Add Statevector-based BaseSamplerV2 implementation

Co-authored-by: Ian Hincks <ian.hincks@gmail.com>

qiskit/primitives/__init__.py

@@ -62,3 +62,4 @@
 from .containers import BindingsArray, ObservablesArray, PrimitiveResult, PubResult, SamplerPub
 from .estimator import Estimator
 from .sampler import Sampler
+from .statevector_sampler import Sampler as StatevectorSampler

qiskit/primitives/base/base_sampler.py

@@ -278,7 +278,7 @@ class BaseSamplerV2:
 
     @abstractmethod
     def run(
-        self, pubs: Iterable[SamplerPubLike], shots: int | None = None
+        self, pubs: Iterable[SamplerPubLike], *, shots: int | None = None
     ) -> BasePrimitiveJob[PrimitiveResult[PubResult]]:
         """Run and collect samples from each pub.
 

qiskit/primitives/statevector_sampler.py

@@ -0,0 +1,215 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2023, 2024.
+#
+# This code is licensed under the Apache License, Version 2.0. You may
+# obtain a copy of this license in the LICENSE.txt file in the root directory
+# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# Any modifications or derivative works of this code must retain this
+# copyright notice, and modified files need to carry a notice indicating
+# that they have been altered from the originals.
+"""
+Statevector Sampler class
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Iterable
+
+import numpy as np
+from numpy.typing import NDArray
+
+from qiskit import ClassicalRegister, QiskitError, QuantumCircuit
+from qiskit.circuit import ControlFlowOp
+from qiskit.quantum_info import Statevector
+
+from .base import BaseSamplerV2
+from .base.validation import _has_measure
+from .containers import (
+    BitArray,
+    PrimitiveResult,
+    PubResult,
+    SamplerPub,
+    SamplerPubLike,
+    make_data_bin,
+)
+from .containers.bit_array import _min_num_bytes
+from .primitive_job import PrimitiveJob
+from .utils import bound_circuit_to_instruction
+
+
+@dataclass
+class _MeasureInfo:
+    creg_name: str
+    num_bits: int
+    num_bytes: int
+    qreg_indices: list[int]
+
+
+class Sampler(BaseSamplerV2):
+    """
+    Simple implementation of :class:`BaseSamplerV2` with Statevector.
+    """
+
+    _DEFAULT_SHOTS: int = 512
+
+    def __init__(self, *, seed: np.random.Generator | int | None = None):
+        """
+        Args:
+            seed: The seed for random number generator.
+        """
+        self._seed = seed
+        if isinstance(self._seed, np.random.Generator):
+            self._rng = self._seed
+        else:
+            self._rng = np.random.default_rng(self._seed)
+
+    @property
+    def seed(self) -> np.random.Generator | int | None:
+        """Return the seed for random number generator.
+
+        Returns:
+            np.random.Generator | int | None: The seed for random number generator.
+        """
+        return self._seed
+
+    def run(
+        self, pubs: Iterable[SamplerPubLike], shots: int | None = None
+    ) -> PrimitiveJob[PrimitiveResult[PubResult]]:
+        job: PrimitiveJob[PubResult] = PrimitiveJob(self._run, pubs, shots)
+        job._submit()
+        return job
+
+    def _run(
+        self, pubs: Iterable[SamplerPubLike], shots: int | None = None
+    ) -> PrimitiveResult[PrimitiveResult[PubResult]]:
+        if shots is None:
+            shots = self._DEFAULT_SHOTS
+        coerced_pubs = [SamplerPub.coerce(pub, shots) for pub in pubs]
+        for pub in coerced_pubs:
+            pub.validate()
+
+        results = []
+        for pub in coerced_pubs:
+            circuit, qargs, meas_info = _preprocess_circuit(pub.circuit)
+            bound_circuits = pub.parameter_values.bind_all(circuit)
+            arrays = {
+                item.creg_name: np.zeros(
+                    bound_circuits.shape + (pub.shots, item.num_bytes), dtype=np.uint8
+                )
+                for item in meas_info
+            }
+            for index in np.ndindex(*bound_circuits.shape):
+                bound_circuit = bound_circuits[index]
+                final_state = Statevector(bound_circuit_to_instruction(bound_circuit))
+                final_state.seed(self._rng)
+                if qargs:
+                    samples = final_state.sample_memory(shots=pub.shots, qargs=qargs)
+                else:
+                    samples = [""] * pub.shots
