤  Measure_2 ├
          │            │
c_0: ═════╡0           ╞
          └────────────┘
c_1: ═══════════════════
Important notes
  • There must be at least one classical register in order to use measurements.
  • The Sampler primitive requires circuit measurements. You can add circuit measurements with the Estimator primitive, but they are ignored.

Next steps​

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