Combining readout circuits for QWC pauli strings in the measure_pauli_strings method (quantumlib#7416)

[PR quantumlib#7236](quantumlib#7236) adds the
feature to measure groups of qubit-wise-commuting Pauli operators
simultaneously.

However, the current implementation generates a separate readout circuit
for each individual Pauli operator within a group. This leads to
non-simultaneous measurements, which is corrected in this PR.

Author: ddddddanni

cirq-core/cirq/contrib/paulistring/pauli_string_measurement_with_readout_mitigation.py

@@ -28,7 +28,9 @@
 from cirq.experiments.readout_confusion_matrix import TensoredConfusionMatrices
 
 if TYPE_CHECKING:
-    from cirq.experiments import SingleQubitReadoutCalibrationResult
+    from cirq.experiments.single_qubit_readout_calibration import (
+        SingleQubitReadoutCalibrationResult,
+    )
     from cirq.study import ResultDict
 
 
@@ -217,6 +219,11 @@ def _normalize_input_paulis(
     return cast(dict[circuits.FrozenCircuit, list[list[ops.PauliString]]], circuits_to_pauli)
 
 
+def _extract_readout_qubits(pauli_strings: list[ops.PauliString]) -> list[ops.Qid]:
+    """Extracts unique qubits from a list of QWC Pauli strings."""
+    return sorted(set(q for ps in pauli_strings for q in ps.qubits))
+
+
 def _pauli_strings_to_basis_change_ops(
     pauli_strings: list[ops.PauliString], qid_list: list[ops.Qid]
 ):
@@ -315,16 +322,38 @@ def _process_pauli_measurement_results(
     for pauli_group_index, circuit_result in enumerate(circuit_results):
         measurement_results = circuit_result.measurements["m"]
         pauli_strs = pauli_string_groups[pauli_group_index]
+        pauli_readout_qubits = _extract_readout_qubits(pauli_strs)
+
+        calibration_result = (
+            calibration_results[tuple(pauli_readout_qubits)]
+            if disable_readout_mitigation is False
+            else None
+        )
 
         for pauli_str in pauli_strs:
             qubits_sorted = sorted(pauli_str.qubits)
             qubit_indices = [qubits.index(q) for q in qubits_sorted]
 
-            confusion_matrices = (
-                _build_many_one_qubits_confusion_matrix(calibration_results[tuple(qubits_sorted)])
-                if disable_readout_mitigation is False
-                else _build_many_one_qubits_empty_confusion_matrix(len(qubits_sorted))
-            )
+            if disable_readout_mitigation:
+                pauli_str_calibration_result = None
+                confusion_matrices = _build_many_one_qubits_empty_confusion_matrix(
+                    len(qubits_sorted)
+                )
+            else:
+                if calibration_result is None:
+                    # This case should be logically impossible if mitigation is on,
+                    # so we raise an error.
+                    raise ValueError(
+                        f"Readout mitigation is enabled, but no calibration result was "
+                        f"found for qubits {pauli_readout_qubits}."
+                    )
+                pauli_str_calibration_result = calibration_result.readout_result_for_qubits(
+                    qubits_sorted
+                )
+                confusion_matrices = _build_many_one_qubits_confusion_matrix(
+                    pauli_str_calibration_result
+                )
+
             tensored_cm = TensoredConfusionMatrices(
                 confusion_matrices,
                 [[q] for q in qubits_sorted],
@@ -356,11 +385,7 @@ def _process_pauli_measurement_results(
                     mitigated_stddev=d_m_with_coefficient,
                     unmitigated_expectation=unmitigated_value_with_coefficient,
                     unmitigated_stddev=d_unmit_with_coefficient,
-                    calibration_result=(
-                        calibration_results[tuple(qubits_sorted)]
-                        if disable_readout_mitigation is False
-                        else None
-                    ),
+                    calibration_result=pauli_str_calibration_result,
                 )
             )
 
@@ -428,8 +453,7 @@ def measure_pauli_strings(
     unique_qubit_tuples = set()
     for pauli_string_groups in normalized_circuits_to_pauli.values():
         for pauli_strings in pauli_string_groups:
-            for pauli_string in pauli_strings:
-                unique_qubit_tuples.add(tuple(sorted(pauli_string.qubits)))
+            unique_qubit_tuples.add(tuple(_extract_readout_qubits(pauli_strings)))
     # qubits_list is a list of qubit tuples
     qubits_list = sorted(unique_qubit_tuples)
 

cirq-core/cirq/contrib/paulistring/pauli_string_measurement_with_readout_mitigation_test.py

@@ -23,7 +23,10 @@
 
 import cirq
 from cirq.contrib.paulistring import measure_pauli_strings
-from cirq.experiments import SingleQubitReadoutCalibrationResult
+from cirq.contrib.paulistring.pauli_string_measurement_with_readout_mitigation import (
+    _process_pauli_measurement_results,
+)
+from cirq.experiments.single_qubit_readout_calibration import SingleQubitReadoutCalibrationResult
 from cirq.experiments.single_qubit_readout_calibration_test import NoisySingleQubitReadoutSampler
 
 
@@ -867,3 +870,37 @@ def test_group_paulis_type_mismatch() -> None:
         measure_pauli_strings(
             circuits_to_pauli, cirq.Simulator(), 1000, 1000, 1000, np.random.default_rng()
         )
+
+
+def test_process_pauli_measurement_results_raises_error_on_missing_calibration() -> None:
+    """Test that the function raises an error if the calibration result is missing."""
+    qubits: list[cirq.Qid] = [q for q in cirq.LineQubit.range(5)]
+
+    measurement_op = cirq.measure(*qubits, key='m')
+    test_circuits = list[cirq.Circuit]()
+    for _ in range(3):
+        circuit_list = []
+
+        circuit = _create_ghz(5, qubits) + measurement_op
+        circuit_list.append(circuit)
+    test_circuits.extend(circuit_list)
+
+    pauli_strings = [_generate_random_pauli_string(qubits, True) for _ in range(3)]
+    sampler = cirq.Simulator()
+
+    circuit_results = sampler.run_batch(test_circuits, repetitions=1000)
+
+    empty_calibration_result_dict = {tuple(qubits): None}
+
+    with pytest.raises(
+        ValueError,
+        match="Readout mitigation is enabled, but no calibration result was found for qubits",
+    ):
+        _process_pauli_measurement_results(
+            qubits,
+            [pauli_strings],
+            circuit_results[0],  # type: ignore[arg-type]
+            empty_calibration_result_dict,  # type: ignore[arg-type]
+            1000,
+            1.0,
+        )

cirq-core/cirq/experiments/single_qubit_readout_calibration.py

@@ -179,6 +179,17 @@ def plot_integrated_histogram(
         ax.set_ylabel('Percentile')
         return ax
 
+    def readout_result_for_qubits(
+        self, readout_qubits: list[ops.Qid]
+    ) -> SingleQubitReadoutCalibrationResult:
+        """Builds a calibration result for the specific readout qubits."""
+        return SingleQubitReadoutCalibrationResult(
+            zero_state_errors={qubit: self.zero_state_errors[qubit] for qubit in readout_qubits},
+            one_state_errors={qubit: self.one_state_errors[qubit] for qubit in readout_qubits},
+            timestamp=self.timestamp,
+            repetitions=self.repetitions,
+        )
+
     @classmethod
     def _from_json_dict_(
         cls, zero_state_errors, one_state_errors, repetitions, timestamp, **kwargs