Add support for parameterized qobj

[chriseclectic]

Summary

Allows specifying a parameterized qobj which will be expanded into multiple circuits when loading the qobj JSON.

Details and comments

qobj paramterizations are specified in the QobjConfig as

"parameterizations": [exp_0_params, ... ]

where exp_k_params = [[[i, j], [val_0, val_1, ....]], ...]
where i is the instruction position in the k qobj circuit, j is the param position for that instruction, and val_n is the value of the n-th parameterization.

For a given exp_param each parameterization instruction must have the same number of parameterization value, however the number of parameterizations can be different for each circuit in the qobj.

If non-empty the length of "parameterizations" must match the number of experiments in the qobj or an invalid qobj exception will be raised (if one of the circuits shouldn't be parameterized then it's exp_k_params should be set to an empty list []).

Example

import numpy as np
from qiskit import *

sim = Aer.get_backend("qasm_simulator")
n_params0 = 5
n_params1 = 10
parameterizations = [
    [[[0, 0], np.linspace(0, 2*np.pi, n_params0)]],
    [[[1, 2], np.linspace(0, 2*np.pi, n_params1)]]
]

qc1 = QuantumCircuit(1, 1)
qc1.ry(1, 0)
qc1.measure(0, 0)

qc2 = QuantumCircuit(2, 2)
qc2.h(1)
qc2.u1(1, 1)
qc2.h(1)
qc2.measure([0, 1], [0, 1])

qobj = assemble(transpile([qc1, qc2], basis_gates=['h', 'u3'],optimization_level=0),
               parameterizations=parameterizations)
result = sim.run(qobj).result()

print('Status: {}'.format(result.status))
print("Results for parameterized circuit 0:")
for j in range(n_params0):
    print(result.get_counts(j))

print("Results for parameterized circuit 0:")
for j in range(n_params0, n_params0 + n_params1):
    print(result.get_counts(j))

returns

Status: COMPLETED
Results for parameterized circuit 0:
{'0': 1024}
{'0': 502, '1': 522}
{'1': 1024}
{'0': 502, '1': 522}
{'0': 1024}
Results for parameterized circuit 0:
{'00': 1024}
{'10': 111, '00': 913}
{'10': 412, '00': 612}
{'10': 755, '00': 269}
{'10': 990, '00': 34}
{'10': 990, '00': 34}
{'10': 755, '00': 269}
{'10': 412, '00': 612}
{'10': 111, '00': 913}
{'00': 1024}

[ghost]

Is it necessary to copy the circuit here? You create circuit, and then in the case that there are params, you copy the circuit, edit that copy, and then push the copy and throw out the original - why not just edit the original?
Yes you need to copy it - you could still use reserve, emplace_back, and in-place editing of the circuits in circuits but perhaps there's not much difference

[ghost]

circuits.reserve(num_circs)

[kdk]

For a given exp_param each parameterization instruction must have the same number of parameterization value, however the number of parameterizations can be different for each circuit in the qobj.

Not an issue, just pointing out that this is different than the current Terra API which limits all circuits in a job to run under the same parameterizations.

qobj = assemble(transpile([qc1, qc2], basis_gates=['h', 'u3'],optimization_level=0),
               parameterizations=parameterizations)

There's an issue here in indexing parameterizations based on circuit gate order. There's no guarantee (even at optimization_level=0) that gates will be in the same order when the are listed in the qobj. (Using 0 as a filler param makes this somewhat more problematic, in that if param substitutions happen on the wrong gate, the circuit will likely still be executable.)

parameterizations = [
    [[[0, 0], np.linspace(0, 2*np.pi, n_params0)]],
    [[[1, 2], np.linspace(0, 2*np.pi, n_params1)]]
]

Is there an extra set of [] in the example here?

[ghost]

how does the typing work here? Do we only allow a single parameter type? Or am I missing some clever templating?

[chriseclectic]

At the moment this only works for complex or double params (basically u1, u2, u3 gates).

[chriseclectic]

@kdk It looks like an extra set of brackets because there is only 1 parameterized instruction in each of the example circuits.

Also yes, it is sensitive to order in the assembled qobj and assumes whomever made the parameterization configuration knew the index of each instruction in the qobj experiments lists.

As you say, even with optimization level zero there is no guarantee this will be the same order as circuits (Maybe if you call assemble on the circuit without any transpiling you can guarantee this though?)

[kdk]

As you say, even with optimization level zero there is no guarantee this will be the same order as circuits (Maybe if you call assemble on the circuit without any transpiling you can guarantee this though?)

This will be true as long as there aren't any classical conditionals (which would add instructions for the qasm to qobj conditional bfunc conversion: https://github.com/Qiskit/qiskit-terra/blob/2ee7a3a/qiskit/assembler/assemble_circuits.py#L121 ).