quantumlib · tanujkhattar · Jun 5, 2023 · Jun 5, 2023 · Jun 5, 2023 · Jun 5, 2023
diff --git a/cirq-core/cirq/__init__.py b/cirq-core/cirq/__init__.py
@@ -284,6 +284,7 @@
     qft,
     Qid,
     QuantumFourierTransformGate,
+    QubitManager,
     QubitOrder,
     QubitOrderOrList,
     QubitPermutationGate,
@@ -566,6 +567,7 @@
     decompose,
     decompose_once,
     decompose_once_with_qubits,
+    DecompositionContext,
     DEFAULT_RESOLVERS,
     definitely_commutes,
     equal_up_to_global_phase,

diff --git a/cirq-core/cirq/ops/__init__.py b/cirq-core/cirq/ops/__init__.py
@@ -120,6 +120,8 @@
 
 from cirq.ops.controlled_operation import ControlledOperation
 
+from cirq.ops.qubit_manager import BorrowableQubit, CleanQubit, QubitManager, SimpleQubitManager
+
 from cirq.ops.qubit_order import QubitOrder
 
 from cirq.ops.qubit_order_or_list import QubitOrderOrList

diff --git a/cirq-core/cirq/ops/classically_controlled_operation.py b/cirq-core/cirq/ops/classically_controlled_operation.py
@@ -105,7 +105,12 @@ def with_qubits(self, *new_qubits):
         )
 
     def _decompose_(self):
-        result = protocols.decompose_once(self._sub_operation, NotImplemented, flatten=False)
+        return self._decompose_with_context_()
+
+    def _decompose_with_context_(self, context: Optional['cirq.DecompositionContext'] = None):
+        result = protocols.decompose_once(
+            self._sub_operation, NotImplemented, flatten=False, context=context
+        )
         if result is NotImplemented:
             return NotImplemented
 

diff --git a/cirq-core/cirq/ops/controlled_gate.py b/cirq-core/cirq/ops/controlled_gate.py
@@ -151,6 +151,11 @@ def _qid_shape_(self) -> Tuple[int, ...]:
 
     def _decompose_(
         self, qubits: Tuple['cirq.Qid', ...]
+    ) -> Union[None, NotImplementedType, 'cirq.OP_TREE']:
+        return self._decompose_with_context_(qubits)
+
+    def _decompose_with_context_(
+        self, qubits: Tuple['cirq.Qid', ...], context: Optional['cirq.DecompositionContext'] = None
     ) -> Union[None, NotImplementedType, 'cirq.OP_TREE']:
         if (
             protocols.has_unitary(self.sub_gate)
@@ -192,15 +197,21 @@ def _decompose_(
                 )
             )
             if self != controlled_z:
-                return protocols.decompose_once_with_qubits(controlled_z, qubits, NotImplemented)
+                return protocols.decompose_once_with_qubits(
+                    controlled_z, qubits, NotImplemented, context=context
+                )
 
         if isinstance(self.sub_gate, matrix_gates.MatrixGate):
             # Default decompositions of 2/3 qubit `cirq.MatrixGate` ignores global phase, which is
             # local phase in the controlled variant and hence cannot be ignored.
             return NotImplemented
 
         result = protocols.decompose_once_with_qubits(
-            self.sub_gate, qubits[self.num_controls() :], NotImplemented, flatten=False
+            self.sub_gate,
+            qubits[self.num_controls() :],
+            NotImplemented,
+            flatten=False,
+            context=context,
         )
         if result is NotImplemented:
             return NotImplemented

diff --git a/cirq-core/cirq/ops/controlled_operation.py b/cirq-core/cirq/ops/controlled_operation.py
@@ -146,8 +146,11 @@ def with_qubits(self, *new_qubits):
         )
 
     def _decompose_(self):
+        return self._decompose_with_context_()
+
+    def _decompose_with_context_(self, context: Optional['cirq.DecompositionContext'] = None):
         result = protocols.decompose_once_with_qubits(
-            self.gate, self.qubits, NotImplemented, flatten=False
+            self.gate, self.qubits, NotImplemented, flatten=False, context=context
         )
         if result is not NotImplemented:
             return result
@@ -157,7 +160,9 @@ def _decompose_(self):
             # local phase in the controlled variant and hence cannot be ignored.
             return NotImplemented
 
