Qiskit

Qiskit

Use current Qiskit 2.x APIs to build circuits, prepare hardware-compatible instruction set architecture (ISA) circuits, and execute them through V2 primitives.

Reviewed 2026-10-01 with local tests on qiskit==2.5.2, qiskit-ibm-runtime==0.50.0, and qiskit-aer==0.17.2. IBM account, QPU, session, and batch examples are illustrative and documentation/source-checked; no remote workloads were submitted. See references/sources.md for the verification boundary.

Runtime 0.50 deprecates the top-level SamplerV2/EstimatorV2 implementations. New code imports Sampler from qiskit_ibm_runtime.executor_sampler and Estimator from qiskit_ibm_runtime.executor_estimator. These still implement the V2 PUB interface, using client-side processing and Executor. Use option models from qiskit_ibm_runtime.options_models.

Choose the Right Path

Installation

Create an isolated environment and install only the components needed:

uv venv --python 3.13
source .venv/bin/activate

# Core SDK plus plotting support
uv pip install "qiskit[visualization]==2.5.2"

# Add only when needed
uv pip install "qiskit-ibm-runtime==0.50.0"
uv pip install "qiskit-aer==0.17.2"

Do not install qiskit-terra; it was superseded by the qiskit distribution. Qiskit Runtime, Aer, Nature, Machine Learning, Optimization, and Algorithms are separate distributions.

For IBM account setup, CI-safe credential handling, optional packages, and environment repair, read references/setup.md.

Core Workflow

Follow this sequence for every hardware-oriented workload:

1. Map the problem to a circuit and, for Estimator, one or more observables.
2. Optimize the parameterized circuit once for the selected backend.
3. Apply the layout to every observable.
4. Execute ISA circuits through a V2 primitive using Primitive Unified Blocs (PUBs).
5. Analyze register-aware results, metadata, uncertainty, and resource usage.

Do not bind and retranspile a parameterized circuit inside every optimizer iteration. Transpile the parameterized circuit once, then pass parameter arrays in PUBs.

Quick Local Sampling

from qiskit import QuantumCircuit
from qiskit.primitives import StatevectorSampler

circuit = QuantumCircuit(2)
circuit.h(0)
circuit.cx(0, 1)
circuit.measure_all()  # creates the classical register named "meas"

sampler = StatevectorSampler(seed=7)
pub_result = sampler.run([circuit], shots=1024).result()[0]
counts = pub_result.data.meas.get_counts()
print(counts)

Sampler V2 preserves shots and classical-register structure. Access the register by its actual name; measure_all() uses meas.

For circuits with multiple classical registers, each register’s counts are a marginal distribution. Preserve shot alignment when computing cross-register correlations; multiplying marginal frequencies destroys those correlations. Use SamplerPubResult.join_data with an explicit register order for joint bitstrings and record that order in the result labels. Qiskit 2.5.2 puts the first joined BitArray register in the least-significant bits, contrary to the current docstring; verify with an asymmetric state. See references/primitives.md.

Quick Local Estimation

import numpy as np
from qiskit import QuantumCircuit
from qiskit.circuit import Parameter
from qiskit.primitives import StatevectorEstimator
from qiskit.quantum_info import SparsePauliOp

theta = Parameter("theta")
circuit = QuantumCircuit(2)
circuit.ry(theta, 0)
circuit.cx(0, 1)

observable = SparsePauliOp.from_list([("ZZ", 1.0), ("XX", 0.5)])
parameter_values = [[0.0], [np.pi / 4], [np.pi / 2]]

estimator = StatevectorEstimator(seed=7)
pub = (circuit, observable, parameter_values)
pub_result = estimator.run([pub]).result()[0]
print(pub_result.data.evs)

Estimator circuits should not contain final measurements. PUB arrays broadcast; verify circuit parameter order before constructing large sweeps.

IBM QPU Sampling

This example assumes credentials were saved securely as described in references/setup.md. It never embeds or prints an API key.

from qiskit import QuantumCircuit
from qiskit.transpiler import generate_preset_pass_manager
from qiskit_ibm_runtime import QiskitRuntimeService
from qiskit_ibm_runtime.executor_sampler import Sampler

service = QiskitRuntimeService()
backend = service.least_busy(
    operational=True,
    simulator=False,
    min_num_qubits=2,
)

circuit = QuantumCircuit(2)
circuit.h(0)
circuit.cx(0, 1)
circuit.measure_all()

pass_manager = generate_preset_pass_manager(
    backend=backend,
    optimization_level=1,
    seed_transpiler=7,
)
isa_circuit = pass_manager.run(circuit)

sampler = Sampler(mode=backend)
job = sampler.run([isa_circuit], shots=1024)
print("job_id:", job.job_id())
counts = job.result()[0].data.meas.get_counts()

Save the job ID before waiting for results so the job can be retrieved later.

IBM QPU Estimation

Runtime Estimator requires both an ISA circuit and observables mapped through the transpiler layout:

from qiskit import QuantumCircuit
from qiskit.quantum_info import SparsePauliOp
from qiskit.transpiler import generate_preset_pass_manager
from qiskit_ibm_runtime.executor_estimator import Estimator

circuit = QuantumCircuit(2)
circuit.h(0)
circuit.cx(0, 1)
observable = SparsePauliOp.from_list([("ZZ", 1.0)])

pass_manager = generate_preset_pass_manager(
    backend=backend,
    optimization_level=1,
    seed_transpiler=7,
)
isa_circuit = pass_manager.run(circuit)
isa_observable = observable.apply_layout(isa_circuit.layout)

estimator = Estimator(
    mode=backend,
    options={"resilience_level": 1},
)
pub_result = estimator.run(
    [(isa_circuit, isa_observable)],
    precision=0.02,
).result()[0]
print(pub_result.data.evs, pub_result.data.stds)

Error mitigation is not guaranteed to improve every workload and increases cost. Record finalized options (estimator.finalize_options().model_dump()), result metadata, and usage. A Runtime dry_run=True call is a server submission returning randomized mock data, not a local simulator or scientific validation.

Non-Negotiable Qiskit 2.x Rules

• Use V2 primitive interfaces and PUB inputs. Do not write new V1 Sampler, Estimator, or QuantumInstance code.
• Runtime primitives accept ISA circuits; they do not perform layout, routing, and basis translation for you.
• Apply the transpiler layout to Estimator observables with observable.apply_layout(isa_circuit.layout).
• Use mode=backend, mode=session, or mode=batch for Runtime primitives.
• Use the Runtime client-side Estimator for resilience levels and expectation-value mitigation. Sampler has different noise-management options and no Estimator-style resilience levels.
• Treat BackendV2.target, backend.operation_names, backend.coupling_map, and direct backend attributes as the source of hardware constraints. Do not use backend.configuration() or BackendProperties.
• Read Sampler output by classical register name. Bitstrings are displayed most-significant bit first; Qiskit qubit 0 is conventionally the least-significant bit.
• Use a fixed seed_transpiler when comparing compilation settings. A simulator seed does not make QPU results deterministic.
• qiskit.pulse was removed in Qiskit 2.0. Use supported fractional gates for IBM hardware. Qiskit Dynamics is archived; isolate legacy pulse-model research and verify its dependency stack separately.
• QPY is the Qiskit-native circuit serialization format. Do not use Python pickle for untrusted circuit artifacts.

See references/migration.md for a detailed old-to-current API map.