Improve Parameter handling in SparsePauliOp

qiskit/algorithms/time_evolvers/trotterization/trotter_qrte.py

@@ -26,8 +26,6 @@
 from qiskit.quantum_info import Pauli, SparsePauliOp
 from qiskit.synthesis import ProductFormula, LieTrotter
 
-from qiskit.algorithms.utils.assign_params import _assign_parameters, _get_parameters
-
 
 class TrotterQRTE(RealTimeEvolver):
     """Quantum Real Time Evolution using Trotterization.
@@ -165,16 +163,25 @@ def evolve(self, evolution_problem: TimeEvolutionProblem) -> TimeEvolutionResult
                 "The time evolution problem contained ``aux_operators`` but no estimator was "
                 "provided. The algorithm continues without calculating these quantities. "
             )
+
+        # ensure the hamiltonian is a sparse pauli op
         hamiltonian = evolution_problem.hamiltonian
         if not isinstance(hamiltonian, (Pauli, PauliSumOp, SparsePauliOp)):
             raise ValueError(
-                f"TrotterQRTE only accepts Pauli | PauliSumOp, {type(hamiltonian)} provided."
+                f"TrotterQRTE only accepts Pauli | PauliSumOp | SparsePauliOp, {type(hamiltonian)} "
+                "provided."
             )
+        if isinstance(hamiltonian, PauliSumOp):
+            hamiltonian = hamiltonian.primitive * hamiltonian.coeff
+        elif isinstance(hamiltonian, Pauli):
+            hamiltonian = SparsePauliOp(hamiltonian)
+
         t_param = evolution_problem.t_param
-        if t_param is not None and _get_parameters(hamiltonian.coeffs) != ParameterView([t_param]):
+        free_parameters = hamiltonian.parameters
+        if t_param is not None and free_parameters != ParameterView([t_param]):
             raise ValueError(
-                "Hamiltonian time parameter does not match evolution_problem.t_param "
-                "or contains multiple parameters"
+                f"Hamiltonian time parameters ({free_parameters}) do not match "
+                f"evolution_problem.t_param ({t_param})."
             )
 
         # make sure PauliEvolutionGate does not implement more than one Trotter step
@@ -213,9 +220,9 @@ def evolve(self, evolution_problem: TimeEvolutionProblem) -> TimeEvolutionResult
             # evolution for next step
             if t_param is not None:
                 time_value = (n + 1) * dt
-                bound_coeffs = _assign_parameters(hamiltonian.coeffs, [time_value])
+                bound_hamiltonian = hamiltonian.assign_parameters([time_value])
                 single_step_evolution_gate = PauliEvolutionGate(
-                    SparsePauliOp(hamiltonian.paulis, bound_coeffs),
+                    bound_hamiltonian,
                     dt,
                     synthesis=self.product_formula,
                 )

qiskit/algorithms/time_evolvers/variational/solvers/var_qte_linear_solver.py

@@ -23,8 +23,6 @@
 from qiskit.quantum_info import SparsePauliOp
 from qiskit.quantum_info.operators.base_operator import BaseOperator
 
-from qiskit.algorithms.utils.assign_params import _assign_parameters
-
 from ..variational_principles import VariationalPrinciple
 
 
@@ -115,13 +113,12 @@ def solve_lse(
 
         if self._time_param is not None:
             if time_value is not None:
-                bound_params_array = _assign_parameters(self._hamiltonian.coeffs, [time_value])
-                hamiltonian = SparsePauliOp(self._hamiltonian.paulis, bound_params_array)
+                hamiltonian = hamiltonian.assign_parameters([time_value])
             else:
                 raise ValueError(
-                    f"Providing a time_value is required for time-dependant hamiltonians, "
+                    "Providing a time_value is required for time-dependent hamiltonians, "
                     f"but got time_value = {time_value}. "
-                    f"Please provide a time_value to the solve_lse method."
+                    "Please provide a time_value to the solve_lse method."
                 )
 
         evolution_grad_lse_rhs = self._var_principle.evolution_gradient(

qiskit/quantum_info/operators/symplectic/sparse_pauli_op.py

@@ -13,14 +13,21 @@
 N-Qubit Sparse Pauli Operator class.
 """
 
