Truncate save_expval (Qiskit#2216)

* truncate save_expval

* fix truncation

* fix truncation

* add num_original_qubits to aer_circuit to get num_qubits without ancilla qubits

* Fix adding qubitset

* add test case, release note and fix docs

* fix doc

* fix doc

* fix doc

* fix doc

* fix doc

* fix doc

* fix doc

* fix doc

* change truncation strategy

* format

* remove print

* no truncation when circuit is empty

* Update VERSION.txt

revert to 0.15.0

---------

Co-authored-by: Hiroshi Horii <hhorii@users.noreply.github.com>

releasenotes/notes/truncate_expval-7fa814c732cca8db.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    This fix truncates qubits of save_expval operation when EstimatorV2
+    is made from existing backends using `from_backend`.
+    By transpiling on the existing backends, ancilla qubits are filled
+    to all the qubits that causes memory error on the simulator.
+    So Aer removes unused qubits for save_expval operation.

src/framework/circuit.hpp

@@ -46,6 +46,7 @@ class Circuit {
   uint_t num_qubits = 0;    // maximum number of qubits needed for ops
   uint_t num_memory = 0;    // maximum number of memory clbits needed for ops
   uint_t num_registers = 0; // maximum number of registers clbits needed for ops
+  uint_t num_original_qubits = 0; // number of qubits without ancilla qubits
 
   // Measurement params
   bool has_conditional = false; // True if any ops are conditional
@@ -419,7 +420,20 @@ void Circuit::reset_metadata() {
 void Circuit::add_op_metadata(const Op &op) {
   has_conditional |= op.conditional;
   opset_.insert(op);
-  qubitset_.insert(op.qubits.begin(), op.qubits.end());
+  if (!qubitset_.empty() &&
+      (op.type == OpType::save_expval || op.type == OpType::save_expval_var)) {
+    for (int_t j = 0; j < op.expval_params.size(); j++) {
+      const std::string &pauli = std::get<0>(op.expval_params[j]);
+      for (int_t i = 0; i < op.qubits.size(); i++) {
+        // add qubit with non-I operator
+        if (pauli[pauli.size() - 1 - i] != 'I') {
+          qubitset_.insert(op.qubits[i]);
+        }
+      }
+    }
+  } else {
+    qubitset_.insert(op.qubits.begin(), op.qubits.end());
+  }
   memoryset_.insert(op.memory.begin(), op.memory.end());
   registerset_.insert(op.registers.begin(), op.registers.end());
 
@@ -589,6 +603,19 @@ void Circuit::set_params(bool truncation) {
     }
     if (remapped_qubits) {
       remap_qubits(ops[pos]);
+    } else if (truncation && qubitmap_.size() < ops[pos].qubits.size()) {
+      // truncate save_expval here when remap is not needed
+      if (ops[pos].type == OpType::save_expval ||
+          ops[pos].type == OpType::save_expval_var) {
+        int_t nparams = ops[pos].expval_params.size();
+        for (int_t i = 0; i < nparams; i++) {
+          std::string &pauli = std::get<0>(ops[pos].expval_params[i]);
+          std::string new_pauli;
+          new_pauli.assign(pauli.end() - qubitmap_.size(), pauli.end());
+          pauli = new_pauli;
+        }
+        ops[pos].qubits.resize(qubitmap_.size());
+      }
     }
     if (pos != op_idx) {
       ops[op_idx] = std::move(ops[pos]);
@@ -653,11 +680,29 @@ void Circuit::set_params(bool truncation) {
 }
 
 void Circuit::remap_qubits(Op &op) const {
-  reg_t new_qubits;
-  for (auto &qubit : op.qubits) {
-    new_qubits.push_back(qubitmap_.at(qubit));
+  // truncate save_expval
+  if (op.type == OpType::save_expval || op.type == OpType::save_expval_var) {
+    int_t nparams = op.expval_params.size();
+    for (int_t i = 0; i < nparams; i++) {
+      std::string &pauli = std::get<0>(op.expval_params[i]);
+      std::string new_pauli;
+      new_pauli.resize(qubitmap_.size());
+      for (auto q = qubitmap_.cbegin(); q != qubitmap_.cend(); q++) {
+        new_pauli[qubitmap_.size() - 1 - q->second] =
+            pauli[pauli.size() - 1 - q->first];
+      }
+      pauli = new_pauli;
+    }
+    for (int_t i = 0; i < qubitmap_.size(); i++) {
+      op.qubits[i] = i;
+    }
+  } else {
+    reg_t new_qubits;
+    for (auto &qubit : op.qubits) {
+      new_qubits.push_back(qubitmap_.at(qubit));
+    }
+    op.qubits = std::move(new_qubits);
   }
-  op.qubits = std::move(new_qubits);
 }
 
 bool Circuit::check_result_ancestor(

test/terra/primitives/test_estimator_v2.py

@@ -18,6 +18,7 @@
 from test.terra.common import QiskitAerTestCase
 
 import numpy as np
+from qiskit import transpile
 from qiskit.circuit import Parameter, QuantumCircuit
 from qiskit.circuit.library import RealAmplitudes
 from qiskit.primitives import StatevectorEstimator
@@ -26,6 +27,7 @@
 from qiskit.primitives.containers.observables_array import ObservablesArray
 from qiskit.quantum_info import SparsePauliOp
 from qiskit.transpiler.preset_passmanagers import generate_preset_pass_manager
+from qiskit.providers.fake_provider import GenericBackendV2
 
 from qiskit_aer import AerSimulator
 from qiskit_aer.primitives import EstimatorV2
@@ -407,6 +409,42 @@ def test_metadata(self):
             {"target_precision": 0.1, "circuit_metadata": qc2.metadata},
         )
 
+    def test_truncate(self):
+        """Test for truncation of save_expval"""
+        qc = QuantumCircuit(2, 2)
+        qc.h(0)
+        qc.cx(0, 1)
+        qc.append(RealAmplitudes(num_qubits=2, reps=2), [0, 1])
+        backend_2 = GenericBackendV2(num_qubits=2)
+        backend_5 = GenericBackendV2(num_qubits=5)
+
+        qc_2 = transpile(qc, backend_2, optimization_level=0)
+        qc_5 = transpile(qc, backend_5, optimization_level=0)
+
+        estimator_2 = EstimatorV2.from_backend(backend_2, options=self._options)
+        estimator_5 = EstimatorV2.from_backend(backend_5, options=self._options)
+
+        H1 = self.observable
+        H1_2 = H1.apply_layout(qc_2.layout)
+        H1_5 = H1.apply_layout(qc_5.layout)
+        theta1 = [0, 1, 1, 2, 3, 5]
+
+        result_2 = estimator_2.run(
+            [
+                (qc_2, [H1_2], [theta1]),
+            ],
+            precision=0.01,
+        ).result()
+        result_5 = estimator_5.run(
+            [
+                (qc_5, [H1_5], [theta1]),
+            ],
+            precision=0.01,
+        ).result()
+        self.assertAlmostEqual(
+            result_5[0].data["evs"][0], result_2[0].data["evs"][0], delta=self._rtol
+        )
+
 
 if __name__ == "__main__":
     unittest.main()