Aer Estimator wrongly reads global phase

[A-tA-v]

Informations

• Qiskit Aer version: 0.12.0
• Python version: 3.10
• Operating system: Mac OS Ventura, Intel

What is the current behavior?

Aer Estimator crashes while running simulation.

Steps to reproduce the problem

Script below

What is the expected behavior?

I create a parameter placeholder at the beginning.
After prepending the circuit with ZFeatureMap and several compositions
the final circuit obtains a non-zero global phase represeneted as a linear
combination of placeholder's entries. I do NOT change the global phase
myself, rather this is the outcome of aforementioned operations.
At the time of evaluation, the linear combination of placeholder's entries
cannot not be cast to a float value causing an exception.

In the simple example below, I reproduced the behaviour by explicitely
expressing the global phase as a sum of two placeholder entries.
Note, the default estimator works without a problem.

Suggested solutions

Checking of global phase before evaluation is needed.

Script reproducing the issue:

import time
import numpy as np
from qiskit import QuantumCircuit
from qiskit.circuit import ParameterVector
from qiskit.primitives import Estimator as BackendEstimator
from qiskit.providers.fake_provider import FakeKolkataV2
from qiskit.quantum_info import SparsePauliOp
from qiskit.utils import algorithm_globals
from qiskit_aer.primitives import Estimator as AerEstimator


def report():
    num_qubits = 10
    algorithm_globals.random_seed = 1234567
    np.random.seed(algorithm_globals.random_seed)

    # Create a circuit with parameter placeholder.
    params = ParameterVector(name="r", length=num_qubits - 1)
    qc = QuantumCircuit(num_qubits)
    for i in range(num_qubits - 1):
        qc.cx(i, i + 1)
        qc.rz(params[i], i)
    qc.global_phase = params[0] + params[1]  # <<<<< M A K E S  T H E  P R O B L E M

    params = np.random.rand(len(params))
    observable = SparsePauliOp.from_list([("Z" * qc.num_qubits, 1)])

    tic = time.perf_counter()
    backend_options = {"seed_simulator": algorithm_globals.random_seed, "shots": 4096}
    use_aer = 1  # <<<<< choose estimator here
    if use_aer != 0:
        transpile_options = {"seed_transpiler": algorithm_globals.random_seed}
        run_options = {"shots": 1000}
        estimator = AerEstimator(
            backend_options=backend_options,
            transpile_options=transpile_options,
            run_options=run_options,
        )
    else:
        estimator = BackendEstimator(FakeKolkataV2(), options=backend_options)
    toc = time.perf_counter() - tic
    print(f"Estimator created in {toc:.2f} seconds")

    for i in range(20):
        tic = time.perf_counter()
        result = estimator.run(qc, observable, params).result()
        toc = time.perf_counter() - tic
        print(f"Attempt {i}, result obtained in {toc:.2f} seconds")
        print(f"Result: {result}\n----------------------------------\n")

report()

Script's output:

% py3 bug_report.py
Number of qubits: 10
Estimator created in 0.00 seconds
Traceback (most recent call last):
  File "/Users/username/venv/qu/lib/python3.10/site-packages/qiskit/circuit/parameterexpression.py", line 472, in __float__
    return float(self._symbol_expr)
  File "symengine_wrapper.pyx", line 1155, in symengine.lib.symengine_wrapper.Basic.__float__
  File "symengine_wrapper.pyx", line 980, in symengine.lib.symengine_wrapper.Basic.n
  File "symengine_wrapper.pyx", line 4383, in symengine.lib.symengine_wrapper.evalf
RuntimeError: Symbol cannot be evaluated.

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/Users/username/work/qml_time_evol_research/bug_report.py", line 243, in <module>
    run_circuit_on_simulator(_num_qubits, _num_units, _num_layers, _insert_barriers)
  File "/Users/username/work/qml_time_evol_research/bug_report.py", line 231, in run_circuit_on_simulator
    result = estimator.run(qc, observable, params).result()
  File "/Users/username/venv/qu/lib/python3.10/site-packages/qiskit/primitives/primitive_job.py", line 50, in result
    return self._future.result()
  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/concurrent/futures/_base.py", line 451, in result
    return self.__get_result()
  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/concurrent/futures/_base.py", line 403, in __get_result
    raise self._exception
  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/concurrent/futures/thread.py", line 58, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/Users/username/venv/qu/lib/python3.10/site-packages/qiskit_aer/primitives/estimator.py", line 119, in _call
    return self._compute(circuits, observables, parameter_values, run_options)
  File "/Users/username/venv/qu/lib/python3.10/site-packages/qiskit_aer/primitives/estimator.py", line 222, in _compute
    .result()
  File "/Users/username/venv/qu/lib/python3.10/site-packages/qiskit_aer/jobs/utils.py", line 41, in _wrapper
    return func(self, *args, **kwargs)
  File "/Users/username/venv/qu/lib/python3.10/site-packages/qiskit_aer/jobs/aerjob.py", line 106, in result
    return self._future.result(timeout=timeout)
  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/concurrent/futures/_base.py", line 451, in result
    return self.__get_result()
  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/concurrent/futures/_base.py", line 403, in __get_result
    raise self._exception
  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/concurrent/futures/thread.py", line 58, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/Users/username/venv/qu/lib/python3.10/site-packages/qiskit_aer/backends/aerbackend.py", line 437, in _execute_circuits_job
    aer_circuits = assemble_circuits(circuits)
  File "/Users/username/venv/qu/lib/python3.10/site-packages/qiskit_aer/backends/aer_compiler.py", line 552, in assemble_circuits
    return [assemble_circuit(circuit) for circuit in circuits]
  File "/Users/username/venv/qu/lib/python3.10/site-packages/qiskit_aer/backends/aer_compiler.py", line 552, in <listcomp>
    return [assemble_circuit(circuit) for circuit in circuits]
  File "/Users/username/venv/qu/lib/python3.10/site-packages/qiskit_aer/backends/aer_compiler.py", line 376, in assemble_circuit
    global_phase = float(circuit.global_phase)
  File "/Users/username/venv/qu/lib/python3.10/site-packages/qiskit/circuit/parameterexpression.py", line 475, in __float__
    raise TypeError(
TypeError: ParameterExpression with unbound parameters ({ParameterVectorElement(r[0]), ParameterVectorElement(r[1])}) cannot be cast to a float.

[ikkoham]

Thank you for your report. This is a bug. Latest Aer cannot handle parameters in the global phase.
Related to: #1781

[hhorii]

#1814 closes this issue.