+from __future__ import annotations
+
 from collections import defaultdict
+from collections.abc import Mapping, Sequence
 from numbers import Number
 from typing import Dict, Optional
+from copy import deepcopy
 
 import numpy as np
 import rustworkx as rx
 
 from qiskit._accelerate.sparse_pauli_op import unordered_unique
+from qiskit.circuit.parameter import Parameter
+from qiskit.circuit.parameterexpression import ParameterExpression
+from qiskit.circuit.parametertable import ParameterView
 from qiskit.exceptions import QiskitError
 from qiskit.quantum_info.operators.custom_iterator import CustomIterator
 from qiskit.quantum_info.operators.linear_op import LinearOp
@@ -112,10 +119,18 @@ def __init__(self, data, coeffs=None, *, ignore_pauli_phase=False, copy=True):
 
         pauli_list = PauliList(data.copy() if copy and hasattr(data, "copy") else data)
 
-        dtype = object if isinstance(coeffs, np.ndarray) and coeffs.dtype == object else complex
+        if isinstance(coeffs, np.ndarray) and coeffs.dtype == object:
+            dtype = object
+        elif coeffs is not None:
+            if not isinstance(coeffs, (np.ndarray, Sequence)):
+                coeffs = [coeffs]
+            if any(isinstance(coeff, ParameterExpression) for coeff in coeffs):
+                dtype = object
+            else:
+                dtype = complex
 
         if coeffs is None:
-            coeffs = np.ones(pauli_list.size, dtype=dtype)
+            coeffs = np.ones(pauli_list.size, dtype=complex)
         else:
             coeffs = np.array(coeffs, copy=copy, dtype=dtype)
 
@@ -997,6 +1012,58 @@ def group_commuting(self, qubit_wise=False):
             groups[color].append(idx)
         return [self[group] for group in groups.values()]
 
+    @property
+    def parameters(self) -> ParameterView:
+        r"""Return the free ``Parameter``\s in the coefficients."""
+        ret = set()
+        for coeff in self.coeffs:
+            if isinstance(coeff, ParameterExpression):
+                ret |= coeff.parameters
+        return ParameterView(ret)
+
+    def assign_parameters(
+        self,
+        parameters: Mapping[Parameter, complex | ParameterExpression]
+        | Sequence[complex | ParameterExpression],
+        inplace: bool = False,
+    ) -> SparsePauliOp | None:
+        r"""Bind the free ``Parameter``\s in the coefficients to provided values.
+
+        Args:
+            parameters: The values to bind the parameters to.
+            inplace: If ``False``, a copy of the operator with the bound parameters is returned.
+                If ``True`` the operator itself is modified.
+
+        Returns:
+            A copy of the operator with bound parameters, if ``inplace`` is ``False``, otherwise
+            ``None``.
+        """
+        if inplace:
+            bound = self
+        else:
+            bound = deepcopy(self)
+
+        # turn the parameters to a dictionary
+        if isinstance(parameters, Sequence):
+            free_parameters = bound.parameters
+            if len(parameters) != len(free_parameters):
+                raise ValueError(
+                    f"Mismatching number of values ({len(parameters)}) and parameters "
+                    f"({len(free_parameters)}). For partial binding please pass a dictionary of "
+                    "{parameter: value} pairs."
+                )
+            parameters = dict(zip(free_parameters, parameters))
+
+        for i, coeff in enumerate(bound.coeffs):
+            if isinstance(coeff, ParameterExpression):
+                for key in coeff.parameters & parameters.keys():
+                    coeff = coeff.assign(key, parameters[key])
+                if len(coeff.parameters) == 0:
+                    coeff = complex(coeff)
+                bound.coeffs[i] = coeff
+
+        return None if inplace else bound
+
 
 # Update docstrings for API docs
 generate_apidocs(SparsePauliOp)

releasenotes/notes/sparsepauliop-improved-parameter-support-413f7598bac72166.yaml

