Add Statevector-based SamplerV2 (Qiskit#11566)

* Add Statevector-based BaseSamplerV2 implementation

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

* Apply suggestions from code reivew

Co-authored-by: Ian Hincks <ian.hincks@gmail.com>
Co-authored-by: Christopher J. Wood <cjwood@us.ibm.com>

* Apply suggestions from code review

* shots should be positive

* add tests of zero shots

---------

Co-authored-by: Ian Hincks <ian.hincks@gmail.com>
Co-authored-by: Christopher J. Wood <cjwood@us.ibm.com>

qiskit/primitives/__init__.py

@@ -42,6 +42,8 @@
    BaseSampler
    Sampler
    BackendSampler
+   BaseSamplerV2
+   StatevectorSampler
    SamplerPub
 
 Results
@@ -58,7 +60,7 @@
 
 from .backend_estimator import BackendEstimator
 from .backend_sampler import BackendSampler
-from .base import BaseEstimator, BaseSampler
+from .base import BaseEstimator, BaseSampler, BaseSamplerV2
 from .base.estimator_result import EstimatorResult
 from .base.sampler_result import SamplerResult
 from .containers import (
@@ -71,3 +73,4 @@
 )
 from .estimator import Estimator
 from .sampler import Sampler
+from .statevector_sampler import StatevectorSampler

qiskit/primitives/containers/sampler_pub.py

@@ -93,8 +93,8 @@ def coerce(cls, pub: SamplerPubLike, shots: int | None = None) -> SamplerPub:
         if shots is not None:
             if not isinstance(shots, Integral) or isinstance(shots, bool):
                 raise TypeError("shots must be an integer")
-            if shots < 0:
-                raise ValueError("shots must be non-negative")
+            if shots <= 0:
+                raise ValueError("shots must be positive")
 
         if isinstance(pub, SamplerPub):
             if pub.shots is None and shots is not None:
@@ -129,8 +129,8 @@ def validate(self):
         if self.shots is not None:
             if not isinstance(self.shots, Integral) or isinstance(self.shots, bool):
                 raise TypeError("shots must be an integer")
-            if self.shots < 0:
-                raise ValueError("shots must be non-negative")
+            if self.shots <= 0:
+                raise ValueError("shots must be positive")
 
         # Cross validate circuits and parameter values
         num_parameters = self.parameter_values.num_parameters

qiskit/primitives/statevector_sampler.py

@@ -0,0 +1,206 @@
+# 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 StatevectorSampler(BaseSamplerV2):
+    """
+    Simple implementation of :class:`BaseSamplerV2` using full state vector simulation.
+    """
+
+    def __init__(self, *, default_shots: int = 1024, seed: np.random.Generator | int | None = None):
+        """
+        Args:
+            default_shots: The default shots for the sampler if not specified during run.
+            seed: The seed or Generator object for random number generation.
+                If None, a random seeded default RNG will be used.
+        """
+        self._default_shots = default_shots
+        self._seed = seed
+
+    @property
+    def default_shots(self) -> int:
+        """Return the default shots"""
+        return self._default_shots
+
+    @property
+    def seed(self) -> np.random.Generator | int | None:
+        """Return the seed or Generator object for random number generation."""
+        return self._seed
+
+    def run(
+        self, pubs: Iterable[SamplerPubLike], *, shots: int | None = None
+    ) -> PrimitiveJob[PrimitiveResult[PubResult]]:
+        if shots is None:
+            shots = self._default_shots
+        coerced_pubs = [SamplerPub.coerce(pub, shots) for pub in pubs]
+
+        job = PrimitiveJob(self._run, coerced_pubs)
+        job._submit()
+        return job
+
+    def _run(self, pubs: Iterable[SamplerPub]) -> PrimitiveResult[PubResult]:
+        results = [self._run_pub(pub) for pub in pubs]
+        return PrimitiveResult(results)
+
+    def _run_pub(self, pub: SamplerPub) -> PubResult:
+        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, bound_circuit in np.ndenumerate(bound_circuits):
+            final_state = Statevector(bound_circuit_to_instruction(bound_circuit))
+            final_state.seed(self._seed)
+            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)
+        return PubResult(data_bin, metadata={"shots": pub.shots})
+
+
+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
+            qbit = circuit.find_bit(item.qubits[0]).index
+            if cbit in active_cbits and qbit in active_qubits:
+                for creg in loc.registers:
+                    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)

test/python/primitives/containers/test_sampler_pub.py

@@ -60,10 +60,11 @@ def test_invalidate_shots_type(self, shots):
         with self.assertRaises(TypeError, msg=f"shots type {type(shots)} should raise TypeError"):
             SamplerPub(QuantumCircuit(), shots=shots)
 
-    def test_invalidate_shots_value(self):
+    @ddt.data(-1, 0)
+    def test_invalidate_shots_value(self, shots):
         """Test invalid shots argument value"""
-        with self.assertRaises(ValueError, msg="negative shots should raise ValueError"):
-            SamplerPub(QuantumCircuit(), shots=-1)
+        with self.assertRaises(ValueError, msg="non-positive shots should raise ValueError"):
+            SamplerPub(QuantumCircuit(), shots=shots)
 
     @ddt.idata(range(5))
     def test_validate_no_parameters(self, num_params):