Bug fix in conversion of PauliNoise to Qulacs

packages/qulacs/tests/qulacs/circuit/test_noise_model_qulacs.py

@@ -29,6 +29,7 @@
     KrausNoise,
     MeasurementNoise,
     NoiseModel,
+    PauliNoise,
     PhaseFlipNoise,
 )
 from quri_parts.qulacs.circuit.noise import convert_circuit_with_noise_model
@@ -288,3 +289,40 @@ def test_convert_empty_circuit() -> None:
     noise_model_readout = NoiseModel([MeasurementNoise([BitFlipNoise(1.0)])])
     converted_readout = convert_circuit_with_noise_model(circuit, noise_model_readout)
     assert converted_readout.get_gate_count() == 1
+
+
+def test_convert_pauli_noise() -> None:
+    circuit = QuantumCircuit(3)
+    circuit.add_H_gate(2)
+    circuit.add_X_gate(0)
+    circuit.add_CNOT_gate(2, 1)
+    circuit.add_Z_gate(2)
+
+    noises = [
+        PauliNoise(
+            pauli_list=[[1, 2], [2, 3]],
+            prob_list=[0.001, 0.002],
+            qubit_indices=[1, 2],
+            target_gates=[names.CNOT],
+        )
+    ]
+    model = NoiseModel(noises)
+
+    converted = convert_circuit_with_noise_model(circuit, model)
+    expected_gates = [
+        qulacs.gate.H(2),
+        qulacs.gate.X(0),
+        qulacs.gate.CNOT(2, 1),
+        qulacs.gate.Probabilistic(
+            [0.001, 0.002],
+            [
+                qulacs.gate.Pauli([1, 2], [1, 2]),
+                qulacs.gate.Pauli([1, 2], [2, 3]),
+            ],
+        ),
+        qulacs.gate.Z(2),
+    ]
+
+    assert converted.get_gate_count() == len(expected_gates)
+    for i, expected_gate in enumerate(expected_gates):
+        assert converted.get_gate(i).to_json() == expected_gate.to_json()

packages/rust/src/qulacs/noise.rs

@@ -18,30 +18,36 @@ fn make_dense_matrix<'py>(
 
 fn convert_add_pauli_noise<'py>(
     py: Python<'py>,
-    qubits: &Vec<usize>,
+    _qubits: &Vec<usize>,
     pauli_noise: &GateNoiseInstruction,
     qulacs_circuit: Bound<'py, PyAny>,
+    fill_identity: bool,
 ) -> PyResult<Bound<'py, PyAny>> {
     let module = py.import_bound("qulacs.gate")?;
     let mut probs = pauli_noise.prob_list.clone();
-    let paulies = pauli_noise.pauli_list.clone();
     let mut gates = vec![];
     for pauli in &pauli_noise.pauli_list {
         gates.push(
             module
                 .getattr("Pauli")?
-                .call1((qubits.clone(), pauli.clone()))?,
+                .call1((pauli_noise.qubit_indices.clone(), pauli.clone()))?,
         );
     }
-    let psum: f64 = probs.iter().sum();
-    if psum < 1.0 {
-        gates.push(module.getattr("Identity")?.call1((qubits.len(),))?);
-        probs.push(1.0 - psum);
+    if fill_identity {
+        let psum: f64 = probs.iter().sum();
+        if psum < 1.0 {
+            gates.push(
+                module
+                    .getattr("Identity")?
+                    .call1((pauli_noise.qubit_indices[0],))?,
+            );
+            probs.push(1.0 - psum);
+        }
     }
     let prob_gate = py
         .import_bound("qulacs.gate")?
         .getattr("Probabilistic")?
-        .call1((probs, paulies))?;
+        .call1((probs, gates))?;
     qulacs_circuit.call_method1("add_gate", (prob_gate,))?;
     Ok(qulacs_circuit)
 }
@@ -128,7 +134,7 @@ fn convert_add_noise<'py>(
             )?;
         }
         "PauliNoise" | "GeneralDepolarizingNoise" => {
-            circuit = convert_add_pauli_noise(py, qubits, noise, circuit)?;
+            circuit = convert_add_pauli_noise(py, qubits, noise, circuit, false)?;
         }
         "ProbabilisticNoise" => {
             circuit = convert_add_probabilistic_noise(py, qubits, noise, circuit)?;