{ "cells": [ { "cell_type": "markdown", "id": "8cefe278-60a0-4980-a0c4-b2f5806d15b7", "metadata": {}, "source": [ "---\n", "title: Measure qubits\n", "description: Learn how to measure qubits, including constraints on where measurements can be used.\n", "---\n", "\n", "# Measure qubits\n", "\n" ] }, { "cell_type": "markdown", "id": "3b56a673-37b7-4837-b718-9d30d26ad87e", "metadata": { "tags": [ "version-info" ] }, "source": [ "{/*\n", " DO NOT EDIT THIS CELL!!!\n", " This cell's content is generated automatically by a script. Anything you add\n", " here will be removed next time the notebook is run. To add new content, create\n", " a new cell before or after this one.\n", " */}\n", "\n", "\n", " \n", " The code on this page was developed using the following requirements.\n", " We recommend using these versions or newer.\n", "\n", " ```\n", " qiskit[all]~=2.5.2\n", " ```\n", " \n", "\n", "\n" ] }, { "cell_type": "markdown", "id": "99224c00-ef6f-4dad-98a8-df5702aa6bc3", "metadata": {}, "source": [ "To get information about a qubit's state, you can *measure* it onto a [classical bit](/docs/api/qiskit/circuit#qiskit.circuit.Clbit). 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 $|1\\rangle$:\n", "\n", "$$\n", "|q\\rangle \\xrightarrow{measure}\\begin{cases}\n", " 0 (\\text{outcome}+1), \\text{with probability } p_0=|\\langle q|0\\rangle|^{2}\\text{,} \\\\\n", " 1 (\\text{outcome}-1), \\text{with probability } p_1=|\\langle q|1\\rangle|^{2}\\text{.}\n", " \\end{cases}\n", "$$\n", "\n", "## Apply a measurement to a circuit\n", "\n", "There are several ways to apply measurements to a circuit:\n", "\n", "### `QuantumCircuit.measure` method\n", "\n", "Use the [`measure`](/docs/api/qiskit/qiskit.circuit.QuantumCircuit#measure) method to measure a [`QuantumCircuit`](/docs/api/qiskit/qiskit.circuit.QuantumCircuit#quantumcircuit-class).\n", "\n", "Examples:\n", "\n" ] }, { "cell_type": "code", "execution_count": 1, "id": "6cc3aa2f-40a5-4191-8236-a5ffd2e64be0", "metadata": {}, "outputs": [ { "data": { "text/plain": [ "" ] }, "execution_count": 1, "metadata": {}, "output_type": "execute_result" } ], "source": [ "from qiskit import QuantumCircuit\n", "\n", "qc = QuantumCircuit(5, 5)\n", "qc.x(0)\n", "qc.x(1)\n", "qc.x(4)\n", "qc.measure(\n", " range(5), range(5)\n", ") # Measures all qubits into the corresponding clbit." ] }, { "cell_type": "code", "execution_count": 2, "id": "5c629673-9000-47ae-8bd3-f3fef65f8b0c", "metadata": {}, "outputs": [ { "data": { "text/plain": [ "" ] }, "execution_count": 2, "metadata": {}, "output_type": "execute_result" } ], "source": [ "from qiskit import QuantumCircuit\n", "\n", "qc = QuantumCircuit(3, 1)\n", "qc.x([0, 2])\n", "qc.measure(1, 0) # Measure qubit 1 into the classical bit 0." ] }, { "cell_type": "markdown", "id": "d909843d-9c23-4017-83d6-8a09fae4c067", "metadata": {}, "source": [ "### `Measure` class\n", "\n", "The Qiskit [Measure](/docs/api/qiskit/circuit#qiskit.circuit.Measure) class measures the specified qubits.\n", "\n" ] }, { "cell_type": "code", "execution_count": 3, "id": "b8855eed-8238-41ce-a45d-f8e81da05508", "metadata": {}, "outputs": [ { "data": { "text/plain": [ "" ] }, "execution_count": 3, "metadata": {}, "output_type": "execute_result" } ], "source": [ "from qiskit.circuit import Measure\n", "\n", "qc = QuantumCircuit(3, 1)\n", "qc.x([0, 1])\n", "qc.append(Measure(), [0], [0]) # measure qubit 0 into clbit 0" ] }, { "cell_type": "markdown", "id": "6583286c-d8e3-4adf-8bce-0e7d9dc3c44f", "metadata": {}, "source": [ "### `QuantumCircuit.measure_all` method\n", "\n", "To measure all qubits into the corresponding classical bits, use the [`measure_all`](/docs/api/qiskit/qiskit.circuit.QuantumCircuit#measure_all) method. By default, this method adds new classical bits in a `ClassicalRegister` to store these measurements.\n", "\n" ] }, { "cell_type": "code", "execution_count": 4, "id": "cac45158-7e3e-47b6-bc89-8ac3a4bc9539", "metadata": {}, "outputs": [], "source": [ "from qiskit import QuantumCircuit\n", "\n", "qc = QuantumCircuit(3, 1)\n", "qc.x([0, 2])\n", "qc.measure_all() # Measure all qubits." ] }, { "cell_type": "markdown", "id": "8b4a57d5-ffc9-4247-b39c-2e19ab2c185b", "metadata": {}, "source": [ "### `QuantumCircuit.measure_active` method\n", "\n", "To measure all qubits that are not idle, use the [`measure_active`](/docs/api/qiskit/qiskit.circuit.QuantumCircuit#measure_active) method. This method creates a new `ClassicalRegister` with a size equal to the number of non-idle qubits being measured.\n", "\n" ] }, { "cell_type": "code", "execution_count": 5, "id": "3161f70d-5c59-48fa-842f-624ce7ed0bbe", "metadata": {}, "outputs": [], "source": [ "from qiskit import QuantumCircuit\n", "\n", "qc = QuantumCircuit(3, 1)\n", "qc.x([0, 2])\n", "qc.measure_active() # Measure qubits that are not idle, that is, qubits 0 and 2." ] }, { "cell_type": "markdown", "id": "5b049296-7cd5-4074-8473-ad462744e270", "metadata": {}, "source": [ "## Next steps\n", "\n", "\n", " * [`Measure`](/docs/api/qiskit/circuit#qiskit.circuit.Measure) class\n", " * [`measure_all`](/docs/api/qiskit/qiskit.circuit.QuantumCircuit#measure_all) method\n", " * [`measure_active`](/docs/api/qiskit/qiskit.circuit.QuantumCircuit#measure_active) method\n", " * [`random_circuit`](/docs/api/qiskit/circuit_random#qiskit.circuit.random.random_circuit) method\n", " * [Mid-circuit measurements](/docs/guides/execute-dynamic-circuits#midcircuit) (Available only when using IBM Quantum Compute Service.)\n", "\n", "\n" ] }, { "cell_type": "markdown", "metadata": {}, "id": "a1b8767d", "source": "© IBM Corp., 2017-2026" } ], "metadata": { "kernelspec": { "display_name": "Python 3", "language": "python", "name": "python3" }, "language_info": { "codemirror_mode": { "name": "ipython", "version": 3 }, "file_extension": ".py", "mimetype": "text/x-python", "name": "python", "nbconvert_exporter": "python", "pygments_lexer": "ipython3", "version": "3" } }, "nbformat": 4, "nbformat_minor": 4 }