Get started with primitives

The steps in this topic describes how to set up primitives, explore the options you can use to configure them, and invoke them in a program.

Get started with Estimator

1. Initialize the account

Because Qiskit Runtime Estimator is a managed service, you first need to initialize your account. You can then select the QPU you want to use to calculate the expectation value.

Follow the steps in the Install and set up topic if you don't already have an account.

from qiskit_ibm_runtime import QiskitRuntimeService, EstimatorV2 as Estimator
 
service = QiskitRuntimeService()
backend = service.least_busy(operational=True, simulator=False, min_num_qubits=127)
estimator = Estimator(mode=backend)

2. Create a circuit and an observable

You need at least one circuit and one observable as inputs to the Estimator primitive.

import numpy as np
from qiskit.circuit.library import IQP
from qiskit.transpiler.preset_passmanagers import generate_preset_pass_manager
from qiskit.quantum_info import SparsePauliOp, random_hermitian
 
n_qubits = 127
 
mat = np.real(random_hermitian(n_qubits, seed=1234))
circuit = IQP(mat)
observable = SparsePauliOp("Z" * n_qubits)
print(f">>> Observable: {observable.paulis}")

Output

>>> Observable: ['ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ...']

The circuit and observable need to be transformed to only use instructions supported by the QPU (referred to as instruction set architecture (ISA) circuits). We'll use the transpiler to do this.

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

3. Initialize Qiskit Runtime Estimator

When you initialize the Estimator, use the mode parameter to specify the mode you want it to run in. Possible values are batch, session, or backend objects for batch, session, and job execution mode, respectively. For more information, see Introduction to Qiskit Runtime execution modes.

from qiskit_ibm_runtime import EstimatorV2 as Estimator
 
estimator = Estimator(mode=backend)

4. Invoke the Estimator and get results

Next, invoke the run() method to calculate expectation values for the input circuits and observables. The circuit, observable, and optional parameter value sets are input as primitive unified bloc (PUB) tuples.

job = estimator.run([(isa_circuit, isa_observable)])
print(f">>> Job ID: {job.job_id()}")
print(f">>> Job Status: {job.status()}")

Output

>>> Job ID: 8874203f-f5a0-4491-b093-3f29a1adcae4
>>> Job Status: JobStatus.RUNNING

result = job.result()
print(f">>> {result}")
print(f"  > Expectation value: {result[0].data.evs}")
print(f"  > Metadata: {result[0].metadata}")

Output

>>> PrimitiveResult([PubResult(data=DataBin<>(evs=0.013671875, stds=0.0156235396179561), metadata={'target_precision': 0.015625})], metadata={})
  > Expectation value: 0.013671875
  > Metadata: {'target_precision': 0.015625}

Get started with Sampler

1. Initialize the account

Because Qiskit Runtime Sampler is a managed service, you first need to initialize your account. You can then select the QPU you want to use to calculate the expectation value.

Follow the steps in the Install and set up topic if you don't already have an account set up.

from qiskit_ibm_runtime import QiskitRuntimeService
 
service = QiskitRuntimeService()
backend = service.least_busy(operational=True, simulator=False, min_num_qubits=127)

2. Create a circuit

You need at least one circuit as the input to the Sampler primitive.

import numpy as np
from qiskit.circuit.library import IQP
from qiskit.transpiler.preset_passmanagers import generate_preset_pass_manager
from qiskit.quantum_info import random_hermitian
 
n_qubits = 127
 
mat = np.real(random_hermitian(n_qubits, seed=1234))
circuit = IQP(mat)
circuit.measure_all()

Use the transpiler to get an ISA circuit.

pm = generate_preset_pass_manager(optimization_level=1, backend=backend)
isa_circuit = pm.run(circuit)

3. Initialize the Qiskit Runtime Sampler

When you initialize the Sampler, use the mode parameter to specify the mode you want it to run in. Possible values are batch, session, or backend objects for batch, session, and job execution mode, respectively. For more information, see Introduction to Qiskit Runtime execution modes.

from qiskit_ibm_runtime import SamplerV2 as Sampler
 
sampler = Sampler(mode=backend)

4. Invoke the Sampler and get results

Next, invoke the run() method to generate the output. The circuit and optional parameter value sets are input as primitive unified bloc (PUB) tuples.

job = sampler.run([isa_circuit])
print(f">>> Job ID: {job.job_id()}")
print(f">>> Job Status: {job.status()}")

Output

>>> Job ID: 58223448-5100-4dec-a47a-942fb30edced
>>> Job Status: JobStatus.RUNNING

result = job.result()
 
# Get results for the first (and only) PUB
pub_result = result[0]
print(f"Counts for the meas output register: {pub_result.data.meas.get_counts()}")

Output

Counts for the meas output register: {'0111': 50, '0000': 243, '0001': 101, '0101': 93, '0100': 188, '0011': 128, '1011': 22, '0110': 13, '1100': 10, '1000': 24, '0010': 29, '1010': 26, '1101': 20, '1110': 45, '1001': 16, '1111': 16}

Get started with the backend primitives

Unlike provider-specific primitives, backend primitives are generic implementations that can be used with an arbitrary backend object, as long as it implements the Backend interface.

• The Sampler primitive can be run with any provider by using qiskit.primitives.BackendSamplerV2.
• The Estimator primitive can be run with any provider by using qiskit.primitives.BackendEstimatorV2.

Some providers implement primitives natively. See the Qiskit Ecosystem page for details.

Example: BackendEstimator

from qiskit.primitives import BackendEstimatorV2
from <some_qiskit_provider> import QiskitProvider
 
provider = QiskitProvider()
backend = provider.get_backend('backend_name')
estimator = BackendEstimatorV2(backend)

Example: BackendSampler

from qiskit.primitives import BackendSamplerV2
from <some_qiskit_provider> import QiskitProvider
 
provider = QiskitProvider()
backend = provider.get_backend('backend_name')
sampler = BackendSamplerV2(backend)

Similarities and differences between backend and Runtime primitives

• The inputs to and outputs from qiskit.primitives.BackendSamplerV2 and qiskit.primitives.BackendEstimatorV2 follow the same PUB format as the primitives in Qiskit Runtime. See Primitive inputs and outputs for details. However, there can be differences in the fields of the returned metadata.

• The qiskit.primitives.BackendEstimatorV2 class offers no measurement or gate error mitigation implementations out-of-the-box, as backend primitives are designed to run locally in the user's machine.

• The qiskit.primitives.BackendSamplerV2 class requires a backend that supports the memory option.

• The backend primitive interfaces expose custom SamplerV2 and EstimatorV2 Options that are different from the Runtime implementations.

Next steps