+                samples_array = np.array(
+                    [np.fromiter(sample, dtype=np.uint8) for sample in samples]
+                )
+                for item in meas_info:
+                    ary = _samples_to_packed_array(samples_array, item.num_bits, item.qreg_indices)
+                    arrays[item.creg_name][index] = ary
+
+            data_bin_cls = make_data_bin(
+                [(item.creg_name, BitArray) for item in meas_info],
+                shape=bound_circuits.shape,
+            )
+            meas = {
+                item.creg_name: BitArray(arrays[item.creg_name], item.num_bits)
+                for item in meas_info
+            }
+            data_bin = data_bin_cls(**meas)
+            results.append(PubResult(data_bin, metadata={"shots": pub.shots}))
+        return PrimitiveResult(results)
+
+
+def _preprocess_circuit(circuit: QuantumCircuit):
+    num_bits_dict = {creg.name: creg.size for creg in circuit.cregs}
+    mapping = _final_measurement_mapping(circuit)
+    qargs = sorted(set(mapping.values()))
+    qargs_index = {v: k for k, v in enumerate(qargs)}
+    circuit = circuit.remove_final_measurements(inplace=False)
+    if _has_control_flow(circuit):
+        raise QiskitError("StatevectorSampler cannot handle ControlFlowOp")
+    if _has_measure(circuit):
+        raise QiskitError("StatevectorSampler cannot handle mid-circuit measurements")
+    # num_qubits is used as sentinel to fill 0 in _samples_to_packed_array
+    sentinel = len(qargs)
+    indices = {key: [sentinel] * val for key, val in num_bits_dict.items()}
+    for key, qreg in mapping.items():
+        creg, ind = key
+        indices[creg.name][ind] = qargs_index[qreg]
+    meas_info = [
+        _MeasureInfo(
+            creg_name=name,
+            num_bits=num_bits,
+            num_bytes=_min_num_bytes(num_bits),
+            qreg_indices=indices[name],
+        )
+        for name, num_bits in num_bits_dict.items()
+    ]
+    return circuit, qargs, meas_info
+
+
+def _samples_to_packed_array(
+    samples: NDArray[np.uint8], num_bits: int, indices: list[int]
+) -> NDArray[np.uint8]:
+    # samples of `Statevector.sample_memory` will be in the order of
+    # qubit_last, ..., qubit_1, qubit_0.
+    # reverse the sample order into qubit_0, qubit_1, ..., qubit_last and
+    # pad 0 in the rightmost to be used for the sentinel introduced by _preprocess_circuit.
+    ary = np.pad(samples[:, ::-1], ((0, 0), (0, 1)), constant_values=0)
+    # place samples in the order of clbit_last, ..., clbit_1, clbit_0
+    ary = ary[:, indices[::-1]]
+    # pad 0 in the left to align the number to be mod 8
+    # since np.packbits(bitorder='big') pads 0 to the right.
+    pad_size = -num_bits % 8
+    ary = np.pad(ary, ((0, 0), (pad_size, 0)), constant_values=0)
+    # pack bits in big endian order
+    ary = np.packbits(ary, axis=-1)
+    return ary
+
+
+def _final_measurement_mapping(circuit: QuantumCircuit) -> dict[tuple[ClassicalRegister, int], int]:
+    """Return the final measurement mapping for the circuit.
+
+    Parameters:
+        circuit: Input quantum circuit.
+
+    Returns:
+        Mapping of classical bits to qubits for final measurements.
+    """
+    active_qubits = set(range(circuit.num_qubits))
+    active_cbits = set(range(circuit.num_clbits))
+
+    # Find final measurements starting in back
+    mapping = {}
+    for item in circuit[::-1]:
+        if item.operation.name == "measure":
+            loc = circuit.find_bit(item.clbits[0])
+            cbit = loc.index
+            creg = loc.registers[0]
+            qbit = circuit.find_bit(item.qubits[0]).index
+            if cbit in active_cbits and qbit in active_qubits:
+                mapping[creg] = qbit
+                active_cbits.remove(cbit)
+        elif item.operation.name not in ["barrier", "delay"]:
+            for qq in item.qubits:
+                _temp_qubit = circuit.find_bit(qq).index
+                if _temp_qubit in active_qubits:
+                    active_qubits.remove(_temp_qubit)
+
+        if not active_cbits or not active_qubits:
+            break
+
+    return mapping
+
+
+def _has_control_flow(circuit: QuantumCircuit) -> bool:
+    return any(isinstance(instruction.operation, ControlFlowOp) for instruction in circuit)