Qiskit

Warnings

• breaking Major breaking changes occurred between Qiskit 0.x and 1.x, and again between 1.x and 2.x. Upgrading from 0.x to 1.x cannot be done via `pip install -U qiskit` and requires a fresh installation, ideally in a new virtual environment, due to a new packaging structure.
  Fix: For upgrades from Qiskit 0.x to 1.x (or later major versions), create a new virtual environment and install Qiskit clean: `python -m venv .venv && source .venv/bin/activate && pip install qiskit`. Always review the official migration guides for specific version changes.
• breaking The Qiskit Primitives API (Sampler and Estimator) underwent significant changes and is mandatory for Qiskit 2.x. Existing algorithm libraries built on Qiskit 1.x primitives will be incompatible. Additionally, the default `shots` parameter changed (e.g., from implicit unlimited to 10000), which can drastically alter algorithm behavior and reproducibility if not explicitly set.
  Fix: Update code to use the new `qiskit.primitives` interface. Explicitly set the `shots` parameter in `Sampler.run()` or `Estimator.run()` calls to ensure consistent behavior, especially when migrating algorithms.
• breaking The `qiskit.pulse` module, which provided pulse-level control, was removed in Qiskit 2.0.
  Fix: For pulse-level control, consider using Qiskit 1.x (which had bug-fix support until Sept 2024 and security support until March 2025) or exploring direct access to fractional gates on newer IBM Quantum QPUs. There is no direct migration path for `qiskit.pulse` functionality in Qiskit 2.x.
• deprecated The `QuantumCircuit.duration` attribute is deprecated in Qiskit 2.x and is recommended to be replaced by `QuantumCircuit.estimate_duration()` due to evolving complexities in classical control flow.
  Fix: Replace `circuit.duration` with `circuit.estimate_duration()`.
• breaking Legacy `c_if` conditions on individual instructions have been removed in Qiskit 2.0. Classical conditions must now be expressed using `IfElseOp` or the `if_test` context manager.
  Fix: Refactor conditional logic to use `circuit.if_test` (as a context manager) or explicitly construct `IfElseOp` instances. Avoid direct use of `Instruction.condition`.
• gotcha The minimum required NumPy version was bumped to `1.21` in Qiskit 2.3.1. Users with older NumPy installations might encounter compatibility issues.
  Fix: Ensure NumPy is updated to at least version `1.21`: `pip install --upgrade numpy`.

Install

• pip install qiskit Core Qiskit SDK
• pip install qiskit-aer matplotlib For local simulation and visualization
• pip install qiskit-ibm-runtime For running on IBM Quantum hardware

Imports

• QuantumCircuit

  from qiskit import QuantumCircuit

• StatevectorSampler

  from qiskit.primitives import StatevectorSampler

  Qiskit 2.x and later uses the `qiskit.primitives` module for execution, replacing older simulator backends like `AerSimulator` or `BasicAer` for many common workflows. `AerSimulator` still exists in `qiskit_aer` for advanced simulation.
• AerSimulator

  from qiskit_aer import AerSimulator

  As of Qiskit 1.x, `qiskit_aer` is a separate package and its components are imported directly from `qiskit_aer`.
• plot_histogram

  from qiskit.visualization import plot_histogram

• QiskitRuntimeService

  from qiskit_ibm_runtime import QiskitRuntimeService

  The legacy `IBMQ` module for managing IBM Quantum accounts was replaced by `QiskitRuntimeService` in `qiskit-ibm-runtime`.

Quickstart

This quickstart builds a Bell state (two entangled qubits), visualizes the circuit, runs it on a local `StatevectorSampler`, and then displays the measurement outcomes as a histogram. It demonstrates basic circuit creation, execution via primitives, and result visualization.

import matplotlib.pyplot as plt
from qiskit import QuantumCircuit
from qiskit.primitives import StatevectorSampler

# Create a Bell state circuit
qc = QuantumCircuit(2, 2) # 2 qubits, 2 classical bits
qc.h(0) # Apply Hadamard gate to qubit 0
qc.cx(0, 1) # Apply CNOT gate with control qubit 0 and target qubit 1
qc.measure([0, 1], [0, 1]) # Measure both qubits into classical bits

# Visualize the circuit
print("Circuit Diagram:")
print(qc.draw(output='text'))

# Run on a local simulator using the Sampler primitive
sampler = StatevectorSampler()
job = sampler.run([qc], shots=1024)
result = job.result()

# Get measurement counts
counts = result[0].data.meas.get_counts()
print("\nMeasurement Counts:", counts)

# Plot histogram of results
plot_histogram(counts)
plt.title("Bell State Measurement")
plt.show()