The Aer device

The qiskit.aer device provided by the PennyLane-Qiskit plugin allows you to use PennyLane to deploy and run your quantum machine learning models on the backends and simulators provided by Qiskit Aer.

You can instantiate a 'qiskit.aer' device for PennyLane with:

import pennylane as qml
dev = qml.device('qiskit.aer', wires=2)

This device can then be used just like other devices for the definition and evaluation of QNodes within PennyLane. A simple quantum function that returns the expectation value of a measurement and depends on three classical input parameters would look like:

@qml.qnode(dev)
def circuit(x, y, z):
    qml.RZ(z, wires=[0])
    qml.RY(y, wires=[0])
    qml.RX(x, wires=[0])
    qml.CNOT(wires=[0, 1])
    return qml.expval(qml.PauliZ(wires=1))

You can then execute the circuit like any other function to get the quantum mechanical expectation value.

circuit(0.2, 0.1, 0.3)

Backends

Qiskit’s Aer layer has several backends, for example 'qasm_simulator', 'statevector_simulator', 'unitary_simulator'. For more information on backends, please visit the Aer provider documentation and the Qiskit Aer tutorials.

To get a current overview what backends are available you can query

dev.capabilities()['backend']

or, alternatively,

from qiskit import Aer
Aer.backends()

Note

Currently, PennyLane does not support the 'pulse_simulator' backend.

You can change a 'qiskit.aer' device’s backend with the backend argument when creating the device:

dev = qml.device('qiskit.aer', wires=2, backend='unitary_simulator')

Backend options

Qiskit’s backends can take different backend options, for example to specify the numerical precision of the simulation. You can find a list of backend options in the backends’ respective API documentations:

• ‘qasm_simulator’

• ‘statevector_simulator’

• ‘unitary_simulator’

The options are set via

dev = qml.device('qiskit.aer', wires=2, backend='unitary_simulator',
                 backend_options={"validation_threshold": 1e-6})

Noise models

One great feature of the 'qiskit.aer' device is the ability to simulate noise. There are different noise models, which you can instantiate and apply to the device as follows (adapting this qiskit tutorial):

import pennylane as qml

import qiskit
import qiskit.providers.aer.noise as noise

# Error probabilities
prob_1 = 0.001  # 1-qubit gate
prob_2 = 0.01   # 2-qubit gate

# Depolarizing quantum errors
error_1 = noise.depolarizing_error(prob_1, 1)
error_2 = noise.depolarizing_error(prob_2, 2)

# Add errors to noise model
noise_model = noise.NoiseModel()
noise_model.add_all_qubit_quantum_error(error_1, ['u1', 'u2', 'u3'])
noise_model.add_all_qubit_quantum_error(error_2, ['cx'])

# Create a PennyLane device
dev = qml.device('qiskit.aer', wires=2, noise_model=noise_model)

# Create a PennyLane quantum node run on the device
@qml.qnode(dev)
def circuit(x, y, z):
    qml.RZ(z, wires=[0])
    qml.RY(y, wires=[0])
    qml.RX(x, wires=[0])
    qml.CNOT(wires=[0, 1])
    return qml.expval(qml.PauliZ(wires=1))

# Result of noisy simulator
print(circuit(0.2, 0.1, 0.3))

Please refer to the Qiskit documentation for more information on noise models.