-        result = protocols.decompose_once(self.sub_operation, NotImplemented, flatten=False)
+        result = protocols.decompose_once(
+            self.sub_operation, NotImplemented, flatten=False, context=context
+        )
         if result is NotImplemented:
             return NotImplemented
 

diff --git a/cirq-core/cirq/ops/gate_operation.py b/cirq-core/cirq/ops/gate_operation.py
@@ -160,8 +160,13 @@ def _num_qubits_(self):
         return len(self._qubits)
 
     def _decompose_(self) -> 'cirq.OP_TREE':
+        return self._decompose_with_context_()
+
+    def _decompose_with_context_(
+        self, context: Optional['cirq.DecompositionContext'] = None
+    ) -> 'cirq.OP_TREE':
         return protocols.decompose_once_with_qubits(
-            self.gate, self.qubits, NotImplemented, flatten=False
+            self.gate, self.qubits, NotImplemented, flatten=False, context=context
         )
 
     def _pauli_expansion_(self) -> value.LinearDict[str]:

diff --git a/cirq-core/cirq/ops/qubit_manager.py b/cirq-core/cirq/ops/qubit_manager.py
@@ -0,0 +1,91 @@
+# Copyright 2023 The Cirq Developers
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import abc
+import dataclasses
+from typing import Iterable, List, TYPE_CHECKING
+from cirq.ops import raw_types
+
+if TYPE_CHECKING:
+    import cirq
+
+
+class QubitManager(metaclass=abc.ABCMeta):
+    @abc.abstractmethod
+    def qalloc(self, n: int, dim: int = 2) -> List['cirq.Qid']:
+        """Allocate `n` clean qubits, i.e. qubits guaranteed to be in state |0>."""
+
+    @abc.abstractmethod
+    def qborrow(self, n: int, dim: int = 2) -> List['cirq.Qid']:
+        """Allocate `n` dirty qubits, i.e. the returned qubits can be in any state."""
+
+    @abc.abstractmethod
+    def qfree(self, qubits: Iterable['cirq.Qid']) -> None:
+        """Free pre-allocated clean or dirty qubits managed by this qubit manager."""
+
+
+@dataclasses.dataclass(frozen=True)
+class _BaseAncillaQid(raw_types.Qid):
+    id: int
+    dim: int = 2
+
+    def _comparison_key(self) -> int:
+        return self.id
+
+    @property
+    def dimension(self) -> int:
+        return self.dim
+
+    def __repr__(self) -> str:
+        dim_str = f', dim={self.dim}' if self.dim != 2 else ''
+        return f"cirq.ops.{type(self).__name__}({self.id}{dim_str})"
+
+
+class CleanQubit(_BaseAncillaQid):
+    """An internal qid type that represents a clean ancilla allocation."""
+
+    def __str__(self) -> str:
+        dim_str = f' (d={self.dimension})' if self.dim != 2 else ''
+        return f"_c({self.id}){dim_str}"
+
+
+class BorrowableQubit(_BaseAncillaQid):
+    """An internal qid type that represents a dirty ancilla allocation."""
+
+    def __str__(self) -> str:
+        dim_str = f' (d={self.dimension})' if self.dim != 2 else ''
+        return f"_b({self.id}){dim_str}"
+
+
+class SimpleQubitManager(QubitManager):
+    """Allocates a new `CleanQubit`/`BorrowableQubit` for every `qalloc`/`qborrow` request."""
+
+    def __init__(self):
+        self._clean_id = 0
+        self._borrow_id = 0
+
+    def qalloc(self, n: int, dim: int = 2) -> List['cirq.Qid']:
+        self._clean_id += n
+        return [CleanQubit(i, dim) for i in range(self._clean_id - n, self._clean_id)]
+
+    def qborrow(self, n: int, dim: int = 2) -> List['cirq.Qid']:
+        self._borrow_id = self._borrow_id + n
+        return [BorrowableQubit(i, dim) for i in range(self._borrow_id - n, self._borrow_id)]
+
+    def qfree(self, qubits: Iterable['cirq.Qid']) -> None:
+        for q in qubits:
+            good = isinstance(q, CleanQubit) and q.id < self._clean_id
+            good |= isinstance(q, BorrowableQubit) and q.id < self._borrow_id
+            if not good:
+                raise ValueError(f"{q} was not allocated by {self}")
diff --git a/cirq-core/cirq/ops/qubit_manager_test.py b/cirq-core/cirq/ops/qubit_manager_test.py
@@ -0,0 +1,66 @@
+# Copyright 2023 The Cirq Developers
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import cirq
+from cirq.ops import qubit_manager as cqi
+import pytest
+
+
+def test_clean_qubits():
+    q = cqi.CleanQubit(1)
+    assert q.id == 1
+    assert q.dimension == 2
+    assert str(q) == '_c(1)'
+    assert repr(q) == 'cirq.ops.CleanQubit(1)'
+
+    q = cqi.CleanQubit(2, dim=3)
+    assert q.id == 2
+    assert q.dimension == 3
+    assert str(q) == '_c(2) (d=3)'
+    assert repr(q) == 'cirq.ops.CleanQubit(2, dim=3)'
+
+    assert cqi.CleanQubit(1) < cqi.CleanQubit(2)
+
+
+def test_borrow_qubits():
+    q = cqi.BorrowableQubit(10)
+    assert q.id == 10
+    assert q.dimension == 2
+    assert str(q) == '_b(10)'
+    assert repr(q) == 'cirq.ops.BorrowableQubit(10)'
+
+    q = cqi.BorrowableQubit(20, dim=4)
+    assert q.id == 20
+    assert q.dimension == 4
+    assert str(q) == '_b(20) (d=4)'
+    assert repr(q) == 'cirq.ops.BorrowableQubit(20, dim=4)'
+
+    assert cqi.BorrowableQubit(1) < cqi.BorrowableQubit(2)
+
+
+@pytest.mark.parametrize('_', range(2))
+def test_simple_qubit_manager(_):
+    qm = cirq.ops.SimpleQubitManager()
+    assert qm.qalloc(1) == [cqi.CleanQubit(0)]
+    assert qm.qalloc(2) == [cqi.CleanQubit(1), cqi.CleanQubit(2)]
+    assert qm.qalloc(1, dim=3) == [cqi.CleanQubit(3, dim=3)]
+    assert qm.qborrow(1) == [cqi.BorrowableQubit(0)]
+    assert qm.qborrow(2) == [cqi.BorrowableQubit(1), cqi.BorrowableQubit(2)]
+    assert qm.qborrow(1, dim=3) == [cqi.BorrowableQubit(3, dim=3)]
+    qm.qfree([cqi.CleanQubit(i) for i in range(3)] + [cqi.CleanQubit(3, dim=3)])
+    qm.qfree([cqi.BorrowableQubit(i) for i in range(3)] + [cqi.BorrowableQubit(3, dim=3)])
+    with pytest.raises(ValueError, match="not allocated"):
+        qm.qfree([cqi.CleanQubit(10)])
+    with pytest.raises(ValueError, match="not allocated"):
+        qm.qfree([cqi.BorrowableQubit(10)])
diff --git a/cirq-core/cirq/ops/raw_types.py b/cirq-core/cirq/ops/raw_types.py
@@ -830,7 +830,14 @@ def _json_dict_(self) -> Dict[str, Any]:
         return protocols.obj_to_dict_helper(self, ['sub_operation', 'tags'])
 
