Fix circuit ordering in InterleavedRB (backport #1191)

qiskit_experiments/library/randomized_benchmarking/interleaved_rb_experiment.py

@@ -12,6 +12,7 @@
 """
 Interleaved RB Experiment class.
 """
+import itertools
 import warnings
 from typing import Union, Iterable, Optional, List, Sequence, Tuple
 
@@ -25,6 +26,7 @@
 from qiskit.quantum_info import Clifford
 from qiskit.transpiler.exceptions import TranspilerError
 from qiskit_experiments.warnings import deprecate_arguments
+from qiskit_experiments.framework import Options
 from qiskit_experiments.framework.backend_timing import BackendTiming
 from .clifford_utils import _truncate_inactive_qubits
 from .clifford_utils import num_from_1q_circuit, num_from_2q_circuit
@@ -63,6 +65,7 @@ def __init__(
         num_samples: int = 3,
         seed: Optional[Union[int, SeedSequence, BitGenerator, Generator]] = None,
         full_sampling: bool = False,
+        circuit_order: str = "RIRIRI",
     ):
         """Initialize an interleaved randomized benchmarking experiment.
 
@@ -87,6 +90,9 @@ def __init__(
                            all lengths. If False for sample of lengths longer
                            sequences are constructed by appending additional
                            Clifford samples to shorter sequences.
+            circuit_order: How to order the reference and the interleaved circuits.
+                ``"RIRIRI"`` (default) - Alternate a reference and an interleaved circuit. Or
+                ``"RRRIII"`` - Push all reference circuits first, then all interleaved ones.
 
         Raises:
             QiskitError: When interleaved_element has different number of qubits
@@ -153,9 +159,25 @@ def __init__(
             self._interleaved_cliff = interleaved_clifford.to_circuit()
         self._interleaved_element = interleaved_element  # Original interleaved element
         self._interleaved_op = None  # Transpiled interleaved element for speed
+        self.set_experiment_options(circuit_order=circuit_order)
         self.analysis = InterleavedRBAnalysis()
         self.analysis.set_options(outcome="0" * self.num_qubits)
 
+    @classmethod
+    def _default_experiment_options(cls) -> Options:
+        """Default InterleavedRB experiment options.
+
+        Experiment Options:
+            circuit_order (str): How to order the reference and the interleaved circuits.
+                ``"RIRIRI"`` (alternate a reference and an interleaved circuit) or
+                ``"RRRIII"`` (push all reference circuits first, then all interleaved ones).
+        """
+        options = super()._default_experiment_options()
+        options.update_options(
+            circuit_order="RIRIRI",
+        )
+        return options
+
     def circuits(self) -> List[QuantumCircuit]:
         """Return a list of RB circuits.
 
@@ -195,7 +217,11 @@ def circuits(self) -> List[QuantumCircuit]:
                 "physical_qubits": self.physical_qubits,
                 "interleaved": True,
             }
-        return reference_circuits + interleaved_circuits
+
+        if self.experiment_options.circuit_order == "RRRIII":
+            return reference_circuits + interleaved_circuits
+        # Default order: RIRIRI
+        return list(itertools.chain.from_iterable(zip(reference_circuits, interleaved_circuits)))
 
     def _to_instruction(
         self, elem: SequenceElementType, basis_gates: Optional[Tuple[str]] = None

releasenotes/notes/irb-circuit-order-619845a707519c44.yaml

@@ -0,0 +1,15 @@
+---
+features:
+  - |
+    A new experiment option ``circuit_order`` was added to :class:`~.InterleavedRB`.
+    It allows to change the order of the reference and the interleaved circuits
+    and hence slightly alter the impact of noise on interleaved RB results.
+    The default value is set to ``"RIRIRI"`` that alternate a reference and
+    an interleaved circuit.
+fixes:
+  - |
+    Changed the ordering of circuits generated by :class:`~.InterleavedRB` back to
+    RIRIRI (R: Reference, I: Interleaved) order.
+    It was accidentally changed into RRRIII order in
+    `#898 <https://github.com/Qiskit/qiskit-experiments/pull/898>`_.
+    Before that, it had been RIRIRI order.

test/library/randomized_benchmarking/test_interleaved_rb.py

@@ -104,10 +104,16 @@ def test_generate_interleaved_circuits(self, interleaved_element, qubits, length
             interleaved_element=interleaved_element,
             physical_qubits=qubits,
             lengths=[length],
-            num_samples=1,
+            num_samples=2,
         )
         circuits = exp.circuits()
         self.assertAllIdentity(circuits)
+        # check order of circuits
+        for i, circ in enumerate(circuits):
+            if i % 2 == 0:  # even <=> reference sequence
+                self.assertFalse(circ.metadata["interleaved"])
+            else:  # odd <=> interleaved sequence
+                self.assertTrue(circ.metadata["interleaved"])
 
     @data([SXGate(), [3], 4], [CXGate(), [4, 7], 5])
     @unpack