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Provide a workaround to #422, Sampler failing when no measurements #426

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Oct 10, 2023
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Provide a workaround to #422, Sampler failing when no measurements #426

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9443b8a
Provide a workaround to #422, Sampler failing when no measurements
garrison Sep 29, 2023
eadcf2c
Add test for workaround to Issue #422.
ibrahim-shehzad Oct 3, 2023
0609cee
Redo previous commit after running tox.
ibrahim-shehzad Oct 4, 2023
b77f362
Fix tox error from previous build.
ibrahim-shehzad Oct 4, 2023
0575fbb
Fix tox error from last build.
ibrahim-shehzad Oct 4, 2023
2853f2c
Un-modify the metadata in a notebook
garrison Oct 5, 2023
4d28704
Update test/cutting/test_cutting_roundtrip.py
ibrahim-shehzad Oct 9, 2023
60830a7
Update test/cutting/test_cutting_roundtrip.py
ibrahim-shehzad Oct 9, 2023
930b51e
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garrison Oct 9, 2023
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11 changes: 9 additions & 2 deletions circuit_knitting/cutting/cutting_experiments.py
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Original file line number Diff line number Diff line change
Expand Up @@ -276,14 +276,21 @@ def _append_measurement_circuit(
if not inplace:
qc = qc.copy()

# If the circuit has no measurements, the Sampler will fail. So, we
# measure one qubit as a temporary workaround to
# https://github.com/Qiskit-Extensions/circuit-knitting-toolbox/issues/422
pauli_indices = cog.pauli_indices
if not pauli_indices:
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@caleb-johnson caleb-johnson Oct 5, 2023 •
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What can pauli_indices be here? Should we just explicitly state what we expect? (e.g. if pauli_indices == [])

Or we could just assert pauli_indices == [] inside the if? (assuming an empty list is what is returned in I observable case)

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What can pauli_indices be here? Should we just explicitly state what we expect? (e.g. if pauli_indices == [])

It's a sequence of ints. It contains the qubit indices that need to be measured.

The choice of style here is influenced by PEP 8:

For sequences, (strings, lists, tuples), use the fact that empty sequences are false:

# Correct:
if not seq:
if seq:
# Wrong:
if len(seq):
if not len(seq):

The "nice" thing about doing it this way is it doesn't matter what type the sequence is. For instance, if it were to be a tuple of indices, then it will still behave correctly with the current code, but it would not if we were to compare with [] because () != [].

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nice, thanks for explanation

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@ibrahim-shehzad ibrahim-shehzad Oct 6, 2023 •
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Wow, this is more profound than I thought. Nice!

pauli_indices = [0]

# Append the appropriate measurements to qc
obs_creg = ClassicalRegister(len(cog.pauli_indices), name="observable_measurements")
obs_creg = ClassicalRegister(len(pauli_indices), name="observable_measurements")
qc.add_register(obs_creg)
# Implement the necessary basis rotations and measurements, as
# in BackendEstimator._measurement_circuit().
genobs_x = cog.general_observable.x
genobs_z = cog.general_observable.z
for clbit, subqubit in enumerate(cog.pauli_indices):
for clbit, subqubit in enumerate(pauli_indices):
# subqubit is the index of the qubit in the subsystem.
# actual_qubit is its index in the system of interest (if different).
actual_qubit = qubit_locations[subqubit]
Expand Down
28 changes: 28 additions & 0 deletions test/cutting/test_cutting_roundtrip.py
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Expand Up @@ -42,6 +42,7 @@
from qiskit.extensions import UnitaryGate
from qiskit.quantum_info import PauliList, random_unitary
from qiskit.primitives import Estimator
from qiskit_aer.primitives import Sampler

from circuit_knitting.utils.simulation import ExactSampler
from circuit_knitting.cutting import (
Expand Down Expand Up @@ -179,3 +180,30 @@ def test_cutting_exact_reconstruction(example_circuit):
logger.info("Max error: %f", np.max(np.abs(exact_expvals - simulated_expvals)))

assert np.allclose(exact_expvals, simulated_expvals, atol=1e-8)


def test_sampler_with_identity_subobservable(example_circuit):
"""This test ensures that the sampler does not throw an error if you pass it a subcircuit with no observable measurements.

Tests temporary workaround to Issue #422.

This test passes if no exceptions are raised.

"""

qc = example_circuit
observable_to_test = PauliList(
["IIZ"]
) # Without the workaround to Issue #422, this observable causes a Sampler error.
subcircuits, bases, subobservables = partition_problem(
qc, "AAB", observables=observable_to_test
)
subexperiments, coefficients = generate_cutting_experiments(
subcircuits, subobservables, num_samples=np.inf
)
samplers = {label: Sampler() for label in subexperiments.keys()}
results = {
label: sampler.run(subexperiments[label]).result()
for label, sampler in samplers.items()
}
_ = results