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Qiskit gates: Support PermutationGate #443

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Qiskit gates: Support PermutationGate #443

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6d2c9b5
Add support for PermutationGate in state vector evolution
q-inho Aug 7, 2025
57be164
Add PermutationGate test
q-inho Aug 7, 2025
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38 changes: 38 additions & 0 deletions python/ffsim/qiskit/sim.py
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Original file line number Diff line number Diff line change
Expand Up @@ -30,6 +30,7 @@
DiagonalGate,
GlobalPhaseGate,
Measure,
PermutationGate,
PhaseGate,
RZGate,
RZZGate,
Expand Down Expand Up @@ -379,6 +380,19 @@ def _evolve_state_vector_spinless(
vec = _apply_iswap(vec, (i, j), norb=norb, nelec=nelec, copy=False)
return states.StateVector(vec=vec, norb=norb, nelec=nelec)

if isinstance(op, PermutationGate):
perm = list(op.pattern)
qs = qubit_indices.copy()
for k in range(len(perm)):
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I think this can be improved by factoring out a generator function that yields the swap pairs of the permutation decomposition. Then this can look something like

for i, j in _decompose_permutation(perm):
    vec = _apply_swap(vec, (qs[i], qs[j]), ... )

while perm[k] != k:
j = perm[k]
vec = _apply_swap(
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This will introduce many "swap defect" sign fixing operations. I wonder if we can use FSWAPS instead of SWAPS, and then only fix the signs a single time at the end. Or maybe it is not straightforward ...

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Thank you for pointing this out!
I have to switch the implementation to use FSWAPs for each transposition so the $−1$ on $|11\rangle$ is handled locally. So it can removes those repeated sign corrections.

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If it's possible to use FSWAPs instead of SWAPs, then the entire permutation can actually be performed as a single orbital rotation.

vec, (qs[k], qs[j]), norb=norb, nelec=nelec, copy=False
)
perm[k], perm[j] = perm[j], perm[k]
qs[k], qs[j] = qs[j], qs[k]
return states.StateVector(vec=vec, norb=norb, nelec=nelec)

if isinstance(op, XXPlusYYGate):
i, j = qubit_indices
theta, beta = op.params
Expand Down Expand Up @@ -715,6 +729,30 @@ def _evolve_state_vector_spinful(
)
return states.StateVector(vec=vec, norb=norb, nelec=nelec)

if isinstance(op, PermutationGate):
perm = list(op.pattern)
qs = qubit_indices.copy()
for k in range(len(perm)):
while perm[k] != k:
j = perm[k]
if (qs[k] < norb) != (qs[j] < norb):
raise ValueError(
f"Gate of type '{op.__class__.__name__}' must be applied on "
"orbitals of the same spin."
)
spin = Spin.ALPHA if qs[k] < norb else Spin.BETA
vec = _apply_swap(
vec,
(qs[k] % norb, qs[j] % norb),
norb=norb,
nelec=nelec,
spin=spin,
copy=False,
)
perm[k], perm[j] = perm[j], perm[k]
qs[k], qs[j] = qs[j], qs[k]
return states.StateVector(vec=vec, norb=norb, nelec=nelec)

if isinstance(op, XXPlusYYGate):
i, j = qubit_indices
if (i < norb) != (j < norb):
Expand Down
4 changes: 4 additions & 0 deletions tests/python/qiskit/sim_test.py
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Original file line number Diff line number Diff line change
Expand Up @@ -28,6 +28,7 @@
CZGate,
DiagonalGate,
GlobalPhaseGate,
PermutationGate,
PhaseGate,
RZGate,
RZZGate,
Expand Down Expand Up @@ -230,6 +231,8 @@ def test_qiskit_gates_spinful(norb: int, nelec: tuple[int, int]):
circuit.append(DiagonalGate(diag), [qubits[i] for i in chosen])
diag = np.exp(1j * rng.uniform(-np.pi, np.pi, size=1 << 2 * norb))
circuit.append(DiagonalGate(diag), qubits)
circuit.append(PermutationGate(list(rng.permutation(norb))), qubits[:norb])
circuit.append(PermutationGate(list(rng.permutation(norb))), qubits[norb:])
circuit.append(GlobalPhaseGate(rng.uniform(-10, 10)))

# Compute state vector using ffsim
Expand Down Expand Up @@ -304,6 +307,7 @@ def test_qiskit_gates_spinless(norb: int, nocc: int):
circuit.append(DiagonalGate(diag), [qubits[i] for i in chosen])
diag = np.exp(1j * rng.uniform(-np.pi, np.pi, size=1 << norb))
circuit.append(DiagonalGate(diag), qubits)
circuit.append(PermutationGate(list(rng.permutation(norb))), qubits)
circuit.append(GlobalPhaseGate(rng.uniform(-10, 10)))

# Compute state vector using ffsim
Expand Down
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