Configurable backends

[chriseclectic]

Summary

Makes simulator backends have configurable options. Allows configuring a noisy backend from

Closes #645

Details and comments

Backend options initialization of the simulator object, or after, and still overridden using execute/run kwargs as previously.

All backend options may be passed in as kwargs, rather than a dict (though dict is maintained for backwards compatibility).

Example

Configuring a simulator after initializing

from qiskit.providers.aer import QasmSimulator
from qiskit.providers.aer.noise import *

# Make a NoiseModel
noise_model = NoiseModel()
error1 = depolarizing_error(1e-3, 1)
error2 = depolarizing_error(1e-2, 2)
noise_model.add_all_qubit_quantum_error(error1, 'u3')
noise_model.add_all_qubit_quantum_error(error2, 'cx')

# Make a backend then configure
backend = QasmSimulator()
backend.set_options(noise_model=noise_model, method='density_matrix')

# Equivalent to configuring during init
backend = QasmSimulator(noise_model=noise_model, method='density_matrix')

# Run noisy density matrix simulation on backend
qc = QuantumCircuit(2, 2)
qc.h(0)
qc.cx(0, 1)
qc.measure([0, 1], [0, 1])
result = execute(qc, backend).result()
result.get_counts(0)

Make a noisy backend from backend noise model

from qiskit import IBMQ

# Create noisy QasmSimulator backend from provider noise model
provider = IBMQ.load_account()
real_backend = provider.get_backend('ibmq_vigo')
fake_backend = QasmSimulator.from_backend(real_backend)

# Run noisy simulation on backend
qc = QuantumCircuit(2, 2)
qc.h(0)
qc.cx(0, 1)
qc.measure([0, 1], [0, 1])
result = execute(qc, fake_backend).result()
result.get_counts(0)

TODO:

• Add API documentation examples
• Add deprecation warning for backend_options kwarg
• Add basic tests
• Add tests for QasmSimulator.from_backend
• Add test for PulseSimulator.from_backend
• Add release note

Follow up

• Update tests to avoid deprecated interface
• Update tutorials
• Improve documentation
• Add mock backends

[nonhermitian]

This is cool.

It would be extra cool if Aer could autogen the noisy sims from those found doing IBMQ.load_account() or from Terra mock backends if no IBMQ such that one can simply do:

sim = Aer.get_backend('vigo_simulator')
job = execute([qc], sim)
...

[vvilpas]

Looks good. I have some very minor comments/sugggestions (most of them remainders to add 'cy' in list of basis)

[vvilpas]

Shouldn't we raise an exception in this case?

[chriseclectic]

Im not sure, @DanPuzzuoli what do you think?

[DanPuzzuoli]

I figured a warning gives a bit more flexibility, enabling the user to independently specify a system model without having to modify other backend attributes. At the same time, I don't have a very clear/obvious use-case for this, and in any case if we change it to an exception, the user could still specify an independent system model by first deleting the Hamiltonian. I think personally I'm leaning towards keeping it as is and leaving it up to the user to choose what to do, though I acknowledge changing it to an exception does keep the object state "cleaner".

[mtreinish]

At a high level this looks good to me. It aligns with the model we're moving towards in Qiskit/qiskit#5086 and puts us in a good place to switch over to that in the next aer release. The only piece missing is the default options (which being none is fine for a default) to define which options are available for a backend. Right now there isn't an easily discoverable way since everything is done via kwargs besides reading the docs. But, that's fine to do in a follow up when we migrate this Aer to be a v2 provider.

I just had a question inline and a doc suggestion, but easy enough to fix in a follow up.