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Optimise QubitPermutationGate decomposition (quantumlib#6588)
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migueltorrescosta authored and jselig-rigetti committed May 28, 2024
1 parent 4769f83 commit a21219b
Showing 1 changed file with 16 additions and 18 deletions.
34 changes: 16 additions & 18 deletions cirq-core/cirq/ops/permutation_gate.py
Original file line number Diff line number Diff line change
Expand Up @@ -12,7 +12,7 @@
# See the License for the specific language governing permissions and
# limitations under the License.

from typing import Any, Dict, Iterable, Sequence, Tuple, TYPE_CHECKING
from typing import Any, Dict, Sequence, Tuple, TYPE_CHECKING

from cirq import protocols, value
from cirq.ops import raw_types, swap_gates
Expand Down Expand Up @@ -74,23 +74,21 @@ def _has_unitary_(self):
return True

def _decompose_(self, qubits: Sequence['cirq.Qid']) -> 'cirq.OP_TREE':
n = len(qubits)
qubit_ids = [*range(n)]
is_sorted = False

def _swap_if_out_of_order(idx: int) -> Iterable['cirq.Operation']:
nonlocal is_sorted
if self._permutation[qubit_ids[idx]] > self._permutation[qubit_ids[idx + 1]]:
yield swap_gates.SWAP(qubits[idx], qubits[idx + 1])
qubit_ids[idx + 1], qubit_ids[idx] = qubit_ids[idx], qubit_ids[idx + 1]
is_sorted = False

while not is_sorted:
is_sorted = True
for i in range(0, n - 1, 2):
yield from _swap_if_out_of_order(i)
for i in range(1, n - 1, 2):
yield from _swap_if_out_of_order(i)
permutation = [p for p in self.permutation]

for i in range(len(permutation)):

if permutation[i] == -1:
continue
cycle = [i]
while permutation[cycle[-1]] != i:
cycle.append(permutation[cycle[-1]])

for j in cycle:
permutation[j] = -1

for idx in cycle[1:]:
yield swap_gates.SWAP(qubits[cycle[0]], qubits[idx])

def _apply_unitary_(self, args: 'cirq.ApplyUnitaryArgs'):
# Compute the permutation index list.
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