@@ -0,0 +1,28 @@
+features:
+  - |
+    Natively support the construction of :class:`.SparsePauliOp` objects with
+    :class:`.ParameterExpression` coefficients, without requiring the explicit construction
+    of an object-array. Now the following is supported::
+
+      from qiskit.circuit import Parameter
+      from qiskit.quantum_info import SparsePauliOp
+
+      x = Parameter("x")
+      op = SparsePauliOp(["Z", "X"], coeffs=[1, x])
+
+  - |
+    Added the :meth:`.SparsePauliOp.assign_parameters` method and
+    :attr:`.SparsePauliOp.parameters` attribute to assign and query unbound parameters
+    inside a :class:`.SparsePauliOp`. This function can for example be used as::
+
+      from qiskit.circuit import Parameter
+      from qiskit.quantum_info import SparsePauliOp
+
+      x = Parameter("x")
+      op = SparsePauliOp(["Z", "X"], coeffs=[1, x])
+
+      # free_params will be: ParameterView([x])
+      free_params = op.parameters
+
+      # assign the value 2 to the parameter x
+      bound = op.assign_parameters([2])

test/python/algorithms/time_evolvers/test_trotter_qrte.py

@@ -20,7 +20,6 @@
 from numpy.testing import assert_raises
 
 from qiskit.algorithms.time_evolvers import TimeEvolutionProblem, TrotterQRTE
-from qiskit.algorithms.utils.assign_params import _assign_parameters
 from qiskit.primitives import Estimator
 from qiskit import QuantumCircuit
 from qiskit.circuit.library import ZGate
@@ -245,11 +244,8 @@ def _get_expected_trotter_qrte(operator, time, num_timesteps, init_state, observ
         for n in range(num_timesteps):
             if t_param is not None:
                 time_value = (n + 1) * dt
-                bound_coeffs = _assign_parameters(operator.coeffs, [time_value])
-                ops = [
-                    Pauli(op).to_matrix() * np.real(coeff)
-                    for op, coeff in SparsePauliOp(operator.paulis, bound_coeffs).to_list()
-                ]
+                bound = operator.assign_parameters([time_value])
+                ops = [Pauli(op).to_matrix() * np.real(coeff) for op, coeff in bound.to_list()]
             for op in ops:
                 psi = expm(-1j * op * dt).dot(psi)
             observable_results.append(

test/python/quantum_info/operators/symplectic/test_sparse_pauli_op.py

@@ -21,6 +21,7 @@
 
 from qiskit import QiskitError
 from qiskit.circuit import Parameter, ParameterVector
+from qiskit.circuit.parametertable import ParameterView
 from qiskit.quantum_info.operators import Operator, Pauli, PauliList, PauliTable, SparsePauliOp
 from qiskit.test import QiskitTestCase
 
@@ -947,6 +948,33 @@ def test_dot_real(self):
         iz = SparsePauliOp("Z", 1j)
         self.assertEqual(x.dot(y), iz)
 
+    def test_get_parameters(self):
+        """Test getting the parameters."""
+        x, y = Parameter("x"), Parameter("y")
+        op = SparsePauliOp(["X", "Y", "Z"], coeffs=[1, x, x * y])
+
+        with self.subTest(msg="all parameters"):
+            self.assertEqual(ParameterView([x, y]), op.parameters)
+
+        op.assign_parameters({y: 2}, inplace=True)
+        with self.subTest(msg="after partial binding"):
+            self.assertEqual(ParameterView([x]), op.parameters)
+
+    def test_assign_parameters(self):
+        """Test assign parameters."""
+        x, y = Parameter("x"), Parameter("y")
+        op = SparsePauliOp(["X", "Y", "Z"], coeffs=[1, x, x * y])
+
+        # partial binding inplace
+        op.assign_parameters({y: 2}, inplace=True)
+        with self.subTest(msg="partial binding"):
+            self.assertListEqual(op.coeffs.tolist(), [1, x, 2 * x])
+
+        # bind via array
+        bound = op.assign_parameters([3])
+        with self.subTest(msg="fully bound"):
+            self.assertTrue(np.allclose(bound.coeffs.astype(complex), [1, 3, 6]))
+
 
 if __name__ == "__main__":
     unittest.main()