     def _decompose_(self) -> 'cirq.OP_TREE':
-        return protocols.decompose_once(self.sub_operation, default=None, flatten=False)
+        return self._decompose_with_context_()
+
+    def _decompose_with_context_(
+        self, context: Optional['cirq.DecompositionContext'] = None
+    ) -> 'cirq.OP_TREE':
+        return protocols.decompose_once(
+            self.sub_operation, default=None, flatten=False, context=context
+        )
 
     def _pauli_expansion_(self) -> value.LinearDict[str]:
         return protocols.pauli_expansion(self.sub_operation)
@@ -979,7 +986,14 @@ def __pow__(self, power):
         return NotImplemented
 
     def _decompose_(self, qubits):
-        return protocols.inverse(protocols.decompose_once_with_qubits(self._original, qubits))
+        return self._decompose_with_context_(qubits)
+
+    def _decompose_with_context_(
+        self, qubits: Sequence['cirq.Qid'], context: Optional['cirq.DecompositionContext'] = None
+    ) -> 'cirq.OP_TREE':
+        return protocols.inverse(
+            protocols.decompose_once_with_qubits(self._original, qubits, context=context)
+        )
 
     def _has_unitary_(self):
         from cirq import protocols, devices

diff --git a/cirq-core/cirq/ops/raw_types_test.py b/cirq-core/cirq/ops/raw_types_test.py
@@ -201,6 +201,8 @@ def __repr__(self):
     assert i**-1 == t
     assert t**-1 == i
     assert cirq.decompose(i) == [cirq.X(a), cirq.S(b) ** -1, cirq.Z(a)]
+    assert [*i._decompose_()] == [cirq.X(a), cirq.S(b) ** -1, cirq.Z(a)]
+    assert [*i.gate._decompose_([a, b])] == [cirq.X(a), cirq.S(b) ** -1, cirq.Z(a)]
     cirq.testing.assert_allclose_up_to_global_phase(
         cirq.unitary(i), cirq.unitary(t).conj().T, atol=1e-8
     )
@@ -618,6 +620,7 @@ def test_tagged_operation_forwards_protocols():
     np.testing.assert_equal(cirq.unitary(tagged_h), cirq.unitary(h))
     assert cirq.has_unitary(tagged_h)
     assert cirq.decompose(tagged_h) == cirq.decompose(h)
+    assert [*tagged_h._decompose_()] == cirq.decompose(h)
     assert cirq.pauli_expansion(tagged_h) == cirq.pauli_expansion(h)
     assert cirq.equal_up_to_global_phase(h, tagged_h)
     assert np.isclose(cirq.kraus(h), cirq.kraus(tagged_h)).all()

diff --git a/cirq-core/cirq/protocols/__init__.py b/cirq-core/cirq/protocols/__init__.py
@@ -50,6 +50,7 @@
     decompose,
     decompose_once,
     decompose_once_with_qubits,
+    DecompositionContext,
     SupportsDecompose,
     SupportsDecomposeWithQubits,
 )