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Allow Spinless JW #479

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27 changes: 21 additions & 6 deletions python/ffsim/qiskit/jordan_wigner.py
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Original file line number Diff line number Diff line change
Expand Up @@ -19,7 +19,9 @@
from ffsim.operators import FermionOperator


def jordan_wigner(op: FermionOperator, norb: int | None = None) -> SparsePauliOp:
def jordan_wigner(
op: FermionOperator, norb: int | None = None, allow_spinless: bool = True
) -> SparsePauliOp:
r"""Jordan-Wigner transformation.

Transform a fermion operator to a qubit operator using the Jordan-Wigner
Expand All @@ -41,6 +43,8 @@ def jordan_wigner(op: FermionOperator, norb: int | None = None) -> SparsePauliOp
op: The fermion operator to transform.
norb: The total number of spatial orbitals. If not specified, it is determined
by the largest-index orbital present in the operator.
allow_spinless: Flag enabling spinless FermionOperators to be
encoded with norb qubits.

Returns:
The qubit operator as a Qiskit SparsePauliOp.
Expand Down Expand Up @@ -69,25 +73,36 @@ def jordan_wigner(op: FermionOperator, norb: int | None = None) -> SparsePauliOp
f"only {norb} were specified."
)

qubit_terms = [SparsePauliOp.from_sparse_list([("", [], 0.0)], num_qubits=2 * norb)]
is_spinless = allow_spinless
if allow_spinless:
for term in op.keys():
if any(t[1] for t in term):
is_spinless = False
break

num_qubits = norb if is_spinless else 2 * norb
qubit_terms = [
SparsePauliOp.from_sparse_list([("", [], 0.0)], num_qubits=num_qubits)
]

for term, coeff in op.items():
qubit_op = SparsePauliOp.from_sparse_list(
[("", [], coeff)], num_qubits=2 * norb
[("", [], coeff)], num_qubits=num_qubits
)
for action, spin, orb in term:
qubit_op @= _qubit_action(action, orb + spin * norb, norb)
qubit_op @= _qubit_action(action, orb + spin * norb, num_qubits)
qubit_terms.append(qubit_op)

return SparsePauliOp.sum(qubit_terms).simplify()


@functools.cache
def _qubit_action(action: bool, qubit: int, norb: int):
def _qubit_action(action: bool, qubit: int, num_qubits: int):
qubits = list(range(qubit + 1))
return SparsePauliOp.from_sparse_list(
[
("Z" * qubit + "X", qubits, 0.5),
("Z" * qubit + "Y", qubits, -0.5j if action else 0.5j),
],
num_qubits=2 * norb,
num_qubits=num_qubits,
)
48 changes: 46 additions & 2 deletions tests/python/qiskit/jordan_wigner_test.py
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Original file line number Diff line number Diff line change
Expand Up @@ -17,6 +17,50 @@
import ffsim.random.random


@pytest.mark.parametrize("spinless", [True, False])
@pytest.mark.parametrize("norb, nelec", ffsim.testing.generate_norb_nelec(range(1, 5)))
def test_random_molecular_hamiltonian(
norb: int, nelec: tuple[int, int], spinless: bool
):
"""Test on random fermion Hamiltonian."""
rng = np.random.default_rng(4482)

if spinless:
nelec = nelec[0]
mol_ham = ffsim.random.random_molecular_hamiltonian_spinless(norb, seed=rng)
else:
mol_ham = ffsim.random.random_molecular_hamiltonian(norb, seed=rng)

op = ffsim.fermion_operator(mol_ham)
linop = ffsim.linear_operator(op, norb=norb, nelec=nelec)
vec = ffsim.random.random_state_vector(ffsim.dim(norb, nelec), seed=rng)
expected_result = ffsim.qiskit.ffsim_vec_to_qiskit_vec(linop @ vec, norb, nelec)
qubit_op = ffsim.qiskit.jordan_wigner(op)
qubit_op_sparse = qubit_op.to_matrix(sparse=True)
actual_result = qubit_op_sparse @ ffsim.qiskit.ffsim_vec_to_qiskit_vec(
vec, norb, nelec
)
np.testing.assert_allclose(actual_result, expected_result, atol=1e-12)

qubit_op = ffsim.qiskit.jordan_wigner(op, norb=norb)
qubit_op_sparse = qubit_op.to_matrix(sparse=True)
actual_result = qubit_op_sparse @ ffsim.qiskit.ffsim_vec_to_qiskit_vec(
vec, norb, nelec
)
np.testing.assert_allclose(actual_result, expected_result, atol=1e-12)
actual_result = qubit_op_sparse @ ffsim.qiskit.ffsim_vec_to_qiskit_vec(
vec, norb, nelec
)
np.testing.assert_allclose(actual_result, expected_result, atol=1e-12)

qubit_op = ffsim.qiskit.jordan_wigner(op, norb=norb)
qubit_op_sparse = qubit_op.to_matrix(sparse=True)
actual_result = qubit_op_sparse @ ffsim.qiskit.ffsim_vec_to_qiskit_vec(
vec, norb, nelec
)
np.testing.assert_allclose(actual_result, expected_result, atol=1e-12)


@pytest.mark.parametrize("norb, nelec", ffsim.testing.generate_norb_nelec(range(5)))
def test_random(norb: int, nelec: tuple[int, int]):
"""Test on random fermion Hamiltonian."""
Expand All @@ -26,14 +70,14 @@ def test_random(norb: int, nelec: tuple[int, int]):
vec = ffsim.random.random_state_vector(ffsim.dim(norb, nelec), seed=rng)
expected_result = ffsim.qiskit.ffsim_vec_to_qiskit_vec(linop @ vec, norb, nelec)

qubit_op = ffsim.qiskit.jordan_wigner(op)
qubit_op = ffsim.qiskit.jordan_wigner(op, allow_spinless=False)
qubit_op_sparse = qubit_op.to_matrix(sparse=True)
actual_result = qubit_op_sparse @ ffsim.qiskit.ffsim_vec_to_qiskit_vec(
vec, norb, nelec
)
np.testing.assert_allclose(actual_result, expected_result, atol=1e-12)

qubit_op = ffsim.qiskit.jordan_wigner(op, norb=norb)
qubit_op = ffsim.qiskit.jordan_wigner(op, norb=norb, allow_spinless=False)
qubit_op_sparse = qubit_op.to_matrix(sparse=True)
actual_result = qubit_op_sparse @ ffsim.qiskit.ffsim_vec_to_qiskit_vec(
vec, norb, nelec
Expand Down