add spinless givens ansatz gate

python/ffsim/qiskit/__init__.py

@@ -14,6 +14,7 @@
 
 from ffsim.qiskit.gates import (
     DiagCoulombEvolutionJW,
+    GivensAnsatzOperatorSpinlessJW,
     OrbitalRotationJW,
     OrbitalRotationSpinlessJW,
     PrepareHartreeFockJW,
@@ -37,6 +38,7 @@
 __all__ = [
     "DiagCoulombEvolutionJW",
     "DropNegligible",
+    "GivensAnsatzOperatorSpinlessJW",
     "MergeOrbitalRotations",
     "OrbitalRotationJW",
     "OrbitalRotationSpinlessJW",

python/ffsim/qiskit/gates/__init__.py

@@ -11,6 +11,7 @@
 """Qiskit fermionic quantum gates."""
 
 from ffsim.qiskit.gates.diag_coulomb import DiagCoulombEvolutionJW
+from ffsim.qiskit.gates.givens_ansatz import GivensAnsatzOperatorSpinlessJW
 from ffsim.qiskit.gates.orbital_rotation import (
     OrbitalRotationJW,
     OrbitalRotationSpinlessJW,
@@ -25,6 +26,7 @@
 
 __all__ = [
     "DiagCoulombEvolutionJW",
+    "GivensAnsatzOperatorSpinlessJW",
     "OrbitalRotationJW",
     "OrbitalRotationSpinlessJW",
     "PrepareHartreeFockJW",

python/ffsim/qiskit/gates/givens_ansatz.py

@@ -0,0 +1,68 @@
+# (C) Copyright IBM 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.
+
+"""Givens rotation ansatz gate."""
+
+from __future__ import annotations
+
+import itertools
+import math
+from collections.abc import Iterator, Sequence
+
+from qiskit.circuit import (
+    CircuitInstruction,
+    Gate,
+    QuantumCircuit,
+    QuantumRegister,
+    Qubit,
+)
+from qiskit.circuit.library import XXPlusYYGate
+
+from ffsim.variational import GivensAnsatzOperator
+
+
+class GivensAnsatzOperatorSpinlessJW(Gate):
+    """Givens rotation ansatz operator under the Jordan-Wigner transformation.
+
+    See :class:`ffsim.GivensAnsatzOperator` for a description of this gate's unitary.
+    """
+
+    def __init__(
+        self, givens_ansatz_operator: GivensAnsatzOperator, *, label: str | None = None
+    ):
+        """Create a new Givens ansatz operator gate.
+
+        Args:
+            givens_ansatz_operator: The Givens rotation ansatz operator.
+            label: The label of the gate.
+        """
+        self.givens_ansatz_operator = givens_ansatz_operator
+        super().__init__(
+            "givens_ansatz_jw", givens_ansatz_operator.norb, [], label=label
+        )
+
+    def _define(self):
+        """Gate decomposition."""
+        qubits = QuantumRegister(self.num_qubits)
+        self.definition = QuantumCircuit.from_instructions(
+            _givens_ansatz_jw(qubits, self.givens_ansatz_operator), qubits=qubits
+        )
+
+
+def _givens_ansatz_jw(
+    qubits: Sequence[Qubit], givens_ansatz_operator: GivensAnsatzOperator
+) -> Iterator[CircuitInstruction]:
+    for (i, j), theta in zip(
+        itertools.cycle(givens_ansatz_operator.interaction_pairs),
+        givens_ansatz_operator.thetas,
+    ):
+        yield CircuitInstruction(
+            XXPlusYYGate(2 * theta, -0.5 * math.pi), (qubits[i], qubits[j])
+        )

python/ffsim/variational/givens.py

@@ -43,6 +43,7 @@ class GivensAnsatzOperator:
     norb: int
     interaction_pairs: list[tuple[int, int]]
     thetas: np.ndarray
+    # TODO add phis for complex phases
 
     def _apply_unitary_(
         self, vec: np.ndarray, norb: int, nelec: tuple[int, int], copy: bool

tests/python/qiskit/givens_ansatz_test.py

@@ -0,0 +1,55 @@
+# (C) Copyright IBM 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.
+
+"""Tests for Givens rotation ansatz gate."""
+
+from __future__ import annotations
+
+import numpy as np
+import pytest
+from qiskit.quantum_info import Statevector
+
+import ffsim
+
+
+def brickwork(norb: int, n_layers: int):
+    for i in range(n_layers):
+        for j in range(i % 2, norb - 1, 2):
+            yield (j, j + 1)
+
+
+@pytest.mark.parametrize("norb, nocc", ffsim.testing.generate_norb_nocc(range(5)))
+def test_random_givens_ansatz_operator(norb: int, nocc: int):
+    """Test random Givens rotation ansatz gives correct output state."""
+    rng = np.random.default_rng()
+    nelec = (nocc, 0)
+    dim = ffsim.dim(norb, nelec)
+    for _ in range(3):
+        n_layers = norb
+        interaction_pairs = list(brickwork(norb, n_layers))
+        thetas = rng.uniform(-np.pi, np.pi, size=len(interaction_pairs))
+        givens_ansatz_op = ffsim.GivensAnsatzOperator(norb, interaction_pairs, thetas)
+        gate = ffsim.qiskit.GivensAnsatzOperatorSpinlessJW(givens_ansatz_op)
+
+        small_vec = ffsim.random.random_statevector(dim, seed=rng)
+        big_vec = ffsim.qiskit.ffsim_vec_to_qiskit_vec(
+            small_vec, norb=norb, nelec=nelec
+        )
+
+        statevec = Statevector(big_vec).evolve(gate)
+        result = ffsim.qiskit.qiskit_vec_to_ffsim_vec(
+            np.array(statevec), norb=norb, nelec=nelec
+        )
+
+        expected = ffsim.apply_unitary(
+            small_vec, givens_ansatz_op, norb=norb, nelec=nelec
+        )
+
+        np.testing.assert_allclose(result, expected)