Add meas_level, meas_return, and noise_model options to BackendSamaplerV2

This change adds support to the `BackendSamplerV2` class so that it will
pass through the `meas_level`, `meas_return`, and `noise_model` options
passed to it through to the underlying `BackendV2`'s `run()` method. For
the sake of compatibility with backends that might not expect those
options, it does not pass default values for them (like the previously
defined options for the class) if the default `None` value is not
overridden.

Additionally, to support `meas_level=1`, the results processing code
checks the `meas_level` option and handles `meas_level=1` data
appropriately, rather than always assuming the returned data is level 2.

Author: wshanks

qiskit/primitives/backend_sampler_v2.py

@@ -17,12 +17,13 @@
 import warnings
 from collections import defaultdict
 from dataclasses import dataclass
-from typing import Iterable
+from typing import Any, Iterable
 
 import numpy as np
 from numpy.typing import NDArray
 
 from qiskit.circuit import QuantumCircuit
+from qiskit.exceptions import QiskitError
 from qiskit.primitives.backend_estimator import _run_circuits
 from qiskit.primitives.base import BaseSamplerV2
 from qiskit.primitives.containers import (
@@ -53,6 +54,21 @@ class Options:
     Default: None.
     """
 
+    noise_model: Any | None = None
+    """A ``NoiseModel`` to pass to pass through a simulator backend (like ones from qiskit-aer)
+    Default: None (option not passed to backend's ``run`` method)
+    """
+
+    meas_level: int | None = None
+    """Measurement level for the backend to return.
+    Default: None (option not passed to backend's ``run`` method)
+    """
+
+    meas_return: str | None = None
+    """Measurement return format for the backend to return.
+    Default: None (option not passed to backend's ``run`` method)
+    """
+
 
 @dataclass
 class _MeasureInfo:
@@ -165,13 +181,21 @@ def _run_pubs(self, pubs: list[SamplerPub], shots: int) -> list[SamplerPubResult
         for circuits in bound_circuits:
             flatten_circuits.extend(np.ravel(circuits).tolist())
 
+        # Put options in dict to unpacked below so that unset options are left
+        # out rather than being passed as None
+        run_opts = {
+            k: getattr(self._options, k)
+            for k in ("noise_model", "meas_return", "meas_level")
+            if getattr(self._options, k) is not None
+        }
         # run circuits
         results, _ = _run_circuits(
             flatten_circuits,
             self._backend,
             memory=True,
             shots=shots,
             seed_simulator=self._options.seed_simulator,
+            **run_opts,
         )
         result_memory = _prepare_memory(results)
 
@@ -189,6 +213,7 @@ def _run_pubs(self, pubs: list[SamplerPub], shots: int) -> list[SamplerPubResult
                     meas_info,
                     max_num_bytes,
                     pub.circuit.metadata,
+                    meas_level=self._options.meas_level,
                 )
             )
             start = end
@@ -203,22 +228,36 @@ def _postprocess_pub(
         meas_info: list[_MeasureInfo],
         max_num_bytes: int,
         circuit_metadata: dict,
+        meas_level: int | None = None,
     ) -> SamplerPubResult:
-        """Converts the memory data into an array of bit arrays with the shape of the pub."""
-        arrays = {
-            item.creg_name: np.zeros(shape + (shots, item.num_bytes), dtype=np.uint8)
-            for item in meas_info
-        }
-        memory_array = _memory_array(result_memory, max_num_bytes)
-
-        for samples, index in zip(memory_array, np.ndindex(*shape)):
-            for item in meas_info:
-                ary = _samples_to_packed_array(samples, item.num_bits, item.start)
-                arrays[item.creg_name][index] = ary
+        """Converts the memory data into a sampler pub result
 
-        meas = {
-            item.creg_name: BitArray(arrays[item.creg_name], item.num_bits) for item in meas_info
-        }
+        For level 2 data, the memory data are stored in an array of bit arrays
+        with the shape of the pub. For level 1 data, the data are stored in a
+        complex numpy array.
+        """
+        if meas_level == 2 or meas_level is None:
+            arrays = {
+                item.creg_name: np.zeros(shape + (shots, item.num_bytes), dtype=np.uint8)
+                for item in meas_info
+            }
+            memory_array = _memory_array(result_memory, max_num_bytes)
+
+            for samples, index in zip(memory_array, np.ndindex(*shape)):
+                for item in meas_info:
+                    ary = _samples_to_packed_array(samples, item.num_bits, item.start)
+                    arrays[item.creg_name][index] = ary
+
+            meas = {
+                item.creg_name: BitArray(arrays[item.creg_name], item.num_bits) for item in meas_info
+            }
+        elif meas_level == 1:
+            raw = np.array(result_memory)
+            cplx = raw[..., 0] + 1j * raw[..., 1]
+            cplx = np.reshape(cplx, (*shape, *cplx.shape[1:]))
+            meas = {item.creg_name: cplx for item in meas_info}
+        else:
+            raise QiskitError(f"Unsupported meas_level: {meas_level}")
         return SamplerPubResult(
             DataBin(**meas, shape=shape),
             metadata={"shots": shots, "circuit_metadata": circuit_metadata},