Improve performance of Estimator and Sampler by avoiding deep cop…

…y at `circuit_to_instruction`. (#8403)

* add simplified circuit_to_instruction

* add a safe guard

* optimize sampler as well as estimator

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

qiskit/primitives/estimator.py

@@ -30,7 +30,7 @@
 from .base_estimator import BaseEstimator
 from .estimator_result import EstimatorResult
 from .primitive_job import PrimitiveJob
-from .utils import init_circuit, init_observable
+from .utils import bound_circuit_to_instruction, init_circuit, init_observable
 
 
 class Estimator(BaseEstimator):
@@ -114,7 +114,7 @@ def _call(
                     f"The number of qubits of a circuit ({circ.num_qubits}) does not match "
                     f"the number of qubits of a observable ({obs.num_qubits})."
                 )
-            final_state = Statevector(circ)
+            final_state = Statevector(bound_circuit_to_instruction(circ))
             expectation_value = final_state.expectation_value(obs)
             if shots is None:
                 expectation_values.append(expectation_value)

qiskit/primitives/sampler.py

@@ -29,7 +29,7 @@
 from .base_sampler import BaseSampler
 from .primitive_job import PrimitiveJob
 from .sampler_result import SamplerResult
-from .utils import final_measurement_mapping, init_circuit
+from .utils import bound_circuit_to_instruction, final_measurement_mapping, init_circuit
 
 
 class Sampler(BaseSampler):
@@ -113,7 +113,7 @@ def _call(
             )
             qargs_list.append(self._qargs_list[i])
         probabilities = [
-            Statevector(circ).probabilities(qargs=qargs)
+            Statevector(bound_circuit_to_instruction(circ)).probabilities(qargs=qargs)
             for circ, qargs in zip(bound_circuits, qargs_list)
         ]
         if shots is not None:

qiskit/primitives/utils.py

@@ -15,7 +15,7 @@
 
 from __future__ import annotations
 
-from qiskit.circuit import ParameterExpression, QuantumCircuit
+from qiskit.circuit import ParameterExpression, QuantumCircuit, Instruction
 from qiskit.extensions.quantum_initializer.initializer import Initialize
 from qiskit.opflow import PauliSumOp
 from qiskit.quantum_info import SparsePauliOp, Statevector
@@ -111,3 +111,35 @@ def final_measurement_mapping(circuit: QuantumCircuit) -> dict[int, int]:
     # Sort so that classical bits are in numeric order low->high.
     mapping = dict(sorted(mapping.items(), key=lambda item: item[1]))
     return mapping
+
+
+def bound_circuit_to_instruction(circuit: QuantumCircuit) -> Instruction:
+    """Build an :class:`~qiskit.circuit.Instruction` object from
+    a :class:`~qiskit.circuit.QuantumCircuit`
+
+    This is a specialized version of :func:`~qiskit.converters.circuit_to_instruction`
+    to avoid deep copy. This requires a quantum circuit whose parameters are all bound.
+    Because this does not take a copy of the input circuit, this assumes that the input
+    circuit won't be modified.
+
+    If https://github.com/Qiskit/qiskit-terra/issues/7983 is resolved,
+    we can remove this function.
+
+    Args:
+        circuit(QuantumCircuit): Input quantum circuit
+
+    Returns:
+        An :class:`~qiskit.circuit.Instruction` object
+    """
+    if len(circuit.qregs) > 1:
+        return circuit.to_instruction()
+
+    # len(circuit.qregs) == 1 -> No need to flatten qregs
+    inst = Instruction(
+        name=circuit.name,
+        num_qubits=circuit.num_qubits,
+        num_clbits=circuit.num_clbits,
+        params=[],
+    )
+    inst.definition = circuit
+    return inst