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Add default decomposition for cirq.QubitPermutationGate in terms of adjacent swaps #5093

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Merged
merged 6 commits into from
Mar 18, 2022
Merged

Add default decomposition for cirq.QubitPermutationGate in terms of adjacent swaps #5093

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1e8f898
Add default decomposition for cirq.QubitPermutationGate in terms of a…
tanujkhattar Mar 17, 2022
54a783e
Merge branch 'master' of https://github.com/quantumlib/cirq into qubi…
tanujkhattar Mar 17, 2022
e3d5f4a
Merge branch 'master' of https://github.com/quantumlib/cirq into qubi…
tanujkhattar Mar 17, 2022
68c8ce9
Change sorting to use odd-even sort
tanujkhattar Mar 17, 2022
ed002f4
Fix mypy types
tanujkhattar Mar 17, 2022
f4c2154
Merge branch 'master' into qubit_permutation_gate
CirqBot Mar 18, 2022
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11 changes: 10 additions & 1 deletion cirq-core/cirq/ops/permutation_gate.py
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Original file line number Diff line number Diff line change
Expand Up @@ -16,7 +16,7 @@

from cirq import protocols, value
from cirq._compat import deprecated
from cirq.ops import raw_types
from cirq.ops import raw_types, swap_gates

if TYPE_CHECKING:
import cirq
Expand Down Expand Up @@ -74,6 +74,15 @@ def num_qubits(self):
def _has_unitary_(self):
return True

def _decompose_(self, qubits: Sequence['cirq.Qid']) -> 'cirq.OP_TREE':
qubit_ids = [*range(len(qubits))]
for i in range(len(qubits)):
q_i = qubit_ids.index(self._permutation.index(i))
for j in range(q_i, i, -1):
yield swap_gates.SWAP(qubits[j], qubits[j - 1])
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Limiting SWAPs to adjacent-index qubits presupposes a linear qubit layout; on a 2D grid or more exotic layout, this will not work:

q = GridQubit.rect(2, 2)
# q[1] and q[2] are not adjacent ((0, 1) and (1, 0))

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Note that the decomposition here is not intended to be architecture aware. The goal here is to simply write some decomposition in terms of other "simpler" gates which eventually ends up decomposing into the native gateset.

For device specific routing, we would need more sophisticated transformers which take care of device topology, but that is out of scope of this PR. The choice to use adjacent index swaps which can be useful for a linear qubit layout is an arbitrary one, mostly because running circuits on a chain of qubits is a common use case and a low hanging fruit.

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If this is meant to be architecture-agnostic, why don't we swap qubits directly (ignoring adjacency)?

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As I said, it's a choice we need to make. Both the approaches would be valid for the scope of this PR.

If a user is relying on the default decomposition of Cirq for compiling their circuit, it means they have already given up fine grained control. The reason I want to stick to this approach is that it's a low hanging fruit which targets a common use case of running circuits on a chain of qubits. It's not going to be enough for every scenario, but still slightly better than assuming all to all connectivity.

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It's not going to be enough for every scenario, but still slightly better than assuming all to all connectivity.

I'm not convinced that linear-swap is better than all-to-all, or even equally valid. Compared to linear-swap, all-to-all is:

  • cheaper to run on simulators (which use default decomposition)
  • easier to convert to efficient circuits in device-specific layout
  • cheaper to construct here (low priority, but worth considering)

I'd support adding a separate method for converting to linear-swap, but the default should be all-to-all.

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cheaper to run on simulators (which use default decomposition)

The class already has a _apply_unitary_ defined, which should be used by simulators over the default decomposition. Is that not the case?

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This is true for Cirq-internal simulators, but not in general. For example, qsim attempts to decompose gates to a specific subset that it natively supports, falling back on conversion to unitary only if no decomposition is defined (code).

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Opened #5097 to track the optimization.

qubit_ids[j], qubit_ids[j - 1] = qubit_ids[j - 1], qubit_ids[j]
assert self._permutation[qubit_ids[i]] == i

def _apply_unitary_(self, args: 'cirq.ApplyUnitaryArgs'):
# Compute the permutation index list.
permuted_axes = list(range(len(args.target_tensor.shape)))
Expand Down
11 changes: 9 additions & 2 deletions cirq-core/cirq/ops/permutation_gate_test.py
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Original file line number Diff line number Diff line change
Expand Up @@ -15,6 +15,7 @@
import pytest

import cirq
import numpy as np
from cirq.ops import QubitPermutationGate


Expand All @@ -30,8 +31,12 @@ def test_permutation_gate_repr():
cirq.testing.assert_equivalent_repr(QubitPermutationGate([0, 1]))


def test_permutation_gate_consistent_protocols():
gate = QubitPermutationGate([1, 0, 2, 3])
rs = np.random.RandomState(seed=1234)


@pytest.mark.parametrize('permutation', [rs.permutation(i) for i in range(3, 7)])
def test_permutation_gate_consistent_protocols(permutation):
gate = QubitPermutationGate(list(permutation))
cirq.testing.assert_implements_consistent_protocols(gate)


Expand Down Expand Up @@ -98,6 +103,8 @@ def test_permutation_gate_maps(maps, permutation):
permutationOp = cirq.QubitPermutationGate(permutation).on(*qs)
circuit = cirq.Circuit(permutationOp)
cirq.testing.assert_equivalent_computational_basis_map(maps, circuit)
circuit = cirq.Circuit(cirq.I.on_each(*qs), cirq.decompose(permutationOp))
cirq.testing.assert_equivalent_computational_basis_map(maps, circuit)


def test_setters_deprecated():
Expand Down